Advanced Civil Engineering

LO - 1 Underst and the methods and techniques used in tunneling activities

LO - 2 Understand the methods and techniques used in Hydraulic structures.

LO - 3 Understand the methods and techniques used in Marine Work.

LO - 4 Understand the methods and techniques used in highway construction and railway works.

LO - 5 Be able to solve problems arising from complex civil engineering activities.

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    LO - 1 Understand the methods and techniques used in tunneling activities

1.1 | List out and analyzing the construction methods for tunneling in soft soil.

A tunnel is an underground passage through mountain, below a city or under waterway Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, and the logistics of supporting the tunnel excavation.

Here I will give a general description of the tunneling techniques such as cut and cover, drill and blast, bored tunneling and sequential mining construction, reviewed for possible use in various projects. A summary of environmental merits and demerits associated with these methods are also given.

Above tunneling techniques are mostly used to construct small tunnels and find their applications in utility projects to a great extent.

v  Cut and Cover Tunneling

Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. Cut and cover tunneling is a common and well-proven technique for constructing shallow tunnels. The method can accommodate changes in tunnel width and non-uniform shapes and is often adopted in construction of underground stations. Several overlapping works are required to be carried out in using this tunneling method. Trench excavation, tunnel construction and soil covering of excavated tunnels are three major integral parts of the tunneling method.  Most of these works are similar to other road construction except that the excavation levels involved are deeper. Bulk excavation is often undertaken under a road deck to minimize traffic disruption as well as environmental impacts in terms of dust and noise emissions and visual impact.

There are many types of cut and cover tunneling, some of them are:

Conventional Method: In the conventional method, excavating a trench in the ground and then back filling and restoring the original roadway or ground is the process used to construct a tunnel. A support system of some sort is also necessary to carry the load of the material used to cover over the tunnel such as shot Crete.

Bottom-up Method: In the cut-and-cover bottom-up or caisson wall method, a drilling rig is used to install caisson walls down to the existing bedrock. Once the caisson walls are in place, soil between the walls is excavated to a depth below the tunnel floor. The tunnel floor, a slab, is poured, followed by the sidewalls of the tunnel from the bottom-up. After the walls of the tunnel are completed, the roof is constructed and the roadway or ground on top of the tunnel restored. Materials used to provide the structure and support in the construction of the tunnel may include concrete, pre-cast concrete, pre-cast arches, or corrugated steel arches.

Top-down Method: In the cut-and-cover top-down or diaphragm wall method, the opposite process takes place in constructing the tunnel. A trencher or trench cutter is typically used to dig a trench out of the ground first before concrete walls are built. This processes consists of using a slurry mixture to build a slurry wall. The slurry wall provides temporary support to the sides of the trench before concrete is poured for a permanent wall structure. Once the concrete walls of the tunnel are completed, the roof of the tunnel is constructed and the surface roadway restored. Excavation of the tunnel is then carried out through openings in the tunnel roof top-down to the tunnel floor. The tunnel floor slab is the last part of construction to be completed.

Cast-in-place Method: Another type of cut-and-cover tunneling is called cast-in-place. In this method, a trench is excavated with forms being built directly inside the trench. Concrete is then poured or cast into the concrete. After the concrete cures the forms are removed. The trench is then back filled and the roadway reinstated. A shoring system is supports the sides of the excavation to prevent the shifting of soil.

v  Sequential Excavation Method

This method is also known as the New Austrian Tunneling Method (NATM). The excavation location of a proposed tunnel is divided into segments first. The segments are then mined sequentially with supports. Some mining equipment such as road-headers and backhoes are commonly used for the tunnel excavation. The ground for excavation must be fully dry for applying the NATM and ground de-watering is also an essential process before the excavation. Another process relates to the ground modifications such as grouting and ground freezing is also common with this method in order to stabilize the soil for tunneling. This method is relatively slow but is found useful in areas where existing structures such as sewer or subway could not be relocated.

v  Drill and Blast

This tunneling method involves the use of explosives. Drilling rigs are used to drill blast holes on the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators (Delay detonators) are then placed in the blast holes. Once blasting is carried out, waste rocks and soils are transported out of the tunnel before further blasting. Most tunneling construction in rock involves ground that is somewhere between two extreme conditions of hard rock and soft ground. Hence adequate structural support measures are required when adopting this method for tunneling. Compared with bored tunneling by Tunnel Boring Machine, blasting generally results in higher but lesser duration of vibration levels. A temporary magazine site is often needed for overnight storage of explosives.

v  Bored Tunneling by Tunnel Boring Machines (TBM):

Boring is viewed as quick and cost effective alternative to laying surface rails and roads. There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, the betonies slurry and earth-pressure balance machines, have pressurized compartments at the front end, allowing them to be used in difficult conditions below the water table.

There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, the betonies slurry and earth-pressure balance machines, have pressurized compartments at the front end, allowing them to be used in difficult conditions below the water table.

Modern TBMs typically consist of the rotating cutting wheel, called a cutter head, followed by a main bearing, a thrust system and trailing support mechanisms. The type of machine used depends on the particular geology of the project, the amount of ground water present and other factors.

In hard rock, either shielded or open-type TBMs can be used. All types of hard rock TBMs excavate rock using disc cutters mounted in the cutter head. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the rock in front of the machine, called the tunnel face. The excavated rock, known as muck, is transferred through openings in the cutter head to a belt conveyor, where it runs through the machine to a system of conveyors or muck cars for removal from the tunnel.

There are three categories of TBMs：

a.      Hard rock TBM

b.      Soft ground TBM

c.     
Mixed ground TBM

1.2 | List out and analyzing the construction methods for tunneling in hard rock.

Tunnel means to dig or force a passage underground or through something. In hilly areas roads are mostly constructed by tunneling. Most widely known methods are stated below with their advantages and disadvantages.

v Cut and cover tunneling

A tunnel is an underground or underwater passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods. A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. The central portions of a rapid transit network are usually in tunnel. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.

Advantages	Disadvantages
In locations with no important constraints on the surface cut and cover, tunnels are inexpensive compared to other tunneling techniques. Approaches to immersed or bored tunnel sections will often be constructed as cut and cover tunnels.	More dust and noise impact may arise, though these can be mitigated through implementation of sufficient control measures;
	Temporary decks are often installed before bulk excavation to minimize the associated environment impacts.
	Larger quantity of C&D materials would be generated from the excavation works, requiring proper handling and disposal.

v Drill and blast

Drilling and Blasting is the controlled use of explosives and other methods such as gas pressure blasting pyrotechnics, to break rock for excavation. It is practiced most often in mining, quarrying and civil engineering such as dam or road construction. The result of rock blasting is often known as a rock cut.

Drilling and blasting currently utilizes many different varieties of explosives with different compositions and performance properties. Higher velocity explosives are used for relatively hard rock in order to shatter and break the rock, while low velocity explosives are used in soft rocks to generate more gas pressure and a greater heaving effect. For instance, an early 20th-century blasting manual compared the effects of black powder to that of a wedge, and dynamite to that of a hammer. The most commonly used explosives in mining today are ANFO based blends due to lower cost than dynamite.

Advantages	Disadvantages
Potential environmental impacts in terms of noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located near the tunnel portal.	Potential hazard associated with establishment of a temporary magazine site for overnight storage of explosives shall be addressed through avoiding populated areas in the site selection process.
Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced.
Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced.
Blasting would significantly reduce the duration of vibration, though the vibration level would be higher compared with bored tunneling (with proper blast design & techniques vibration can be reduced).

v Bored tunneling by TBM:

A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for micro tunneling. They can bore through anything from hard rock to sand. Tunnel diameters can range from a meter (done with micro-TBMs) to 19.25 meters to date. Tunnels of less than a meter or so in diameter are typically done using trench less construction methods or horizontal directional drilling rather than TBMs.

Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods in rock and conventional "hand mining" in soil. TBMs have the advantages of limiting the disturbance to the surrounding ground and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in heavily urbanized areas. The major disadvantage is the upfront cost. TBMs are expensive to construct, and can be difficult to transport. However, as modern tunnels become longer, the cost of tunnel boring machines versus drill and blast is actually less. This is because tunneling with TBMs is much more efficient and results in shortened completion times (when they operate successfully).

Advantages	Disadvantages
Potential environmental impacts in terms of noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located near the launching and retrieval shafts	The major disadvantage is the upfront capital cost. TBMs are expensive to construct, difficult to transport, require significant backup systems and power.
Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced.
Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced.

v Sequential Excavation Method:

The Sequential Excavation Method (SEM), also known as New Austrian Tunneling method (NATM), describes a popular method of modern tunnel design and construction. This technique first gained attention in the 1960s based on the work of Ladislaus von Rabcewicz, Leopold Müller and Franz Pacher between 1957 and 1965 in Austria. The name NATM was intended to distinguish it from the old Austrian tunneling approach. The fundamental difference between this new methods of tunneling, as opposed to earlier methods, comes from the economic advantages made available by taking advantage of the inherent geological strength available in the surrounding rock mass to stabilize the tunnel.

In the last 20 years the Sequential Excavation Method (SEM) or the New Austrian Tunneling Method (NATM) has been gaining popularity and use in the United States. Its use is versatile, in various ground conditions and at various depths. Although many of the projects were successfully completed, the lack of design guidelines for underground construction and in particular for the SEM construction, in which it relies on observational method and assessment of the ground behavior at the face, has negatively impacted the tunneling industry. Recognizing the need to develop design guidelines for underground construction, in 2007 FHWA awarded a contract to Parsons Brinckerhoff to develop and publish a design manual for road tunnels. As a result of this contract, “Technical Manual for Design and Construction of Road Tunnels—Civil Elements” was published in November 2008. The Manual provided specific guidelines for SEM construction. This paper provides a summary of the guidelines for the design and construction of tunnels using Sequential Excavation Method with emphasis on its technical aspects, contractual issues, and practices in the US based on the recommendations and guidelines made in the above stated publication.

Advantages	Disadvantages
Similar to the drill-and-blast and bored tunneling methods, only localized potential environmental impacts would be generated.	As the method is relatively slow, duration of potential environmental impacts would be longer than that of the other methods.

1.3 | Assessing the most appropriate method for the construction of the tunnel and shaft for the above project.

After considering all the advantages and disadvantages, I choose “Bored tunneling by TBM” method. The only disadvantage of this method is that it is really expensive, but as there was nothing mentioned about any financial limitation I choose this method for the highway project has been planned in a hilly area.

Not only does each tunneling project have its own unique characteristics, but also each proposed shaft within a project will have its own individual characteristics and conditions. This unique set of characteristics and conditions will dictate the selection process of the temporary ground support method. At present, it is common industry practice for clients to transfer the design responsibility and risk of temporary ground support structures onto the Contractor. This is typically carried out due to the temporary nature of the structure and the absence of stringent client functional requirements.

Shafts are the doorways to the underground, serving as the location at which all material enters and exits. They vary in size and depth, and their design and construction are key to the successful completion of any tunneling project.

Before determining shaft construction method, decide our minimum shaft size. During design, the minimum dimensions are typically determined by the physical layout of the final structure to be constructed or space needed for launching a tunnel boring machine (TBM). For water and wastewater tunnels, final structures will include drop shafts, access shafts, pump stations, gate valves and surge chambers. For transit tunnels, shafts can be used for access, elevators, ventilation, transit stations and utility drops. It is difficult to determine exactly what size shaft the contractor will need because you will not know the proposed means and methods and the exact equipment that will be used. When surface space allows, assume the contractor may need to increase the footprint of the shaft.

Contractors will typically transfer the design responsibility and risk of temporary ground support structures to a specialized design consultancy firm who will complete the required calculations, drawings, specifications and design reports. However, in practice, the selection process to lock in the temporary ground support method is typically a highly consultative and combined effort between the Contractor and the Designer, which is highly scrutinized by the Clients Representative. This selection process will typically be based on a set of factors including, but not limited to the following:

*       Functional requirements / purpose

*       Required shaft depth

*       Shape and geometry of the shaft

*       Subsurface geotechnical/ hydrogeological/ and environmental conditions

*       Available shaft compound and working areas

*       Adjacent third party assets (surrounding structures, underground utilities, overhead utilities)

*       Adjacent and direct surcharge loadings (traffic & jacking thrust forces)

*       Allocated budgets

*       Timeline and schedule requirements

*       Local Contractors / Designers experience and capability

*       Local bylaws

*       Client & stakeholder requirements

All of the above factors are critical in selecting a temporary ground support method. Subsequently, the greater the understanding into each of these factors, the more optimal the solution will be for each of the relevant stakeholders.

Traffic patterns and traffic control may also influence allowable shaft size. Some cities require shafts to have traffic plates placed over them at certain times of the day because of rush hour. Other cities want concrete barriers around shafts for public safety. Fall protection provisions must be provided at all shafts. The laydown area around a shaft can require the contractor to conform to some unusual requirements, like working under bridges or restricted access. Aboveground features such as light poles, electrical transmission lines, other utilities, structures, and plants may adversely impact the laydown area required. Consider noise and vibrations from the shaft construction on adjacent neighbors.

Even with a watertight shaft, a sump and sump pump should be installed. Water can enter the shaft from minor leaks, rain or launching of the TBM. Providing power and backup power to the sump pump is important to prevent flooding of the shaft. A project can be down weeks or months if a shaft and tunnel are allowed to flood.

Shaft Construction Methods

There are several different shaft support methods available. A watertight shaft support system should be used below the groundwater table. Watertight shoring systems will not require dewatering to lower the groundwater table. Dewatering can be an expensive process. Discharging the water can be an even greater challenge particularly in an urban environment.

Employee Identification

Entrances to all underground facilities must have a check-in and checkout system that provides the contractor with an accurate record of each person underground. The system must be able to identify each individual and general location. General locations include heading, train crew, track crew, maintenance area, storage area, survey stations, etc. Additionally, when underground, all employees must carry or wear a positive means of identification, such as a metal disk or tag

Personal Protective Equipment

Employees entering underground workings must wear, as a minimum, hardhats, suitable eye protection, and foot protection. Employee’s entering wet areas must wear rubber footwear; underground type rain gear; and eye, face, and head protection as described in the section on "Personal Protective Equipment." When applicable, provide employees with other personal protective equipment, and ensure they wear them

Electrical Equipment

 A professional engineer knowledgeable in underground wiring practices must design and certify the underground electrical distribution system to meet good practice and applicable standards. Install and maintain all electrical equipment, including the section on "Electrical Safety," to meet applicable requirements. Permit only dry-type transformers underground and ensure they are protected from possible damage. Separate or protect power lines from air and water lines, metal ducts, telephone lines, and blasting lines.

Emergency Hoists

Provide an emergency personnel hoist for shafts more than 50 feet deep. Design the hoists that, as a minimum, the load hoist drum is powered in both directions and a brake automatically applies upon power release or failure. Provide the emergency hoist in addition to the primary hoist.

Carry out all drilling and excavation operations in a manner that meets the requirements of this section and control airborne dust concentrations within limits arranged in the section on "Occupational Health." Quantitative testing is required for underground environments and operations to ensure the success of dust control methods.

Many different shaft construction methods are available. The key to deciding on a method is determining if there is a groundwater table presence. If so, use one of the watertight construction methods. Above project.

    LO - 2 Understand the methods and techniques used in Hydraulic structures.

2.1 | Outlining the classification of hydraulic structures on the basis of function.

Hydraulic engineering is a sub-discipline of civil engineering concerned with the flow and conveyance of fluids, principally water. This area of engineering is intimately related to the design of dams, channels,

Canals, levees, elevators, and to environmental engineering. Common topics of design for hydraulic engineers includes hydraulic structures, including dams and levees, water distribution networks, water collection networks, storm water management, sediment transport, and various other topics.

A hydraulic structure is a structure submerged or partially submerged in any body of water, which disrupts the natural flow of water.

Hydraulic structures are anything that can be used to divert, restrict, stop, or otherwise manage the natural flow of water. They can be made from materials ranging from large rock and concrete to unclear items such as wooden timbers or tree trunks. Dam, for instance, is a type of hydraulic structure used to hold water in a reservoir as possible energy, just as a weir is a type of hydraulic structure which can be used to pool water for irrigation, establish control of the bed (grade control) or, as a new technique, to divert flow away from eroding banks or into diversion channels for flood control.

v Classification of hydraulic structures on the basis of material:

§     Earth fill

§     Rock fill

§     Concrete

§     Stone masonry

§     Timber

§     Steel coffer 

v Classification of hydraulic structures on the basis of function:

Ø  Flow control structures: They are used to regulate the flow and pass excess flow. They might be gates, slipways, valves, or outlets.

Ø  Flow measurement structures: They are used to measure discharge. They are weirs, orifices, flumes etc.

Ø  Division structures: They are used to divert the main course of water flow. They are coffer dams, weirs, canal head works, intake works.

Ø  Conveyance structures: They are used to guide the flow from one place to another. They are open channels, pressure conduit, pipes, canals and sewers.

Ø  Collection structures: they are used to collect water for disposal. They are Drain inlets, infiltration galleries, wells.

Ø  Energy dissipation structures: They are used to prevent erosion and structural damage. They are stilling basins, surge dams, check dams.

Ø  River training and water stabilizing structures: they are used to control or remove sediments and other pollutants. They are racks, screens, traps, sedimentation tanks, filters, sluice ways.

Ø  Hydraulics Machines: They are used to convert energy from one form to another. They are turbine, pumps, and ramps.

Ø  Storage structures: They are used for the purpose of storage of water. These may be dams or tanks etc.

Ø   Shore protection structures: They are used to protect banks, Dikes, groins, jetties, revetments.

v Dam

A dam, for instance, is a type of hydraulic structure used to hold water in a reservoir as potential energy, just as a weir is a type of hydraulic structure which can be used to pool water for irrigation, set up control of the bed (grade control) or, as a new innovative technique, to divert flow away from eroding banks or into diversion channels for flood control.

A dam is a barrier that impounds water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for such activities as irrigation, human consumption, industrial use, aquaculture, and navigability. Hydropower is often used in conjunction with dams to generate electricity. A dam can also be used to collect water or for storage of water which can be evenly distributed between locations. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees (also known as dikes) are used to manage or prevent water flow into specific land regions.

Artificial dam created by man for their needs, and this hydroelectric dams, water withdrawals for irrigation systems, dams, bridges, dams, creating a reservoir in its upper reach. Natural Culverts Dam is the result of natural forces: landslides, mudflows, avalanches, landslides, earthquakes.

v Culvert

A culvert is a structure that allows water to flow under a road, railroad, trail, or similar obstruction from one side to the other side. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe, reinforced concrete or other material. In the United Kingdom the word can also be used for a longer artificially buried watercourse. A structure that carries water above land is known as an aqueduct.

Culverts are commonly used both as cross-drains for ditch relief and to pass water under a road at natural drainage and stream crossings. A culvert may be a bridge-like structure designed to allow vehicle or pedestrian traffic to cross over the waterway while allowing adequate passage for the water. Culverts can be constructed of a variety of materials including cast-in-place or precast concrete (reinforced or non-reinforced), galvanized steel, aluminum, or plastic, typically high density polyethylene. Two or more materials may be combined to form composite structures. For example, open-bottom corrugated steel structures are often built on concrete footings Culverts come in many sizes and shapes including round, elliptical, flat-bottomed, pear-shaped, and box-like constructions. The culvert type and shape selection is based on a number of factors including: requirements for hydraulic performance, limitation on upstream water surface elevation, and roadway embankment height.

v Bridge

A bridge is a structure built to span physical obstacles without closing the way underneath such as a body of water, valley, or road, for the purpose of providing passage over the obstacle. There are many different designs that each serve a particular reason and apply to different situations. Designs of bridges vary depending on the function of the bridge, the nature of the terrain where the bridge is constructed and anchored, the material used to make it, and the funds available to build it.

Bridge, structure that spans horizontally between supports, whose function is to carry vertical loads. The prototypical bridge is quite simple—two supports holding up a beam—yet the engineering problems that must be overcome even in this simple form are inherent in every bridge: the supports must be strong enough to hold the structure up, and the span between supports must be strong enough to carry the loads. Spans are generally made as short as possible; long spans are justified where good foundations are limited—for example, over estuaries with deep water.

All major bridges are built with the public’s money. Therefore, bridge design that best serves the public interest has a threefold goal: to be as efficient, as economical, and as elegant as is safely possible. Efficiency is a scientific principle that puts a value on reducing materials while increasing performance. Economy is a social principle that puts value on reducing the costs of construction and maintenance while retaining efficiency. Finally, elegance is a symbolic or visual principle that puts value on the personal expression of the designer without compromising performance or economy. There is little disagreement over what constitutes efficiency and economy, but the definition of elegance has always been controversial.

Modern designers have written about elegance or aesthetics since the early 19th century, beginning with the Scottish engineer Thomas Telford. Bridges ultimately belong to the general public, which is the final arbiter of this issue, but in general there are three positions taken by professionals. The first principle holds that the structure of a bridge is the province of the engineer and that beauty is fully achieved only by the addition of architecture. The second idea, arguing from the standpoint of pure engineering, insists that bridges making the most efficient possible use of materials are by definition beautiful. The third case holds that architecture is not needed but that engineers must think about how to make the structure beautiful. This last principle recognizes the fact that engineers have many possible choices of roughly equal efficiency and economy and can therefore express their own aesthetic ideas without adding significantly to materials or cost.

v Sluice gate

A sluice gate is a mechanism used to control water flow. A sluice (from the Dutch "sluis") is a water channel controlled at its head by a gate. A mill race, flume, penstock or lade is a sluice channeling water toward a water mill. The terms sluice, sluice gate, knife gate, and slide gate are used interchangeably in the water and wastewater control industry. These devices are also often used in water treatment plants, mining, dams, rice fields, and cranberry bogs, among other places. The gates are typically made of wood or metal, and often slide vertically on a frame to open or close, allowing water to flow out of a space or to be contained in it. For this reason, they are also known as a sluice gate valve.

Sluice gate design is not only limited to a vertical sliding system, however. One kind of sluice gate acts like a flap and is moved by water pressure being greater on one side than the other. A sluice gate is traditionally a wood or metal barrier sliding in grooves that are set in the sides of the waterway. Sluice gates commonly control water levels and flow rates in rivers and canals. They are also used in wastewater treatment plants and to recover minerals in mining operations, and in watermills.

The word sluice indicates a man-made channel or modified natural waterway that conducts water. This kind of gate regulates how and where that water is moved. This is especially useful for controlling flooding or water levels in agricultural and other industries. Many sluice gates are moved by means of a threaded rod system, which needs to be regularly cleaned and greased. Often, when these gates are used in applications with a large amount of water pressure, such as dams, they are raised and lowered by hydraulic systems to control the sluice gate flow. Sometimes in smaller uses, such as in cranberry bogs, the gates are raised and lowered manually. At other times, an electrically-driven hoisting system is used.

2.2 | Explaining the construction methods of Earth dam, Aqueducts and Sluice Gate Hydraulic structures.

v Earth Dam

An earth dam is composed of suitable soils obtained from borrow areas or required excavation and compacted in layers by mechanical means. Following preparation of a foundation, earth from borrow areas and from required excavations is transported to the site, dumped, and spread in layers of required depth. The soil layers are then compacted by tamping rollers, sheep foot rollers, heavy pneumatic tired rollers, vibratory rollers, tractors, or earth-hauling equipment. One advantage of an earth dam is that it can be adapted to a weak foundation, provided proper consideration is given to thorough foundation exploration, testing, and design.

The successful design, construction, and operation of a reservoir project over the full range of loading require a comprehensive site characterization, a detailed design of each feature, construction supervision, measurement and monitoring of the performance, and the continuous evaluation of the project features during operation. The design and construction of earth dams is complex because of the nature of the varying foundation conditions and range of properties of the materials available for use in the embankment. The first step is to conduct detailed geological and subsurface explorations, which characterize the foundation, abutments, and potential borrow areas. The next step is to conduct a study of the type and physical properties of materials to be placed in the embankment. This study should include a determination of quantities and the sequence in which they will become available. The design should include all of the studies, testing, analyses, and evaluations to ensure that the embankment meets all technical criteria and the requirements of a dam. Construction supervision, management, and monitoring of the embankment and appurtenant structures are a critical part of the overall project management plan. Once the project is placed into operation, observations, surveillance, inspections, and continuing evaluation are required to assure the satisfactory performance of the dam.

Ø  Risk downstream of a dam

It is the permit holder’s responsibility to ensure a dam is constructed so as to be in a safe condition at all times. This requires a risk assessment of downstream impacts of the potential for dam failure to be developed in the planning stage and submitted with the application to the Assessment Committee for permit approval. It also needs to be recognized that the situation downstream of the dam may change over time and place the dam into a higher hazard category during the life of the dam. For example, downstream changes may include the development of new infrastructure, such as housing development where the Population at Risk (PAR) and the severity of damage and loss becomes greater than when the dam was first built. Hence this may make the dam deficient in its safety requirements, such as the size of the spillway and a higher safety risk. A suitably qualified and experienced dam engineer should be consulted if it is apparent that this may occur.

Ø  Site investigations

Before construction commences the permit holder may need to get their engineer to make a thorough investigation of the site to establish the nature of the foundation and to locate sufficient suitable clay material to use in the embankment.

Depending on the size of the dam, test pits may need to be excavated and soil tests carried out by qualified geotechnical engineers. This may also include a thorough investigation of the foundation of the dam to ensure that it will not fail and cause the embankment to fail. This is especially important where existing dams are to be raised, if this is not addressed the Assessment Committee cannot make a decision on whether to issue a dam works permit.

Ø  Advantages of earth dams

In spite of the structural incompressibility of earth and water, many earth dams have been constructed with various types of earth and rock with stable conditions. The main advantages of earth dams are as follow:

Availability of earth materials

Earth is readily available in most parts of the world close to possible dam sites. Now a day the knowledge of soil mechanics has further enhanced the chances to make unstable earth dam as stable.

Easy handling of earth

Earth can be excavated by hand, transported in baskets and compacted with cattle’s walking over it. In suitable situation, earth can be handled and moved with latest machinery.

Foundation condition

Earth dams are suited to the sites where a masonry dam cannot be used for structural reasons. These dams can be constructed even on compressible foundations. The intensity of foundation stress due to earth is less than that due to solid masonry. The horizontal water pressure on the dams is distributed over greater area because of greater base width and hence, the danger of sliding on a weak foundation is minimized. The greater width of dam foundation also minimizes the leakage through the foundation beneath the dam.

Cost of construction

The most important advantage of an earth dam compared to masonry dam, is its lesser cost. It has been observed that the total cost of an earth dam, is roughly one-half of a concrete dam.

Ø  Disadvantages of earth dams

However, the earth dams possess the following disadvantages.

*       Non-availability of the materials at or near the dam site.

*       Greater maintenance cost as compared to a good concrete dam.

*       Earthen dams are unsuitable for spillways.

Ø  Embankment.

An embankment dam is a massive artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock. It has a semi-pervious waterproof natural covering for its surface and a dense, impervious core. This makes such a dam impervious to surface or seepage erosion. Such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than by the use of a cementing substance.

Many different trial sections for the zoning of an embankment should be prepared to study utilization of fill materials; the influence of variations in types, quantities, or sequences of availability of various fill materials; and the relative merits of various sections and the influence of foundation condition. Although procedures for stability analyses afford a convenient means for comparing various trial sections and the influence of foundation conditions, final selection of the type of embankment and final design of the embankment are based, to a large extent, upon experience and judgment.

v Aqueducts

An aqueduct is a watercourse constructed to convey water. In modern engineering, the term aqueduct is used for any system of pipes, ditches, canals, tunnels, and other structures used for this purpose. The term aqueduct also often refers specifically to a bridge on an artificial watercourse.  Aqueducts were used in ancient Greece, ancient Egypt, and ancient Rome. In modern times, the largest aqueducts of all have been built in the United States to supply the country's biggest cities. The simplest aqueducts are small ditches cut into the earth. Much larger channels may be used in modern aqueducts. Aqueducts sometimes run for some or all of their path through tunnels constructed underground. Modern aqueducts may also use pipelines. Historically, agricultural societies have constructed aqueducts to irrigate crops and supply large cities with drinking water.

Aqueduct, (Latin aqua + ducere, "to lead water”) man-made conduit for carrying water in a restricted sense, aqueducts are structures used to conduct a water stream across a hollow or valley. In modern engineering, however, aqueduct refers to a system of pipes, ditches, canals, tunnels, and supporting structures used to convey water from its source to its main distribution point. Such systems generally are used to supply cities and agricultural lands with water. Aqueducts have been important particularly for the development of areas with limited direct access to freshwater sources. Historically, aqueducts helped keep drinking water free of human waste and other contamination and thus greatly improved public.

It started with a need for extra water in a city, supplementary to other water sources like a river, rainwater catchment, wells, and/or springs. Then a decision was made by the town council or an individual Maecenas, to finance and build an aqueduct by means of a masonry channel or terracotta pipes. A suitable water source must be found, not too far away, at the right level and with water of good quality.

v Sluice Gates

A sluice gate is a mechanism used to control water flow. These devices are often used in water treatment plants, mining, dams, rice fields, and cranberry bogs, among other places. The gates are typically made of wood or metal, and often slide vertically on a frame to open or close, allowing water to flow out of a space or to be contained in it. For this reason, they are also known as a sluice gate valve.

A sluice gate is usually a wood or metal barrier sliding in grooves that are set in the sides of the waterway. Sluice gates commonly control water levels and flow rates in rivers and canals. They are also used in wastewater treatment plants and to recover minerals in mining operations, and in watermills. Sluice gate refers to a movable gate allowing water to flow under it. When a sluice is lowered, water may spill over the top, in which case the gate operates as a weir. Usually, a mechanism drives the sluice up or down. This may be a simple, hand-operated, chain pulled/lowered, worm drive or rack-and-pinion drive, or it may be electrically or hydraulically powered. The sluice gate provides a suitable means of flow regulation, especially in irrigation and drainage schemes where flow has to be distributed in networks of interconnected channels. The gate is provided with a lifting mechanism so that the opening under it may be set to any desired position. When closed, the aperture is sealed so that no flow can pass through the gate. In this experiment measure the discharge under the gate and establish the effective coefficient of discharge.

Many sluice gates are moved by means of a threaded rod system, which needs to be regularly cleaned and greased. Often, when these gates are used in applications with a large amount of water pressure, such as dams, they are raised and lowered by hydraulic systems to control the sluice gate flow. Sometimes in smaller uses, such as in cranberry bogs, the gates are raised and lowered manually. At other times, an electrically-driven hoisting system is used. Historically, sluice gates proved useful in mills. A mill sluice was known as a millrace, and would often turn a waterwheel or turbine, which could be then used to power equipment needed in sawmills and gristmills. The millrace was often regulated by sluice gates to decrease or increase the flow of water, depending on when it was needed.

2.3 | Explaining in detail cross drainage works.

Cross drainage works is defined as when an irrigation canal is intercepted by a drain which is required to be crossed over the canal, some suitable structure is required to be constructed. Thus the engineering works constructed to cross the canal by the drain is called cross drainage works. These also include the crossing of rivers, streams, canals and natural water course. When the network of main canals, branch canals, distributaries, etc are provided, then  these  canals  may  have  to  cross  the  natural drainages  like  rivers,  streams,  mullahs, etc. at different points. The crossing of the canals with such obstacle cannot be avoided. So, suitable structures is constructed at the  crossing  point  for  the  easy  flow  of  water  of  the canal and drainage in the respective directions. These structures are known as cross-drainage works.

v Necessity

The water-shed canals do not cross natural drainages. But in actual orientation of the canal network, this ideal condition may not be available and the obstacles like natural drainages may be present across the canal. So, the cross drainage works must be provided for running the irrigation system. At the crossing point, the water of the canal and the drainage get intermixed. So, far the smooth running of the canal with its design discharge the cross drainage works are required.

The site condition of the crossing point may be such that without any suitable structure, the water of the canal and drainage can not be diverted to their natural directions. So, the cross drainage works must be provided to maintain their natural direction of flow.

v Selection of suitable site for cross drainage works

§     The factors which affect the selection of suitable type of cross drainage works are:

§     Relative bed levels and water levels of canal and drainage

§     Size of the canal and drainage.

§     The following considerations are important

§     When the bed level of the canal is much above the HFL of the drainage, an aqueduct is the obvious choice.

§     When the bed level of the drain is well above FSL of canal, super passage is provided.

§     The necessary headway between the canal bed level and the drainage HFL can be increased by shifting the crossing to the downstream of drainage. If, however, it is not possible to change the canal alignment, a siphon aqueduct may be provided.

§     When canal bed level is much lower, but the FSL of canal is higher than the bed level of drainage, a canal siphon is preferred.

§     When the drainage and canal cross each other practically at same level, a level crossing may be preferred. This type of work is avoided as far as possible.

v Types of Cross Drainage works

There are three types of cross drainage works structures:

1.     Cross drainage work carrying canal over the drain. It can be Aqueduct, Syphon Aqueduct.

2.     Cross drainage works admitting canal water into the canal. It can be Level Crossing, Canal inlets.

3.     Cross Drainage work carrying Drainage over the canal. It can be Super passage, Canal Syphon

Ø  Aqueduct

In an aqueduct, the canal bed level is above the drainage bed level so canal is to be constructed above drainage. When the HFL of the drain is sufficiently below the bottom of the canal such that the drainage water flows freely under gravity, the structure is known as Aqueduct.

A canal trough is to be constructed in which canal water flows from upstream to downstream. This canal trough is to be rested on number of piers. The drained water flows through these piers upstream to downstream. The canal water level is referred as full supply level (FSL) and drainage water level is referred as high flood level (HFL). The HFL is below the canal bed level.

Aqueduct is similar to a bridge, instead of roadway or railway, canal water are carried in the trough and below that the drainage water flows under gravity and possessing atmospheric pressure.

Ø  Syphon Aqueduct

In case of the siphon Aqueduct, the HFL of the drain is much higher above the canal bed, and water runs under siphonic action through the Aqueduct barrels.

The construction of the syphon aqueduct structure is such that, the flooring of drain is depressed downwards by constructing a vertical drop weir to discharge high flow drain water through the depressed concrete floor. The drain bed is generally depressed and provided with pucci floors, on the upstream side, the drainage bed may be joined to the pucca floor either by a vertical drop or by glacis of 3:1. The downstream rising slope should not be steeper than 5:1. When the canal is passed over the drain, the canal remains open for inspection throughout and the damage caused by flood is rare. However during heavy floods, the foundations are susceptible to scour or the waterway of drain may get choked due to debris, tress etc.

Ø  Level Crossing

When the bed of canal and drainage is at same level a level crossing is provided. An escape weir is constructed along the drain at upstream junction. Top of weir is kept at the full reservoir level (F.S.L.) of the canal. When the water level in drains rises above full reservoir level (F.S.L.) of canal it spills over the escape weir and mixes with the canal water and is taken out from the canal at downstream junction by providing regulators.

Ø  Canal inlets

In a canal inlet structure, the drainage water to be admitted into canal is very less. The drainage is taken through the banks of a canal at inlet. And then this drainage mixed with canal travels certain length of the canal, after which an outlet is provided to create suction pressure and suck all the drainage solids, disposing it to the watershed area nearby.

There are many disadvantages in use of canal inlet structure, because the drainage may pollute canal water and also the bank erosion may take place causing the canal structure deteriorate so that maintenance costs are high. Hence this type of structure is rarely constructed.

Ø  Super Passage

Super passage structure carries drainage above canal as the canal bed level is below drainage bed level. The drainage trough is to be constructed at road level and drainage water flows through this from upstream to downstream and the canal water flows through the piers which are constructed below this drainage trough as supports.

The full supply level of canal is below the drainage trough in this structure. The water in canal flows under gravity and possess the atmospheric pressure. This is simply a reverse of Aqueduct structure.

Ø  Canal Syphon

In a canal syphon, drainage is carried over canal similar to a super passage but the full supply level of canal is above than the drainage trough.so the canal water flows under syphonic action and there is no presence of atmospheric pressure in canal.

When compared, super passage is more often preferred than canal Syphon because in a canal Syphon, big disadvantage is that the canal water is under drainage trough so any defective minerals or sediment deposited cannot be removed with ease like in the case of a Syphon Aqueduct. Flooring of canal is depressed and ramp like structure is provided at upstream and downstream to form syphonic action. This structure is a reverse of Syphon aqueduct.

    LO - 3 Understand the methods and techniques used in Marine Work.

3.1 | Outlining the coastal protection structures.

In history, the structures developed for shoreline protection were constructed of durable materials such as rock and reinforced concrete. They were designed to withstand the force of wave action. Such "hard" stabilization methods are still in use today and include seawalls, revetments, breakwaters, impermeable groins, and jetties. Revetments of broken concrete or riprap are powerful devices for reducing the energy from wave action, and they are repaired practically. Their irregular surface offers protection from wave run-up, or the movement of breaking waves up the shore. Revetments often limit access to the beach and, as with seawalls, they can be rather unsightly.

Shore protection in its widest usage refers to the reduction or elimination of damage to the shore and backland as might be caused by flooding, wave attack, and erosion. The shore may consist of cliffs, reefs, beaches, and artificial or engineered structures that form part of the water and land interface. Shore protection structures can be classified as hard, soft, or a combination. Soft structures or soft methods of shore protection usually involve placement of beach-quality sediment, typically sand, directly on the beach, a process called beach nourishment or beach fill. The beach fill may be placed across the upper beach profile and as a dune system. In such designs, the beach berm protects the dune against erosion, and the dune protects the backland from flooding and wave attack. Another type of soft shore protection structure is a “near shore berm,” referring to placement of material in an approximate linear form along the shore to break storm waves and to feed material to the beach during times of accretionary wave conditions.

Types of shore protection structures:

The following discussion focusses on the most commonly deployed protection structures, including groins, seawalls, breakwaters, sills and sand fences. Many structures are hybrids that are intended to incorporate functions of more than one traditional. A variety of experimental or low cost structures exists but they are deployed in limited numbers, are often temporary and have shortcomings similar to traditional or normal approaches.

v Seawalls

When coastal buildings or roads are threatened, usually the first suggestion is to "harden" the coast with a seawall. Seawalls are structures built of concrete, wood, steel or boulders that run parallel to the beach at the land or water interface. They may also be called bulkheads or revetments. They are designed to protect structures by stopping the natural movement of sand by the waves. If the walls are maintained they may hold back the ocean temporarily. The construction of a seawall usually displaces the open beach that it is built upon. They also prevent the natural landward migration of an eroding beach.

When waves hit a smooth, solid seawall, the wave is reflected back towards the ocean. This can make matters worse. The reflected wave takes beach sand with it. Both the beach and the surf may disappear.

Seawalls can cause increased erosion in adjacent areas of the beach that do not have seawalls. This so-called "flanking erosion" takes place at the ends of seawalls. Wave energy can be reflected from a seawall sideways along the shore, causing coastal bluffs without protection to erode faster. When it is necessary to build a seawall, it should have a sloped (not vertical) face. Seawalls should also have pockets and grooves in them that will use up the energy of the waves instead of reflecting it.

v Groins

Groins are the oldest and most common shore-connected, beach stabilization structure. They are structures that extend, fingerlike, perpendicularly or nearly right angles from the shore and are relatively short when compared to navigation jetties at tidal inlets. Groins is a hard shoreline structure designed as so-called "permanent solution" to beach erosion. A groin is a shoreline structure that is perpendicular to the beach. It is usually made of large boulders, but it can be made of concrete, steel or wood. It is designed to interrupt and trap the long shore flow of sand. Sand builds up on one side of the groin at the expense of the other side. If the current direction is constant all year long, a groin "steals" sand that would normally be deposited on the downdrift end of the beach. The amount of sand on the beach stays the same. A groin merely transfers erosion from one place to another further down the beach.  Usually constructed in groups called groin fields, their primary purpose is to trap and retain sand, nourishing the beach compartments between them.

Groins occasionally improve the shape of surfing waves by creating a rip current next to the rocks. The rip can be a hazard to swimmers. The rip can also divert beach sand onto offshore sand bars, thereby accelerating erosion. Groins can also ruin the surf. If the waves are reflected off the rocks, the waves may lose their shape and "close-out."

As soon as one groin is built, property owners downdrift of it may start clamoring for the government to build groins to save "their" beach. Eventually, the beach may become lined with groins. Since no new sand is added to the system, groins simply steal sand from one part of the beach so that it will build up on another part. There will always be beach erosion downdrift of the last groin.

Groins may be a possible component of a shore-protection project and sand-management program under the following situations:

ª     Where there is a divergent region of long shore transport, such as at the center of a crenulated shaped pocket beach, or where the curvature of the coast changes greatly.

ª     Where there is no source of sand, such as the downdrift side of a large harbor or inlet with jetties.

ª     Where intruding sand is to be managed, such as to retain sand on the updrift side of a harbor to prevent shoaling of the channel and to stockpile the sediment for bypassing by land transport.

ª     Where the sand transport rate is to be controlled or gated, such as to prevent undue loss of beach fill to unwanted areas.

ª     Where an entire littoral reach is to be stabilized, such as on a spit, near a submarine canyon.

ª    
Where stabilization of the high-tide shoreline is required under extreme conditions.

v Breakwaters

A breakwater is a large pile of rocks built parallel to the shore. It is designed to block the waves and the surf. Some breakwaters are below the water's surface. Breakwaters used for shore protection are shore-parallel structures placed offshore to reduce or eliminate wave energy and contribute to deposition on beaches landward of them. Sills are low, shore-parallel structures often constructed with top elevations below water level and usually directly connected to the substrate they protect. Sills can hold beaches landward of them at a gentler slope, reducing offshore sand losses and rates of longshore transport out of the area. The beach retained to landward is often called a perched beach. Submerged sills can provide a more aesthetically pleasing setting for beach users than emerged structures Breakwaters are usually built to provide calm waters for harbors and artificial marinas. Submerged breakwaters are built to reduce beach erosion. These may also be referred to as artificial reefs.

Breakwaters can be high crested to act as wave barriers, low crested to allow frequent overtopping and high wave transmission, or submerged to lessen their physical and visual impact. Difficulties of designing breakwaters to perform specific hydraulic functions and costs of constructing them are increased in locations where tidal ranges are great. Breakwaters are more suitable in micro-tidal environments, but they have been deployed in locations with an appreciable tidal range.

A breakwater can be offshore, underwater or connected to the land. As with groins and jetties, when the longshore current is interrupted, a breakwater will dramatically change the profile of the beach. Over time, sand will accumulate towards a breakwater. Downdrift sand will erode. A breakwater can cause millions of dollars in beach erosion in the decades after it is built.

v Detached breakwaters

Detached breakwaters are sometimes referred to as offshore breakwaters. They function to reduce wave energy on their landwards sides. Sediment will accumulate in the wave-sheltered region because the water is calmer and the longshore current behind the detached breakwater weaker than on adjacent shore that is open to full wave energy. Detached breakwaters are often constructed to be partially submerged, for example, at higher tide, to allow wave transmission over and, sometimes, through them. In this way, a certain amount of wave energy reaches the sheltered region and promotes sediment transport alongshore. Detached breakwaters are often built in groups, and this configuration is referred to as a segmented detached breakwater system. Detached breakwaters may be an appropriate shore protection measure in areas where cross-shore transport is large, or where wave energy must be reduced, such as at a change in orientation of the coast.

The response of the beach to detached breakwaters is controlled by at least 14 variables, making these structures more difficult to design than groins. The response of the beach to the presence of a detached breakwater may be as a tombolo, for which the high-tide shoreline reaches the structure, or as a salient that describes a cuspate morphologic form that grows toward, but does not reach the structure. The type of beach response is determined by design requirements. Typically, it is desired to allow some amount of sediment to pass alongshore behind the structure. A potential design deficiency for detached breakwaters is to place them too far offshore. In such a case, the high-tide shoreline will show no response.

v Beach Nourishment

In recent years, the hard structures described above have fallen somewhat out of favor by communities due to the negative impacts we have discussed. Beach nourishment or beach fill is becoming the favored "soft" alternative. Beach nourishment is simply depositing sand on the beach in order to widen it. Although paid for by all taxpayers, it is frequently undertaken to protect private oceanfront buildings. Occasionally the taxpaying public is refused access to beaches that they have paid to protect. Sand nourishment is a costly, temporary solution. The projects are not intended to have a long life span and must be renourished on a regular basis, creating a cycle that will go on until the money runs out or shorefront buildings are relocated.

There are many considerations that must addressed when designing a nourishment project. If the grains of sand are not exactly the same size as that of the natural beach, the newly nourished beach may erode faster than the natural beach was eroding. Beach nourishment can cause bottom organisms and habitats to be smothered by turbid water that has sand and mud suspended in it. The shoreline is moved seaward into deeper water, causing the beach to drop off quickly, posing a hazard to swimmers. This may also impact the surf for a period of time, causing the waves to break as shore break, until the beach and sandbars can reestablish a level of equilibrium.

3.2 | Explaining in detail the use of Cofferdams foundation and its’ application available for marine structures.

A cofferdam may be defined as a temporary structure that is constructed on a river or a lake or any other water-bearing surface for excluding water from a given site to execute the building operation to be performed on dry surface. The walls of the temporary structure should be practically water tight or at least they should be able to exclude water to such an extent that the quantity of water that leaks inside the enclosed area, can be easily pumped out. Cofferdams are classified according to the type of construction. The type of construction is dependent upon the depth, soil conditions fluctuations in the water level, availability of material etc. Cofferdams are advantageously constructed where a large area of site is to be enclosed and the hard bed is at reasonable depth.

In construction of cofferdams maintaining close tolerances is difficult since cofferdams are usually constructed offshore and sometimes under severe weather conditions. Under these circumstances, significant deformations of cofferdam elements may happen during the course of construction, and therefore it may be necessary to deviate from the design dimensions in order to complete the project according to plan.

However, designers of marine structures are tending to use larger and larger diameter piles, both drilled shafts and driven piles. Designers are finding that piles in the range of 2 to 3-m diameter are more efficient than a high number of smaller piles. These larger piles with their higher stiffness make it possible from a design standpoint to position the pile cap at any point in the water column rather than at or below the midline as required with the use of smaller piles. In most cases, the pile-top structures are not located completely out of the water because of aesthetics and concern with splash-zone corrosion at the pile to pile cap connection.

v Types of cofferdam

§     Braced: It is formed from a single wall of sheet piling which is driven into the ground to form a box around the excavation site. The box is then braced on the inside and the interior is dewatered. It is primarily used for bridge piers in shallow water (30 - 35 feet depth)

§     Earth-Type: It is the simplest type of cofferdam. It consists of an earth bank with a clay core or vertical sheet piling enclosing the excavation. It is used for low-level waters with low velocity and easily scoured by water rising over the top.

§     Timber Crib: Constructed on land and floated into place. Lower portion of each cell is matched with contour of river bed. It uses rock ballast and soil to decrease seepage and sink into place, also known as “Gravity Dam”. It usually consists of 12’x12’ cells and is used in rapid currents or on Rocky River beds. It must be properly designed to resist lateral forces such as tipping / overturning and sliding.

§     Double-Walled Sheet Pile: They are double wall cofferdams comprising two parallel rows of sheet piles driven into the ground and connected together by a system of tie rods at one or more levels. The space between the walls is generally filled with granular material such as sand, gravel or broken rock.

§     Cellular: Cellular cofferdams are used only in those circumstances where the excavation size precludes the use of cross-excavation bracing. In this case, the cofferdam must be stable by virtue of its own resistance to lateral forces.

v Advantages of Cofferdam

Performing work over water has always been more difficult and costly than performing the same work on land. And when the work is performed below water, the difficulties and cost difference can increase geometrically with the depth at which the work is performed. The key to performing marine construction work efficiently is to minimize work over water, and perform as much of the work as possible on land.

Some of the advantages of cofferdams are:

*       Allow excavation and construction of structures in otherwise poor environment

*       Provides safe environment to work

*       Contractors typically have design responsibility

*       Steel sheet piles are easily installed and removed

*       Materials can typically be reused on other projects

*

v Use of cofferdams

For dam construction, two cofferdams are usually built, one upstream and one downstream of the proposed dam, after an alternative diversion tunnel or channel has been provided for the river flow to bypass the dam foundation area. These cofferdams are typically a conventional embankment dam of both earth- and rock-fill, but concrete or some sheet piling also may be used. Typically, upon completion of the dam and associated structures, the downstream coffer is removed and the upstream coffer is flooded as the diversion is closed and the reservoir begins to fill. Dependent upon the geography of a dam site, in some applications, a "U"-shaped cofferdam is used in the construction of one half of a dam. When complete, the cofferdam is removed and a similar one is created on the opposite side of the river for the construction of the dam's other half.

The cofferdam is also used on occasion in the shipbuilding and ship repair industry, when it is not practical to put a ship in drydock for repair or alteration. An example of such an application is certain ship lengthening operations. In some cases a ship is actually cut in two while still in the water, and a new section of ship is floated in to lengthen the ship. Torch cutting of the hull is done inside a cofferdam attached directly to the hull of the ship; the cofferdam is then detached before the hull sections are floated apart. The cofferdam is later replaced while the hull sections are welded together again. As expensive as this may be to accomplish, use of a drydock may be even more expensive.

3.3 | Assessing the most appropriate method of construction of breakwater for harbor construction.

As I mentioned before that breakwaters are structures constructed on coasts as part of coastal defense or to protect an anchorage from the effects of both weather and longshore drift.

Breakwaters reduce the intensity of wave action in inshore waters and thereby reduce coastal erosion or provide safe harborage. Breakwaters may also be small structures designed to protect a gently sloping beach and placed one to three hundred feet offshore in relatively shallow water. An anchorage is only safe if ships anchored there are protected from the force of high winds and powerful waves by some large underwater barrier which they can shelter behind. Natural harbors are formed by such barriers as headlands or reefs. Artificial harbors can be created with the help of breakwaters. Mobile harbors, such as the D-Day Mulberry harbors, were floated into position and acted as breakwaters. Some natural harbors, such as those in Plymouth Sound, Portland Harbor and Cherbourg, have been enhanced or extended by breakwaters made of rock.

Breakwaters are required for the protection of artificial and semi-natural harbors. Their location and extent will depend on the direction of the maximum waves, the configuration of the shore line and the minimum size of the harbor required for the anticipated traffic in the port.

Breakwaters may consist of two arms out from the shore, plus a single breakwater, more or less parallel to the shore line, thereby providing two openings to the harbor. When a breakwater is connected to one of the arms, resulting in a curved alignment, only one opening is provided. The harbor can be protected by two arms converging near their out shore ends and overlapping to form a protected entrance to the harbor.

v Construction

There are two main types of fixed breakwaters, the mound type and the wall (vertical) type.

The following four types fall under the first classification and are identified by the materials out of which they are constructed:

·       Natural rock,

·       Concrete block,

·       Combination of rock and concrete block,

·       Concrete shapes such as tetra pods

·       Quadrupeds’dolosse and others.

In the second main classification of breakwaters there are such types as:

·       Concrete block gravity walls,

·       concrete caisons,

·       rock filled sheet pile walls,

·       Concrete or steel pile walls.

The type of breakwaters is usually determined by:

·       The availability of materials at or near the site

·       The depth of water

·       The condition of the sea bottom

·       The function of the breakwater in the harbor

·       The equipment suitable and available for its construction

·       The depth of water and the character of the bottom are important factors in the design of breakwater since most breakwaters are gravity structures, which depend upon their weight for stability.

One common application of breakwaters is to create harbors, where breakwaters are arranged to create an enclosed area of water adjacent to the coast. Harbors provide a safe location where boats can be moored while being protected from the harsh conditions of the open water. The barriers block waves and minimize the wave energy inside. Harbors, however, cannot be entirely closed from the open water. They also require gaps open to the near body of water. These gaps are important to harbors primarily to allow boats to travel in and out, but also to allow fresh water to circulate through the harbor.

v Rubble Mound Breakwaters:

Rubble mound breakwaters are usually a pile of rubble fitted together loosely, therefore creating empty spaces that help to ease the force of the wave. By using concrete Armor or rock outside of the structure, the force is absorbed on the outside, and to further dissipate the force of the way, sands are help stop the wave energy from going through the breakwater core. Building a shallow water breakwater is less costly than deeper water, as the deeper water requires more rubble.

Rubble mound breakwaters is important to protecting the coast and crucial to the design of any port. Rocks will vary in size and the method of dumping the rocks will vary depending on the needs of the particular project. The various methods of building a rubble mound breakwater include building on land using a dump truck, from the sea by barges or from the sea with fall pipes by barges.

v Artificial Concrete Blocks:

They are used where natural rock is not available, or it cannot be produced economically or it large enough size required for armoring the breakwater. Artificial blocks are usually made of plain concrete but rarely are reinforced. They are formed of devised irregular shaped concrete units tested before being used in the field.

The more common ones which have been testes quite extensively are tetra pods. Quadrupeds, hexapods, tribars, modified cubes, akmons, and dolosse.

Other new shapes of concrete units have been developed, tested and used in protective cover layers of rubble mound breakwaters in the past few years; the irregular r slopes and units of lighter weight. This is due to their better (shape factor) and superior adsorption of wave energy.

The properties of the irregular concrete shaped units make it possible to reduce the unit weight of the armor block for the same degree of stability. In addition, the steeper slope and lower run up value of the waves, because of the high degree of irregularity of the surface, enable a reduction in the volume of the remainder of the supporting structure.

v Geotextile Tubes

Southern Dredging & Marine Geotextile Tubes and Bags are UV resistant sand colored geotextile containers filled either hydraulically or mechanically with sand and used instead of stone, concrete or other hardened systems for protecting the coastline. They are especially important where there is minimal local stone or where stone or concrete structures would spoil from the beauty of the area.

Our Geotextile Tubes are considered “soft” armored structures and are generally easier to secure permits than hard armored structures such as various concrete structures or riprap.

Southern Dredging & Marine Geotextile Tubes are used in the design of various shoreline coastal engineering structures such as breakwaters, dykes, dune cores, revetments, reefs, groynes.

    LO - 4 Understand the methods and techniques used in highway construction and railway works.

4.1| Defining the types of pavement with neat sketches.

Pavement is the actual travel surface especially made durable and serviceable to withstand the traffic load commuting upon it. Pavement grants friction for the vehicles thus providing comfort to the driver and transfers the traffic load from the upper surface to the natural soil. A pavement is a structure consisting of superimposed layers of processed materials above the natural soil sub-grade, whose primary function is to distribute the applied vehicle loads to the sub-grade. The pavement structure should be able to provide a surface of acceptable riding quality, adequate skid resistance, favorable light reflecting characteristics, and low noise pollution. The ultimate aim is to ensure that the transmitted stresses due to wheel load are sufficiently reduced, so that they will not exceed bearing capacity of the sub-grade. Two types of pavements are generally recognized as serving this purpose, namely flexible pavements and rigid pavements. This chapter gives an overview of pavement types, layers, and their functions, and pavement failures. Improper design of pavements leads to early failure of pavements affecting the riding quality.

v Types of Pavements

Pavements are primarily to be used by vehicles and pedestrians. Storm water drainage and environmental conditions are a major concern in the designing of a pavement. The first of the constructed roads date back to 4000 BC and consisted of stone paved streets or timber roads. The roads of the earlier times depended solely on stone, gravel and sand for construction and water was used as a binding agent to level and give a finished look to the surface.

The pavements can be classified based on the structural performance into two, flexible pavements and rigid pavements. In flexible pavements, wheel loads are transferred by grain-to-grain contact of the aggregate through the granular structure. The flexible pavement, having less flexural strength, acts like a flexible sheet like bituminous road. On the contrary, in rigid pavements, wheel loads are transferred to sub-grade soil by flexural strength of the pavement and the pavement acts like a rigid plate like cement concrete roads. In addition to these, composite pavements are also available. A thin layer of flexible pavement over rigid pavement is an ideal pavement with most desirable characteristics. However, such pavements are rarely used in new construction because of high cost and complex analysis required.

v Flexible pavements

Are those pavements which reflect the deformation of subgrade and the subsequent layers to the surface. Flexible, usually asphalt, is laid with no reinforcement or with a specialized fabric reinforcement that permits limited flow or repositioning of the roadbed underground changes.

Flexible pavements will transmit wheel load stresses to the lower layers by grain-to-grain transfer through the points of contact in the granular structure. The wheel load acting on the pavement will be distributed to a wider area, and the stress decreases with the depth. Taking advantage of this stress distribution characteristic, flexible pavements normally has many layers. Hence, the design of flexible pavement uses the concept of layered system. Based on this, flexible pavement may be constructed in a number of layers and the top layer has to be of best quality to sustain maximum compressive stress, in addition to wear and tear. The lower layers will experience lesser magnitude of stress and low quality material can be used. Flexible pavements are constructed using bituminous materials. These can be either in the form of surface treatments or asphalt concrete surface courses. Flexible pavement layers reflect the deformation of the lower layers on to the surface layer. In the case of flexible pavement, the design is based on overall performance of flexible pavement, and the stresses produced should
be kept well below the allowable stresses of each pavement layer.

Ø  Layers of a flexible pavement

Layers of a conventional flexible pavement includes surface course, base course, sub-base course, and natural sub-grade.

Surface course

Surface course is the layer directly in contact with traffic loads and generally contains superior quality materials. They are usually constructed with dense graded asphalt concrete(AC). The functions and requirements of this layer are:

ª     It provides characteristics such as friction, smoothness, drainage, etc. Also it will prevent the entrance of excessive quantities of surface water into the underlying base, sub-base and sub-grade,

ª     It must be tough to resist the distortion under traffic and provide a smooth and skid- resistant riding surface,

ª     It must be water proof to protect the entire base and sub-grade from the weakening effect of water.

Base course

The base course is the layer of material immediately beneath the surface of binder course and it provides additional load distribution and contributes to the sub-surface drainage it may be composed of crushed stone, crushed slag, and other untreated or stabilized materials.

Sub-Base course

The sub-base course is the layer of material beneath the base course and the primary functions are to provide structural support, improve drainage, and reduce the intrusion of fines from the sub-grade in the pavement structure If the base course is open graded, then the sub-base course with more fines can serve as a filler between sub-grade and the base course A sub-base course is not always needed or used. For example, a pavement constructed over a high quality, stiff sub-grade may not need the additional features offered by a sub-base course. Therefor I didn’t mentioned the sub-grade course in the figer.  In such situations, sub-base course may not be provided.

Ø  Types of Flexible Pavements

The following types of construction have been used in flexible pavement:

Conventional flexible pavements: Conventional flexible pavements are layered systems with high quality expensive materials are placed in the top where stresses are high, and low quality cheap materials are placed in lower layers.

Full - depth asphalt pavements: Full - depth asphalt pavements are constructed by placing bituminous layers directly on the soil sub-grade. This is more suitable when there is high traffic and local materials are not available.

Contained rock asphalt mats: Contained rock asphalt mats are constructed by placing dense or open graded aggregate layers in between two asphalt layers. Modified dense graded asphalt concrete is placed above the sub-grade will significantly reduce the vertical compressive strain on soil sub-grade and protect from surface water.

Ø  Failure of flexible pavements

The major flexible pavement failures are fatigue cracking, rutting, and thermal cracking. The fatigue cracking of flexible pavement is due to horizontal tensile strain at the bottom of the asphaltic concrete. The failure criterion relates allowable number of load repetitions to tensile strain and this relation can be determined in the laboratory fatigue test on asphaltic concrete specimens. Rutting occurs only on flexible pavements as indicated by permanent deformation or rut depth along wheel load path. Two design methods have been used to control rutting: one to limit the vertical compressive strain on the top of subgrade and other to limit rutting to a tolerable amount (12 mm normally). Thermal cracking includes both low-temperature cracking and thermal fatigue cracking.

v Rigid pavements

Rigid pavements have sufficient flexural strength to transmit the wheel load stresses to a wider area below. Compared to flexible pavement, rigid pavements are placed either directly on the prepared sub-grade or on a single layer of granular or stabilized material. Since there is only one layer of material between the concrete and the sub-grade, this layer can be called as base or sub-base course.

In rigid pavement, load is distributed by the slab action, and the pavement behaves like an elastic plate resting on a viscous medium. Rigid pavements are constructed by Portland cement concrete (PCC) and should be analyzed by plate theory instead of layer theory, assuming an elastic plate resting on viscous foundation. Plate theory is a simplified version of layer theory that assumes the concrete slab as a medium thick plate which is plane before loading and to remain plane after loading. Bending of the slab due to wheel load and temperature variation and the resulting tensile and flexural stress.

Ø  Types of Rigid Pavements

Rigid pavements can be classified into this types:

Jointed Plain Concrete Pavement: Jointed Plain Concrete Pavement are plain cement concrete pavements constructed with closely spaced contraction joints. Dowel bars or aggregate interlocks are normally used for load transfer across joints. They normally has a joint spacing of 5 to 10m.

Jointed Reinforced Concrete Pavement: Although reinforcements do not improve the structural capacity significantly, they can drastically increase the joint spacing to 10 to 30m. Dowel bars are required for load transfer. Reinforcement help to keep the slab together even after cracks.

Continuous Reinforced Concrete Pavement: Complete elimination of joints are achieved by reinforcement.

Ø  Failure criteria of rigid pavements

Traditionally fatigue cracking has been considered as the major, or only criterion for rigid pavement design. The allowable number of load repetitions to cause fatigue cracking depends on the stress ratio between flexural tensile stress and concrete modulus of rupture. Of late, pumping is identified as an important failure criterion. Pumping is the ejection of soil slurry through the joints and cracks of cement concrete pavement, caused during the downward movement of slab under the heavy wheel loads. Other major types of distress in rigid pavements include faulting, spalling, and deterioration.

v Difference between Flexible Pavements and Rigid Pavements

Flexible Pavement	Rigid Pavement
It consists of a series of layers with the highest quality materials at or near the surface of pavement.	It consists of one layer Portland cement concrete slab or relatively high flexural strength.
It reflects the deformations of subgrade and subsequent layers on the surface.	It is able to bridge over localized failures and area of inadequate support.
Its stability depends upon the aggregate interlock, particle friction and cohesion.	Its structural strength is provided by the pavement slab itself by its beam action.
Pavement design is greatly influenced by the subgrade strength.	Flexural strength of concrete is a major factor for design.
It functions by a way of load distribution through the component layers	It distributes load over a wide area of subgrade because of its rigidity and high modulus of elasticity.
Temperature variations due to change in atmospheric conditions do not produce stresses in flexible pavements.	Temperature changes induce heavy stresses in rigid pavements.
Flexible pavements have self-healing properties due to heavier wheel loads are recoverable due to some extent.	Any excessive deformations occurring due to heavier wheel loads are not recoverable, i.e. settlements are permanent.

4.2 | Analyzing the methods of construction of Railway track.

Ballasted railway tracks mainly comprise two main parts: superstructure and substructure. Steel rails, various types of rail fasteners, timber, steel, or concrete sleepers, and granular ballast, sub ballast, and subgrade materials are major components used in railway track construction. Historically, the understanding of track structural behavior has been facing difficulties. This is due to different mechanical properties of track components from one side, and complex interaction between track components from the other side

Wide range of track configurations could be designed and constructed. This makes railway track structure to be subjected to regular changes. Consequently, based on theoretical and experimental investigations, large number and sometimes contradictory design criteria have been suggested by railway authorities and practitioners. Such a diversity of design criteria usually makes railway track design procedure a difficult task. This highlights the necessity of a need for a thorough review of the currently used railway track analysis and design methods.

v Methods

Generally, railway track systems are designed to provide a smooth and safe running surface for passing trains. They also serve to sustain the loads imposed to track structure mainly as a result of trains passages and temperature changes.

There are mainly three distinct methods of construction of railway track.

Ø  Tram line method

In this method, a temporary line known as a tram line is laid by the side of the proposed track for transporting track material to the site. This method can be useful in flat terrains, where laying of a tram line on the natural ground may be comparatively easier. This method is however, seldom used in actual practice.

This method is used where tram carrier are installed for carrying earthwork or in rainy season due to difficulty in movement of cart. Some tramline is established on with a gauge of 2'-2'-6". The basic difference between this and telescopic method lies in the conveyance and spreading of the sleepers. The track can be assembled at more than one points simultaneously, which is the great advantage of this method. Sometimes an additional track is laid on the side of existing track for which this method is best.

A modification of the above method, called side method, is also in practice, where track and bridge material such as steel girders and RCC slabs is carried to the site in trucks on a service road that runs parallel to the track. These materials are then unloaded near the work site. This method is used only in cases where the terrain is comparatively flat.

Ø  Mechanical Method

This method is also known as American method. This method is extensively used in Britain and America by using special track laying machine. In this method, rails and sleepers are first assembled in the base depot, and the pre-assembled track panels are then conveyed to the site along with the necessary cranes, etc. The track panels are then unloaded at the site of work either manually or with the help of cranes and laid in their final position. There are two types of machines available.  In first type of machine, the track material carried by the material. Train is delivered at the rail head and laid in the required position by means of projecting arm or mounted on the truck nearest to the rail head. The material train moves forward on the assembled track and operation is repeated.

In the second type of machines a long cantilevered arm projecting beyond. The wagon on which is fitted. A panel of assembled track consists of pair of rail with appropriate number of sleepers on the ballast layer. This panel is conveyed by special trolley running over the wagons of material train to the jibs. It is lowered by the jib in the required position and connected to the previous panel. The track laying machine then movies forwarded and operation is repeated.

This procedure is used in many developed countries, particularly where concrete sleepers are laid, which are quite heavy and not very easy to handle manually.

Ø  Telescopic method

This method is widely used on Indian Railways. In this method, rails, sleepers and fastenings are unloaded from the material train as close to the rail head as possible. The track material is then taken to the rail head, where the tracks linked and packed. The rail head is then advanced up to the point where the track has been laid. The track materials are then taken up to the extended rail head with the help of a dip lorry and the track is linked and packed again. The sleepers are carried by carts or men along the adjoining service road and spread on the ballast. The rails are then carried on pairs to the end of last pair of connected rails and linked.

v Rail construction

As the most important part of railway track system, railway track plays a role in giving a reliable surface for train to run. Railway track have other names like railroad track, track and permanent way. Since the first track building in the 1825, railway track go through several reforms. Ballasted track and ballastless track are typical types of railroad track. In general, railway track consist of ballast bed, steel rail, railway sleeper, railway fish plate, rail clip, railroad tie plate and other railway fasteners. How to build a railway track with all these components? Here is the guide to introduce the process step by step.

Pre-construction activities

There are some preparative works before railway track construction. Among all the activities, subgrade drainage and materials preparation are common.

§     The subgrade drainage is a system that is used to prevent he railway from water logging. The subgrade, road bed and slope of railway track are very easy to be washed by water. If the subgrade drainage measures are not proper, this will lead to the subgrade diseases. It is necessary to install drainage before laying the track. Specifically, it mainly adopts drainage pipes, carrier drains and attenuation ponds in some area.

§     The preparation of construction materials is another work before track laying. Ordinary materials include railway sleepers, steel rail, rail fasteners and some construction equipment. Preparation works primarily refer to check the complement and integrity of all materials.

Bottom ballast

The track materials are taken to the base depot and unloaded with the help of material gangs. The first base depot lies at the junction of the existing line and the new line to be constructed. All the track material is taken from the base depot to the rail head with the help of a dip lorry it is a special type of trolley. Ballast bed is the dependable foundation for railway track. According to the construction procedures, ballast bed construction is divided into two parts: bottom ballast and top ballast. There are other procedures between bottom ballast and top ballast laying. So, bottom ballast and top ballast will be separate to introduce.

The rail head goes on advancing till the track is sufficiently linked. After that, a subsidiary depot is established at a distance of about 5 km and track material carried to this depot with the help of a material train. Alternatively, track material is transferred from the base depot with the help of a dip lorry up to a distance of about 2 km and by the means of a material train beyond this distance. The base depot has arrangements for advanced processes such as adzing and boring of sleepers as well as for matching materials, etc. to ensure the speedy linking of the track at the site.

Anchorage

Anchorage means the process that fix railroad spike to railway sleeper. This procedure requires materials as follow: Sulphur, sand, cement, paraffin, screw spike. Learn more specific process at how to fastener screw spike to railway sleeper.

Linking of track

Once the track material is unloaded, the track is linked with the help of linking gangs. The following procedure is normally adopted for this purpose.

*       A string is first stretched along the central line of the alignment and the sleepers are laid with their centres on the string. The sleepers are laid roughly at the desired spacing, keeping the total number of sleepers per rail intact.

*       The rails are carried using rail tongs and laid on the cess of the bank almost near the final position. Carrying rails is a strenuous job, as about 12 to 15 gangmen are required to carry each rail (each rail weighs about 0.6 t or so). A special type of rail carrier known as the Anderson rail carrier, can be used for carrying rails with lesser strain.

*       Next the sleepers are distributed over the length of the formation. The rails have markings to indicate the final position of the sleepers as shown in figure.

*       Small fittings such as fish plates and bolts are kept near the joints. The fittings required for each sleeper are kept near the ends of the sleepers.

*       The rails are then placed on the sleepers and fixed with the help of fittings, which are chosen depending upon the type of sleeper. For example, rail screws are used for fixing rails to wooden sleepers. In the case of steel sleepers, rails are fixed with the help of keys. Bearing plates are also provided wherever required, as per the prescribed track standards.

*      
 The rails are joined with each other after ensuring that there is sufficient gap between them. Normally, the initial laying of the tracks is done using three rail panels. Adequate expansion gaps should be provided in the case of single-rail as well as three-rail panels. The recommended expansion gaps are provided with the help of steel liners or shims of appropriate thickness (1 mm to 4 mm), which are fixed between the two rail ends.

Packing of track

The track is then thoroughly packed with the help of beaters by the Packing-in-gangs. The following aspects should be examined during this process.

ª     The track should have a proper gradient.

ª     If the track is on a curve, it should have proper curvature.

ª     The cross levels should be even. If a track is to be provided with the recommended super elevation, this can be achieved by raising the outer rail.

ª     The track should be thoroughly packed and should be free of hollow spaces.

Ballasting of track

The railway line is normally covered with the ballast after the embankment has settled and has endured at least two monsoons. Ballasting is generally done with the help of a ballast train, which has special hoppers that are used for automatically unloading the ballast onto the track. Alternatively, the ballast is taken to the cess and then placed on the track manually. Either method ensures that the ballast is thoroughly packed and inserted properly under the track.

Rail anchor and rail brace

Rail anchor is used to prevent track from crawling. Rail brace is connected with steel rail through rail bolt and nuts. Both rail anchor and rail brace are designed to keep steel rail in place and ensure the rail safety.

Rail anchors made in one-piece construction from spring steel are designed to fasten the rail tight on the base of the rail to prevent the rail from longitudinal movements, the of the rail, caused by changing temperature, grades, traffic patterns, and braking action of trains. Rail anchors are applied to the rail base directly and lodge up against the tie. Rail anchors provide a large bearing surface against rail base and rail tie, preventing cutting and wear, and eventually to prolong the working life of the rail ties. Anchors are made for a specific rail weight and base width, which can be classified into two types: the Drive-on rail anchors and spring type rail anchor.

4.3 | Explaining in detail the method of construction of flexible pavement

I already discussed the Types, Causes of failure, Layer and more of Flexible pavement. Now I will give more details about the Flexible pavement.

Flexible pavement is composed of a bituminous material surface course and underlying base and subbase courses. The bituminous material is more often asphalt whose viscous nature allows significant plastic deformation. Most asphalt surfaces are built on a gravel base, although some 'full depth' asphalt surfaces are built directly on the subgrade. Depending on the temperature at which it is applied, asphalt is categorized as hot mix asphalt (HMA), warm mix asphalt, or cold mix asphalt. Flexible Pavement is so named as the pavement surface reflects the total deflection of all subsequent layers due to the traffic load acting upon it. The flexible pavement design is based on the load distributing characteristics of a layered system.

It transmits load to the subgrade through a combination of layers. Flexible pavement distributes load over a relatively smaller area of the subgrade beneath. The initial installation cost of a flexible pavement is quite low which is why this type of pavement is more commonly seen universally. However, the flexible pavement requires maintenance and routine repairs every few years. In addition flexible pavement deteriorates rapidly; cracks and potholes are likely to appear due to poor drainage and heavy vehicular traffic.

The wheel load acting on the pavement will be distributed to a wider area, and the stress decreases with the depth. Taking advantage of these stress distribution characteristic, flexible pavements normally has many layers. Hence, the design of flexible pavement uses the concept of layered system.

v Layer of Flexible pavement

I already discussed about layers of flexible pavement in LOC 4.1. Based on this, flexible pavement may be constructed in a number of layers and the top layer has to be of best quality to sustain maximum compressive stress, in addition to wear and tear.

The lower layers will experience lesser magnitude of stress and low quality material can be used. Flexible pavements are constructed using bituminous materials. These can be either in the form of surface treatments (such as bituminous surface treatments generally found on low volume roads) or, asphalt concrete surface courses (generally used on high volume roads such as national highways). Flexible pavement layers reflect the deformation of the lower layers on to the surface layer (e.g., if there is any undulation in sub-grade then it will be transferred to the surface layer). In the case of flexible pavement, the design is based on overall performance of flexible pavement, and the stresses produced should be kept well below the allowable
stresses of each pavement layer.

v Construction

Delivery and Spreading

Pavement base, sub-base and select material shall not be placed on the subgrade or previous layers of pavement until release of the Hold Point(s) associated with those layers. Material shall not be placed over a layer with moisture content exceeding 90% of the laboratory optimum moisture content as determined by AS 1289.5.2.1 or that has become rutted or mixed with foreign matter.

Base, sub-base and select materials, when delivered, shall have a moisture content within ± 2% of the Modified optimum moisture content.

The material shall be spread in uniform layers as near as practicable to the required thickness by direct tipping from suitable vehicles or by the use of a mechanical spreader. Segregation of material during tipping and spreading shall be minimized. If material becomes segregated it shall be remixed to produce a non-segregated material.

Compaction and Finishing

Layers of base, sub-base and select material shall be not less than 100mm in compacted thickness. Maximum layer thickness shall be limited to that which will allow compaction to specified densities by the equipment in use. Where a course of a particular material is composed of several layers they shall be of approximately equal thickness within these limits.

During compaction, the moisture content of pavement materials shall be maintained in the range specified above for delivery. Water spraying equipment used for this purpose shall be capable of uniformly distributing water in controlled quantities over uniform widths.

On sections of pavement with one-way crossfall, compaction shall begin on the low side of the pavement and progress to the high side. On crowned sections, compaction shall begin at the sides of the pavement and progress towards the crown. Each coverage of the rollers shall be approximately parallel with the centreline of the roadway and uniformly overlap each preceding coverage. The outer metre of both sides of the pavement shall receive at least two more coverages by the compaction plant than the remainder of the pavement.

Surfaces of base material like surfaces to receive a bituminous surfacing shall be constructed slightly higher than the specified levels and cut to profile by power grader or trimming machine towards the end of the compaction process. Rolling shall then continue to produce the specified density and a tight, even surface without loose stones or a slurry of fines. Cuttings from the surface may be used to a maximum depth of 50mm in the bottoms of adjoining layers of base material subject to non-contamination by other materials.

Matching to Existing Pavements

Unless specified otherwise, where the pavement is to be joined to an existing pavement, remove a strip of the existing pavement at least 300mm wide for its full depth and trim the edge to an angle of approximately 45º in steps of maximum height 150mm before placing new pavement material. If the existing pavement has a bituminous surface, trim the bituminous surface to a neat edge using a saw cut, pneumatic tools or other suitable means.

    LO - 5 Be able to solve problems arising from complex civil engineering activities.

5.1 | Design an appropriate solution to mitigate the further damage to the dam. What are the problems could be created for complex civil engineering activities and propose appropriate solution for such problem.

Note: Take two such problems from Marine, Highways and Tunneling Activities

Flooding has many impacts. It damages property and endangers the lives of humans and other type. Rapid water runoff causes soil erosion and concomitant sediment evidence somewhere else (such as further downstream or down a coast).

The spawning grounds for fish and other wildlife habitats can become polluted or completely shattered. Some extended high floods can delay traffic in areas which lack elevated roadways. Floods can interfere with drainage and economical use of lands, such as interfering with farming. Structural damage can occur in bridge abutments, bank lines, sewer lines, and other structures within floodways. Waterway navigation and hydroelectric power are often impaired.

The failure of a dam can result in the uncontrollable release of water flooding downstream areas. Dams can fail due to one or more of the following scenarios.

Extreme upstream floods, which can overtop a dam wall causing erosion or the movement of the dam;

Seepage and possible piping of water through the dam wall or its foundations; Earthquakes, which can cause damage to a dam wall or outlet infrastructure Human factors relating to the operation of a dam or willful damage Some environmental problems caused by dams.

Soil Erosion

One of the first problems with dams is the erosion of land. Dams hold back the sediment load normally found in a river flow, depriving the downstream of this. In order to make up for the sediments, the downstream water erodes its channels and banks. This lowering of the river bed threatens vegetation and river wildlife. A major example of soil erosion problems is the Aswan Dam. One of the reasons dams are built is to prevent flooding. However, most ecosystems which experience flooding are adapted to this and many animal species depend on the floods for various lifecycle stages, such as reproduction and hatching.

Changes to Earth's Rotation

NASA geophysicist Dr. Benjamin Fong Chao found evidence that large dams cause changes to the earth's rotation, because of the shift of water weight from oceans to reservoirs. Because of the number of dams which have been built, the Earth's daily rotation has apparently sped up by eight-millionths of a second since the 1950s. Chao said it is the first time human activity has been shown to have a measurable effect on the Earth's motion.

Spread of Disease

Dam reservoirs in tropical areas, due to their slow-movement, are literally breeding grounds for mosquitoes, snails, and flies, the vectors that carry malaria, schistosomiasis, and river blindness.

Species Extinction

As fisheries become an increasingly important source of food supply, more attention is being paid to the harmful effects of dams on many fish and marine mammal populations. The vast majorities of large dams do not include proper bypass systems for these animals, interfering with their lifecycles and sometimes even forcing species to extinction many dams and their associated reservoirs are designed completely or partially to aid in flood protection and control. Many large dams have flood-control reservations in which the level of a reservoir must be kept below a certain elevation before the onset of the rainy/summer melt season to allow a certain amount of space in which floodwaters can fill. The term dry dam refers to a dam that serves purely for flood control without any conservation storage (e.g. Mount Morris Dam, Seven Oaks Dam).

Water-Gate

The Water-Gate Flood barrier is a rapid response barrier which can be rolled out in a matter of minutes. It is unique in the way that it self deploys using the weight of water to hold it back. The product has been FM Approved following testing from the US Army. It is used in 30 countries around the world, and notably by the Environment Agency in the UK.

Self-closing flood barrier

The self-closing flood barrier (SCFB) is a flood defense system designed to protect people and property from inland waterway floods caused by heavy rainfall, gales or rapid melting snow The SCFB can be built to protect residential properties and whole communities, as well as industrial or other strategic areas. The barrier system is constantly ready to deploy in a flood situation, it can be installed in any length and uses the rising flood water to deploy. Barrier systems have already been built and installed in Belgium, Italy, Ireland, the Netherlands, Thailand, United Kingdom, Vietnam, Australia, Russia and the United States. Millions of documents at the National Archives building in Washington DC are protected by two SCFBs

Solution for such problem

*       Sufficient outlets and spillways should be provided to avoid the possibility overtopping during design floods.

*       For frost action, wave action and earthquake motions, sufficient freeboard must be provided.

*        If the stability of foundations and embankments is not impaired by piping, sloughing etc., there should be little harm in seepage through a flood control dam. But a conservation dam should be as water-tight as possible.

*       To avoid sloughing of face of earth dam, the phreatic line i.e. seepage line should be within the downstream face of the dam.

*       The downstream face must be protected properly against rain, waves, up to tail water and the upstream face against wave action. To reduce erosion due to flow of rain water horizontal berms may be provided at suitable intervals in the downstream face. Ripraps may be provided on the entire upstream slope and on the downstream slope near the toe so as to prevent erosion.

*        By providing suitable horizontal filler drain or chimney drain or toe drain, the portion of the dam and downstream of the impervious core should be properly drained.

*       There must be no possibility of free flow of water from upstream to downstream face.

*        The upstream and downstream slopes should be designed so as to be stable under worst conditions of loading. Such critical conditions occur for the upstream slope during sudden downstream of the reservoir and for the downstream slope during steady seepage under full reservoir.

*        The upstream slope and downstream slope must be flat enough to provide sufficient base width at the foundation level, such that the maximum shear stress developed remains well below the corresponding maximum shear strength of the soil so as to provide suitable factor of safety.

*       Due to development of excessive pore pressure and consequent reduction in shear strength of soil, the stability of the embankment and foundations is very critical during construction or even after the construction during the period of consolidation. So under this critical condition the embankment slopes must remain safe.

5.2 | Explaining the importance of resource plan in detail.

In the 2^nd Scenario we got that: During the last week the rain fall was very high and the water level of a reservoir exceed the high flood level and also found there was a seepage water path developed across the dam. This may result a serious damage to the dam. If this seepage is allowed to continue, there will be imminent danger of breaking the dam and which is adversely affected   the people who are living in down –stream of the dam.

Because of this Scenario now I’ll explain the importance of resource plan of Seepage through Earthen Dams.

v Seepage through Earthen Dams

Wet areas downstream from dams are not usually natural springs, but seepage through or under the dam. Even if natural springs exist, they should be treated with suspicion and carefully observed. Flows from ground-water springs in existence prior to the reservoir would probably increase due to the pressure caused by the pool of water behind the dam.

All dams have some seepage as the impounded water seeks paths of least resistance through the dam and its foundation. Seepage becomes a concern if it is carrying material with it, and should be controlled to prevent erosion of the embankment, or foundation, or damage to concrete structures.

Seepage can emerge anywhere on the downstream face, beyond the toe, or on the downstream abutments at elevations below normal pool. Seepage may vary in appearance from a "soft" wet area to a flowing "spring." It may show up first as an area where the plants is lush and darker green. Cattails, reeds, mosses, and other marsh vegetation often become established in a seepage area. Another indication of seepage is the presence of rust-colored iron bacteria. Due to their nature, the bacteria are found more often where water is discharging from the ground than in surface water. Seepage can make inspection and maintenance difficult. It can also saturate and weaken portions of the embankment and foundation, making the embankment susceptible to earth slides.

The need for seepage control will depend on the quantity, content, and location of the seepage. Reducing the quantity of seepage that occurs after construction is difficult and expensive. It is not usually attempted unless the seepage has lowered the pool level or is endangering the dam or appurtenant structures. Typical methods used to control the quantity of seepage are grouting or installation of an upstream blanket. Of these methods, grouting is probably the least effective and is most applicable to leakage zones in bedrock, abutments, and foundations.

Types of seepage control measures that have been utilized at existing Recovery embankments include internal embankment filters/drains, toe drains; downstream drainage trenches, relief wells, horizontal drains, drainage galleries and tunnels, and conduit filter envelopes.

Internal filter or drain

Internal filter and drainage features for an embankment dam typically include a chimney filter and drain located immediately downstream of the core of the dam, connected to a horizontal filter and drainage blanket that extends to the downstream toe of the dam. Quite often this filter and drain system is comprised of two separate zones to ensure both filter compatibility and adequate drainage capacity.

Drainage trenches

Downstream drainage trenches provide relief of pressures and a filtered outlet for seepage pathways that are located at a greater depth than would be encountered with a typical toe drain. Trenches are excavated and filled with filter/drainage materials of specified gradation to prevent piping of adjacent foundation soils into the trench.

Relief wells: Relief wells are used to reduce excessive pore pressures in pervious foundations to a tolerable level and provide safety against high exit gradients or uplift pressures. Frequently, relief wells are used to reduce artesian pressures in confined aquifers. Carefully designed “filter packs” are placed around the well screen to ensure that foundation materials are not piped into the well.

Drainage galleries and tunnels

These features typically consist of tunnels bored into the foundation from which a series of drain holes fan out into the foundation (which typically is rock). The general intent of the features is to relieve pore pressures and remove and control seepage flows from beneath the embankment, often with a focus at the embankment/foundation contact. Since potentially high gradients might develop in the immediately surrounding soils, drains that are installed near the embankment/foundation contact or in erodible rock should be filtered to minimize any potential for piping of soils into the drains.

Conduit filter envelopes: Historically, concentrated seepage along conduits has been one of the main contributors to internal erosion failures in embankment dams. The key measure for protecting against internal erosion along conduits is a filter envelope that fully surrounds the conduit.

Horizontal drains

Horizontal or semi-horizontal drains can be bored into foundations (frequently in abutment areas) to relieve excessive pore pressures or intercept seepage. Horizontal drains have been constructed in both rock and soil materials. Careful attention to screening and filtering is essential to prevent the potential for internal erosion into the drains.

Toe drains

Toe drains typically serve as the collection system for the internal drainage system in the embankment, as well as a drainage source for foundation seepage. Such drains are designed to satisfy filter criteria for both embankment and foundation soils. An important advantage of toe drains is that they provide a means for quantitative measurement of seepage to aid in observation of seepage-related behavior

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