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Earthwork 81 continue sinking by removing the obstacles manually. The working chamber should have access for entry/exit of workers and lowering of materials, (vide Fig. 4.26). Precaution is to be taken for workers to work in an environment of pressurized air. In Japan, robots are deployed in place of workers. Safe limit of compressed air pressure is 3.1 bar or 310 kN/m2. Sinking of the caisson is controlled as follows:

0 Excavation is to be carried out symmetrically

0 Working chamber should be made leaving berm with cutting edge to avoid compressed air leaking

Air pressure should be maintained depending on the types of soil encountered or to be encountered-sand blowing may cause compressed air leakage resulting in stoppage of work and re-pressurising of the working chamber

0 Building up of static friction could be avoided by continuous sinking-this would require mobilisation of standby compressors and decompression facilities When the required depth is reached and sinking is completed, the floor of the working chamber is plugged with well-vibrated concrete. The sealing is completed by carefully pressure grouting the top gap of the chamber. Reinforced bottom plugging is possible only in pneumatic caissons.

4.6 TRENCHLESS (NO-DIG) TECHNOLOGY

Trenchless technology is adopted for underground utility services in urban or developed areas underneath existing buildings, streets, roads, railways, embankments, installations, or wherever open excavation or trenching is not possible. This technology (no-dig methods) allows underground work to be executed without disrupting the existing utilities/facilities and/or causing inconvenience to the concerned people. This relatively new technology should be exploited to its full potential by more research and development efforts. There are different methods involved in trenchless technology on installation of drains, pipes, or ducts. The method to be selected would depend on:

Sizes/dimensions of cables/pipes/ducts 0 Length involved

0 Soil conditions

0 Existing underground hindrances

Pipe Jacking

Pipe jacking is based on the principle that involves pushing pipes up to 400 mm in diameter over a distance of up to 500 m by engaging one or several hydraulic jacks from jacking pit (also referred to as launch pit). The first length of pipe is fitted with a sharp closed hood and is set on guide frames. Jack pressure is transmitted to the pipe through a thrust bar with an adjustable rod. The distance between the holes on the thrust bar equals the length of the jack-working stroke. Extensions are attached to the pipes as it is forced in. The wall of the launching pit is reinforced with a shield to provide support for the jack. Pipe jacking is applicable at depths of not less than 3 m in soils free of boulders

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and other solid inclusions that could cause the pipe to lose direction. But jacking can be done in water bearing granular materials, soil containing cobbles, and even soft rocks by innovating jacking methods, vide Fig. 4.27. Excavation of soil in the pipe, which is pushed in, is carried out manually or by deploying backhoe wherever possible. In case of smaller-diameter pipes, soil inside the pushed pipe is cleaned by rotary slurry cutting heads or by using augers.

lubricant jacks rotary borer or

excavating arm (backhoe type) Fig. 4.27 Pipe jacking

Thrust Boring

Thrust boring is another method of jacking pipe (diameter 250 mm up to 1800 mm) as well as sewer line of square cross-section (duct) through ground between the launch pit to the receiving pit (also referred to as exit pit)-a distance that could be up to 200 m. The first piece/section of pipe/duct is equipped with a rotating boring head with tungsten carbide cutters, which cut the soil into small pieces as hydraulic jacks push the pipe forward. The whole system uses laser and computer control located in the launch pit to guide the advance of pipe/duct through the ground. The cut soil is channeled back through the boring head into a crusher wherefrom the resulting mixture is sucked to the surface with a vacuum pump. As the boring makes progress, additional pieces of pipe/ duct are added at the launch pit.

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Earthwork 83

Horizontal Direction Drilling

Horizontal direction drilling (HDD), unlike pipe jacking, involves both pushing and pulling. The system consists of a self-contained hydraulic bore rig operated by an integrated diesel engine and a bentonite mixing system. A fresh water tank and a high- pressure cleaning system are also an integral part of the bore rig for cleaning the drill pipes during and after the boring operation. Bentonite slurry is forced at high pressure through a small-diameter nozzle for cutting into soil. The cutting head could also be a mechanical device, something like a rotating drill. An electric rotary motor mounted on a drilling rig capable of producing rotation of the drill head is an alternative to the hydraulic system. A transmitter unit (sonde) fitted in the drill head transmits signal that is detected at the surface through a hand-held antenna. The cutting jet/head is thus steered clear of existing obstacles by constant monitoring of direction and depth of cutting head. The hindrances can be circumvented and curves accommodated. HDD is used for installing underground ducts, cables, and service pipes for crossing roads/ rivers or meandering through known existing underground services. The drill stem consists of flexible steel tubes of 80 mm diameter. The drill bit is started at the launch pit for termination on completion of boring at the receptiodexit pit. The boring is done through clay, but boring through sand and gravels is also possible.

At the reception pit, a back-reaming head replaces the drill head. The back-reamer can work up to bore of 350 mm. With the progress of back reaming, the service pipe is simultaneously pulled ultimately back to the launch pit. Whereas the push force should be limited to so much as not to buckle the drill stem, the pull force can be relatively much more.

Radio link between 'guide man' and surface machine operator

Cutting head

tracked bv radio

I$?

\

Launchoit

Cutting head

I Pulled back through the ground by the surrace machine

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Pipe Bursting

Pipe bursting is a trenchless method of replacing any underground pipe that needs to be replaced. To do this, a larger-diameter pipe is pushed along the old pipeline using hydraulic rams. A metal cone is fitted at the head of the new pipe. The hitting of this metal cone into the old pipe results in bursting of the old pipe into pieces. The impact of the ramming consolidates the pieces into the surrounding ground, thus making way for the new pipe to replace the shattered old pipe. Instead of applying hydraulic force, the metal cone can be pulled by a steel wire powered by mechanical energy. The impact and the consequence of the impact would Fig. 4.30 Pipe bursting be the same as was in the case of hydraulic rams. The principle of pipe bursting is a simple and quick way of replacing old pipes, but this method cannot be applied if the pipe to be replaced is connected to other pipelines joining it at angles.

Percussive Boring

Percussive boring is based on using a chisel head for punching its way through the ground forming a hole along the way. The diameter of this hole is larger than the diameter of PVC pipe that is to be pulled by the chisel head, which is powered by compressed air. This method is used for installing smaller pipes of diameter up to 200 mm. The equipment comprises a piston-driven chisel head. The driving power of the piston is compressed air, which powers the chisel head to punch its way through the ground pulling a PVC pipe along. Bricks or stones that may obstruct the chisel head’s progress are crushed or pushed aside. This method is more suitable up to a distance of 20 m or so but may be used up to 60 m.

The most serious problem with the trenchless technology is the avoidance of existing pipes and services. At present, approximate locations of anticipatedhuspected pipes are to be located by radio or metal detectors and then establish precise locations by manual excavations and identify the services. This can be a difficult, time-consuming and costly proposition, but hitting and disrupting any service could be more costly and hazardous. Drilling and boring equipment can easily cut through an existing service unintentionally, and it is in this area that research and development work is desperately needed. There are basically two aspects to the problem-the first is that the precise location of any existing service is difficult, and the second problem is that the drilling and boring machine cannot see where they are heading and would not know of an existing service until it is too late. The location of existing services is being tackled by an integrated national computerized register of service locations in many countries with the intention of doing the work as accurately as possible, but since many locations are

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Earthwork 85

Piston Chisel head

C

I

Compressed- +

air line +

Sight rail

1

\

Cradle Pressure head Fig. 4.3 I Percussive boring

recorded plus or minus three metres, there is a lot of work to be done in this area before the system can be considered reliable. The detection of underground services from the drill head or surface is an area that requires further research and development. Trenchless technology for pipe and service installation and repair are already very useful, but hold the potential for great benefit to our society in the future if the problem of existing service location can be solved.

4.7

GRADING

Construction project sites need to be properly graded and accurately finished by eliminating natural terrain roughness and providing slopes as shown in design drawings. The site surfaces should be smooth and level without undulations and ridges. In any project, both cutting and filling is usually involved. Grading, therefore, could involve balanced, excessive or deficient amounts of earthwork in excavation and filling. In the first case, the volumes of cutting and filling are balanced, which should be the ideal case. In the second case, the volume of cutting would be in excess of filling, thereby necessitating disposal of the soil mass that is in excess. In the third case, the volume of cutting would be less than the volume of earthwork involved in filling, thereby necessitating hauling of the deficit volume from outside the project site. In site development work, there could be cases where cutting would be nominal or negligible, and there could also be cases where filling would not be required at all. In any case, it is easier to spread several small piles of excavated spoils than spreading a huge pile. Accordingly, excavation should be interrupted from time to time to allow spreading of the excavated spoils.

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Grading of a project site is to be planned on the basis of contour maps and design drawings. While contour lines show the existing level intervals, construction drawings would show design grade levels. How much earthwork is involved in either excavation or filling can be worked out on the basis of existing levels and design grade levels. Design levels are determined keeping technology, drainage, environment, and aesthetic considerations in view. For effecting economy in earthwork and also for compacting time schedule for the same, a project site may be divided into different design grade levels on the basis of contour lines. This grading referred to here is related to the site development. This initial problem of site development would be further compounded as and when excavation for foundations, trenches, and reservoirs are taken into consideration.

Grading is important for both excavation and embankment formation for accurate final levelling work. Lasers and software could be incorporated in the grading operation in order to achieve quick and accurate results. To start with, a laser level is erected at the centre or to one side of area to be graded. A laser detector is attached to the blade of the grader and a diagrammatic readout is given to the operator via a liquid crystal display mounted in front of the operator’s seat. From this control system, the operator would know if the blade is too high or too low and adjust accordingly. This adjustment can be automatically controlled by software.

About the final grade level, certain issues deserve consideration. There should be different considerations for excavation and filling. Clay and clayey soil is compressible in nature. Underlying compressible layers remain in equilibrium under the weight of the overburden above. As and when the overburden is excavated and removed, there would be heaving due to decompression. This heaving process continues over a period of six months to a year. Final grading should be carried out after total heaving and after having been exposed to weather during the decompression period.

In case of embankment formation, the considerations that deserve attention are: (a) the soil materials with which filling is carried out would settle during and after execution (b) the underlying soil would settle due to the weight of filled-up overburden. Such consolidation could continue for a period of 12 months. The final grading should be carried out after consolidation. More details are given in Chapter 12, Section 12.1; and Chapter 13, Section 13.4.

Compaction is the process whereby particles of soil are packed together using construction equipment to reduce the air content. As for water content, it acts as lubricant to achieve greater compaction. Then as the air content decreases, the water tends to keep soil particles apart, thus hindering compaction. Full compaction can be achieved only when the moisture content is at the optimum level. The optimum moisture content can be found out by laboratory tests.

4.8

DREDGING

Dredging is the process of moving/excavating soil/rock (hard rock by blasting) under water. The object of dredging is generally to remove material from a particular location

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Earthwork 87

to place it at another location or let it be moved to a new location by natural forces. It is a highly specialised excavating method carried out for many purposes, as follows:

Moving/excavating soil or rock under water to deepen lake, reservoir, river, or sea for offshore construction, restoration of water reserve capacity, navigation, and submersible pipe/cable laying

0 Filling up to raise level under water or on land with spoil excavated under water (i) to ensure safety of under water foundations/structures and pipeline beds, (ii) to form beaches and dykes, (iii) to raise embankments for roads, (iv) to improve areas around ports

0 Replacing poor quality of soil with good quality soil under water for foundation construction or on land for land reclamation

0 Mobilizing soil materials for construction purposes like obtaining coarse/fine aggregates for concrete mixing or land reclamation

0 Improving environment by dredging and disposing off contaminated materials or sealing up contaminated materials with good materials and filling up wetlands with good dredged materials

A dredger is a vessel or a floating plant fitted with equipmenddevices to excavate or move soil or rock under water. The basic design of a dredger comprises equipment mounted on barge/pontoon for floating and moving on the water surface over the soil to be excavated in a bay, harbour, river, lake, or reservoir. On the pontoon or barge, the projecting frame at the forward end supports the excavating part of the dredger. The arrangement or layout may vary, but frame or boom would be an essential item. Also, there would be a number of spuds (pointed steel pipes that are vertically driven into the bottom and used as braces against digging thrust, wind, and waves-they are vertically retractable anchor posts) for maintaining position or pivoting. Power may be diesel or electric (within reach of power lines). Different types of dredgers deployed for dredging operation are: 0 0 0 0 0 0 0 0 0 0 0 0

Trailing suction hopper dredger Bed leveller

Water injection dredger Suction dredger

Cutting suction dredger Dustpan dredger Clamshell dredger Grab hopper dredger Hydraulic backhoe dredger Dipper dredger

Bucket dredger Special dredgers

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Trailing Suction Hopper Dredger

A trailing suction hopper dredger is a self-propelled (propelled at about 4 k d h ) vessel fitted with among others things trailing suction pipe, and hoppers built into the hull. It is normally rated according to its hopper capacity, which is usually in the range of 500 m3 to 10000 m3 (modern pumps can fill up 10000 m3 hopper in 1 hour). Dredged materials are loaded by one or more (up to 4) pumps into the hopper/s. The pumps may be installed onboard or may also be fitted in the trailing suction pipe as submersible pumps. The maximum depth to which dredging is possible depends on the vacuum head generated by the pumps-more depth means less output for a particular pump. Dredging depth could be as much as 80 m or so. Greater suction depth is possible in case of submersible pumps. The output of a dredger depends on:

Capacity of pump/s Depth of digging/dredging 0 Height of discharge 0 Line friction

Percentage of solids

Solid dredged materials settle down in the hoppers, top water is let out through the overflow pipes. In case of very fine dredged materials, overflowing water contains fine particles and pumping is discontinued as soon as water starts overflowing. In case of coarse or heavy materials, pumping is continued even as water starts overflowing. The bearing pressure of the draghead at the end of the trailing suction pipe on the bed is usually controlled by an adjustable pressure compensation system that acts between the draghead and the hoisting winch. The type of draghead to be deployed is selected on the basis of type of materials to be dredged. Since the suction draghead does not have a rotating cutter, teeth of various types and shapes are fitted on the bottom of the draghead to help breakup compacted materials in front of the suction inlet. When the hopper is loaded full, the suction pipe is restored onboard and the dredger is moved to the designated offshore/onshore unloading site. Offshore hopper bottom discharge using hydraulically operated dump doors is quick, but pump-discharge onshore/upland for land reclamation takes time. The cycle time of this kind of dredger:

0 Loading time

0 Sailing time to discharge point 0 Time of discharge

0 Time for sailing back to next loading area

Approximate limiting factors for this kind of dredger: 0 Minimum water depth for operation

0 Maximum water depth for operation 0 Minimum turning circle

0 Maximum wave height 0 Maximum particle size

4 m 45 m

75 m 5 m 300 mm

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Earthwork 89 Bridge and control room

Pump discharge

Draghead Fig. 4.32 Trailing suction hopper dredger

Bed

Leveller

A bed leveller is a very large tray-like cutter with cutting blade on one side. Cutting side is open for loading materials. Self-propelled tugboats tow bed levellers. Bed levellers are used for two applications: (i) as a dredger to move materials over short distances cost-efficiently - materials close to quays, jetties and in entrances or restricted areas can be moved without turning to conventional dredgers to do the job (ii) to improve efficiency of the other types of dredgers. Bed levellers can be deployed in such areas as are inaccessible to the other types of dredgers. The cycle time of a bed leveller:

0 Positioning of tug boat

0 Lowering of bed leveller with blades 0 Towing blades

0 Raising blades

0 Returning to starting point for repeating the process Approximate limiting factors for this kind of dredger: 0 Minimum water depth for operation 3 m

0 Maximum depth of operation 30 m

0 Maximum wave height l m

0 Maximum swell l m

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To hoist winch Bed leveller

Cutting blade

d'

Open bottom

Fig. 4.33 Bed leveller dredger

Water Injection Dredger

A water injection dredger is a self-propelled vessel. It is deployed to inject water on bed soil materials to fluidize the same for disposal purposes. This type of dredger can be successfully deployed in low strength fine-grained materials for the fluidized material to flow to lower levels. A fixed set of water jet nozzles are lowered to initiate water injection at a pre-determined pressure and flow rate to penetrate the bed materials. The vessel moves ahead slowly driving the fluidized materials before it. If the seabed slopes away from the working area, then fluidized materials would move over considerable distances speedily. The fluidized materials would move also at level or undulating ground. The cycle time would be similar to that of a bed leveller. Approximate limiting factors for this kind of dredger:

0 Minimum water depth for operation 3 m

0 Maximum depth of operation 15 m

0 Maximum wave height 0.5 m

0 Maximum swell 0.75 m

Soil type-very soft and soft cohesive soils and very loose and loose fine granular soils

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Earthwork 91

Fig. 4.34 Water injection dredger

Suction Dredger

A stationary suction dredger, unlike a trailing suction hopper dredger, is anchored at the place of dredging before commencement of dredging operation. The result of dredging is generally an inverted shaped cone in the bed. This type of dredger is deployed for obtaining granular materials for land reclamation or aggregates for concrete work. A stationary suction dredger may be fitted with hoppers for discharging dredged materials. Otherwise, dredged materials are loaded on barges or pumped through pipelines to delivery points over distances of 10 km or more. On larger vessels, over

10, 000 m3/h may be pumped for disposal.

At the point of dredging, suction pipe is lowered from the vessel to the bed for starting dredging operation. High-pressure water jets around suction pipes may be used

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for fluidization of soil to be dredged. The resulting spoil and water mixture is then drawn up the suction pipe for discharging as mentioned above. The cycle time of a stationary suction dredger:

0 Loading

0 Moving to discharge area 0 Discharging

Moving to dredging area to repeat the process. Approximate limiting factors for this kind of dredger:

0 Minimum water depth for operation 3 m

0 Maximum depth of operation 50 m or more

0 Maximum wave height 3 m

0 Soil type-permeable granular soils

Cantilever side discharge arrangement

Forward facing suction pipe arrangement for use only

in stationary dredging Fig. 4.35 Suction dredger

Cutting Suction Dredger

A cutting suction dredger is also anchored at the place of dredging before commencement of dredging operation. In general, it is a pontoon hull structure with no power unit for propulsion. However, larger version of this type of dredger may be self- propelled. Where the effect of normal suction pressures or jetting is insufficient to loosen the soil, a mechanical cutter is preferred. The equipment comprises a rotating cutter head fixed to the end of a stiff jib on which the drive motor and the suction pipe are also carried. The cutter operates by rotating the arm in an arc. The dredging operation is initiated by powerful cutting followed by suction and pumped discharge to either barges or along a floating pipeline to the disposal points. A wide range of materials including rock can be dredged by this type of cutter for pumping the dredged materials directly to the discharge points. A cutter suction dredger is generally rated by the diameter of discharge pipes (1 50 mm to 11 00 mm) or by the power driving the cutter

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Earthwork 93

head (1 5 kW to 4,500 kW). The soil or rock to be dredged is cut, dislodged or broken by a powerful cutter driven by hydraulic or electric power. The cycle time of a cutter suction dredger:

0 Lowering cutter to bed 0 Cutting one across face 0 Further cutting to finish depth 0 Cleaning cut

0 Raising cutter

0 Advancing back to lowering cutter to bed

Approximate limiting factors for this kind of dredger: 0 Minimum water depth for operation 0.75 m 0 Maximum water depth for dredging 35 m 0 Maximum cut width (single pass) 175 m

0 Maximum wave height 2 m

0 Maximum swell l m

0 Maximum particle size 500 mm

0 Maximum compressive strength (rocks) 50 MPa Service crane Control room

Stepping spud

Working spud

Main engine Ladder and pump room P F P

' Cutterhead service platform

'Swing winch wire Fig. 4.36 Cutting suction dredger

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Dustpan Dredger

A dustpan dredger, which is similar to some extent to the cutter suction dredger, is a suction dredger that has an exceptional form of mouth on its suction head. This suction mouthpiece may be 9 m or more wide. Because of its shape and form, it is called

dustpan. Its purpose is to agitate and pump for water jetting for loosening and fluidizing

materials to be dredged without any mechanical cutter. The dustpan mouthpiece has water jets spaced along its width to assist in dislodging or loosening of the soil being dredged. It is particularly useful for excavating in loose, soft, and free-flowing materials over relatively large areas. Its most effective purpose for maintaining navigation channel is to fluidize and pump shoal materials in large rivers and discharge the same through floating pipeline into currents for deposition elsewhere in the water- body. Dustpan dredgers are moved and positioned by winch power. Because of the deployment of winches, this type of dredger can move over the ground freely and quickly. There is no definite cycle time of the dustpan dredger. Approximate limiting factors for this kind of dredger:

0 Minimum water depth for operation 1.5 m 0 Maximum water depth for dredging 20 m 0 Maximum cut width (single pass) 10 m

0 Maximum discharge distance 500 m

Water in \

Water

Side elevation

Fig. 4.37 Dustpan dredger

Clamshell Dredger

uction

A clamshell dredger (also known as grab pontoon dredger) comprises a lattice jib crane to which a grab bucket (clamshell) is attached for dredging. This rope-operated jib crane is mounted at one end of a simple pontoon, which has no hopper. Dredged materials are uninterruptedly loaded into separate hopper barges. The pontoon is held in position by anchors and winches when spuds are not used. A clamshell dredger is rated

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Earthwork 95

by its clamshell bucket capacity (0.75 m3 to 20 m3). The weight of the bucket itself determines the digging effort. A clamshell dredger is very useful in soft to medium-hard dredging operation. Unlike pumped discharge, clamshell buckets load hoppers with more solids. A clamshell bucket is well suited for deep (beyond the capacity of other dredgers) dredging in confined areas such as wharfs and breakwaters. The cycle time of a clamshell dredger:

0 Lowering grab Closing grab 0 Raising grab

0 Swinging for discharging 0 Discharging

Swinging to place of dredging for repeating the process Approximate limiting factors for this kind of dredger:

0 Minimum water depth for operation l m

0 Maximum water depth for dredging 50 m or more

0 Wave height 2 m

0 Maximum compressive strength (rocks) 1 MPa

Fig. 4.38 Clamshell dredger

Grab Hopper Dredger

A grab hopper dredger is generally a ship integrally fitted with a hopper. One or more lattice jib cranes are mounted on the ship. The ship is anchored for dredging operation. The hopper capacity rarely exceeds 1500 m3. The hopper is loaded by one or more clamshell buckets. Hydraulic crane is possible alternative to the rope-operated crane. Because this type of dredger is self-propelled and fitted with hopper, its operation covers much larger areas. The cycle time of a grab hopper dredger:

0 Mooring

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0 Lowering grab Closing bucket 0 Raising grab

0 Swinging for discharging 0 Discharging

Or

0 Releasing anchors 0 Sailing to discharge point 0 Discharging

0 Sailing to working area for repeating the process Approximate limiting factors for this kind of dredger: 0 Minimum water depth for operation 3 m

0 Maximum water depth for dredging 45 m or more

0 Minimum turning circle 75 m

0 Maximum wave height 2 m

Grab bucket

I \

Mooring

winches Mooring winches

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Earthwork 97

Hydraulic Backhoe Dredger

A hydraulic backhoe dredger utilizes a hydraulic backhoe excavator for digging towards the dredger. This backhoe dredger is mounted on a fabricated pedestal at one end of a spud-rigged pontoon. Spud location of the pontoon is generally necessary to provide a positive reaction to the hydraulic digging action. This kind of dredger is rated according to the maximum size of the dredging bucket ranging from 1 m3 to 20 m3 depending upon the material and depth of dredging. Materials dredged include boulders, debris, stiff clay, or weak rocks. The cycle time of a hydraulic pontoon dredger:

0 Lowering bucket

0 Shoving backhoe bucket 0 Raising bucket

0 Swinging for discharging 0 Discharging

0 Swinging to place of dredging for repeating the process Approximate limiting factors for this kind of dredger:

0 Minimum water depth for operation 2 m 0 Maximum water depth for operation 24 m

0 Maximum width of cut 25 m

0 Minimum width of cut Bucket width

0 Maximum wave height 1.5 m

0 Maximum swell l m

0 Maximum compressive strength (rocks) 10 MPa

Bucket (various sizes)

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Dipper Dredger

A dipper (or dipper-bucket) dredger, cable-operated or hydraulic powered, operates by digging forwards and upwards. Both are mounted on spud-rigged pontoons to balance reaction to the digging action. The dipper (bucket) is mounted at the forward end of the dipper stick. Its operation resembles that of a face shovel. The boom and the dipper stick assembly turns through 1 SO" and the materials are discharged to barges moored on either side of the dredger or on shore if close enough. The main advantage of a dipper dredger is its powerful crowding action and, as such, it can dredge a wide range of compact and oversize materials like rocks, weak rocks, and stiff clay without the need of blasting. Even blasted materials could be handled, if required. The rope-operated type compared to hydraulic variety can dredge to greater depth. Dipper dredgers are suitable for deepening harbours and channels in restricted areas or in the areas where the bottom is too hard. These dredgers are also used for digging canals through swamps.

b-

Trailing tilting spud

r"l-

\

Optional drop chisel rock breaker

Dipper arm 11

h o k % - e Top-hinged door Bucket

Fig. 4.4 I Dipper dredger

The cycle time of a dipper dredger: Lowering dipper (bucket) 0 Digging forward

0 Raising

0 Swinging to discharge 0 Discharging

0 Swinging to earmarked dredging area for repeating the process Approximate limiting factors for this kind of dredger:

0 Minimum water depth for operation 3.5 m 0 Maximum water depth for operation 20 m

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Earthwork 99

0 Maximum width of cut 30 m

0 Minimum width of cut Bucket width

0 Maximum wave height 1.5 m

0 Maximum swell l m

0 Maximum compressive strength (rocks) 12 MPa

Bucket Dredger

A bucket dredger operates by using a ladder-mounted endless chain of buckets like a ladder-type trenching machine. The buckets are inverted. They scoop out bed materials (mud or clay, medium sand, gravel, and loose rock fragments) for lifting above water for discharging under gravity onto chutes for conveying to hopper-like containers on the barges moored alongside. The heavy bucket chain is supported by a steel ladder and driven by hydraulic or electric power via a tumbler at the top. The ladder is mounted centrally on a rectangular pontoon. Five or six winches are used for positioning the pontoon. A bucket dredger is generally used only to load barges in quiet waters. The ladder is susceptible to damage by possible shifting of the barge precipitated by currents, passing vessels, or inclement weather. The breakout force, which is dependent on the size and mass of the dredger, is exerted via the bucket cutting edge withlwithout teeth and may be substantial. The powerful head winch provides reaction to digging force. A bucket dredger has the advantage of a continuous dredging process, but it cannot be deployed in shallow water. It is preferred where working in restricted area is required, for example, along quay walls and in dock systems. The chain and buckets er

Fig. 4.42 Bucket dredger

are subjected to considerable wear, and consequently, maintenance costs turn out to be high. The production cycle time of any dredger depends on bed materials, depth of each cut, number of cuts per unit time, time of advancing to new faces for cutting (in case of forward cutting, it is referred to as face cutting), time of replacing barges, and anchoring time of the pontoon at new location.

Approximate limiting factors for this kind of dredger: 0 Minimum water depth for operation 5 m 0 Maximum water depth for dredging 35 m 0 Maximum cut width (single pass) 150 m

0 Maximum wave height 1.5 m

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0 Maximum particle size 1500 mm 0 Maximum compressive strength (rocks) 10 MPa

Special Dredgers

Special dredgers are additional devicedfacilities to enhance production of various types of dredgers. They may also be deployed for dredging. Special dredgers include:

A booster pump like a jet pump may be introduced into any dredger that works on the suction process like a stationary suction dredger so as to force water at considerable speed to produce the required suction. A jet pump comprises a high-pressure water system that forces a jet of water through a venturi near the extreme end of the suction pipe. The resulting additional kinetic energy helps in lifting watedsolid materials to the required height.

Another booster is the aid$ that works by forcing compressed air at the submerged

extremity of the pipe. Its simplicity lies in not having any submerged moving parts. The jet of compressed air reduces density of water-solid mixture resulting in induced upward flow. This kind of lightweight dredging assembly is suitable for divers.

The exhaust stroke of a positive displacement piston pump forces materials from a hopper into a discharge pipeline via a non-return mechanism for disposal of dredged materials.

For dredging in cohesive soils, pneumatic dredgers are used. Soil flows into one or

more chambers by hydrostatic pressure. When a chamber is full, compressed air forces soil to the surface via a discharge pipe fitted with non-return valve.

Small amphibious dredgers are deployed in shallow waters. They operate on the

principle of grab, backhoe, and cutter suction dredgers. Their movement on land is based on track-laying or hydraulic system.

A scraper dredger is another special dredger that works on self-propelling and self-

loading system using power of scraper action instead of usual pumping method. Hydraulic or mechanical (winch-cable) system powers scraping action. Dredging here is restricted to shallow water.

Selection and performance of a dredger depends, among other things, on: Access to the dredging area

Location of reclamation and discharge areas Depth of water

Length of the dredging area Width of the dredging area

Dredging profiles and accuracy depending on dredging technique and site conditions

Proximity to structures

Site conditions: (i) wind (ii) rain (iii) fog (iv) temperature (v) waves and swells- limiting heights for efficient operation (vi) currents (vii) anchorage (viii) disturbing local traffic

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Earthwork 101 To calculate the quantum of dredging done, the area to be dredged is outlined as accurately as possible, and existing bed levels are determined by soundings. Intervals between sounding lines may vary according to the general contours of the area and the nature of the materials to be dredged. In practice, the interval is usually between 7.5 m to 50 m. As precise work is not possible in dredging, it is normal to allow both horizontal and vertical tolerances in over-dredging. The results of dredging operation are checked by a second set of soundings made on completion of dredging work, using the same sounding pattern as for the original survey.

The dredged materials should be dumped in the sea in accordance with the regulations of the appropriate authority having jurisdiction over that part of the sea, or disposed of elsewhere with the written permission of the owners of the disposal area. No dredged materials should be allowed to leak into or be deposited in navigable channels during transportation. Most common methods of disposal of dredged materials are pumping and hauling by trucks. Other modes of disposal of dredged materials are transportation on barges, railways, and belt conveyors. Whereas transportation using barges is economical up to 480 km, truck movement is economical only up to 80 km. Belt conveyor has limited use. Belt specifications vary in width (76 cm to 178 cm), flight length (270 m to 780 m), and speed (11 km/hr to 144 k d h r ) . Again, while pumping is possible in case of spoil in slurry form, only dry materials can be hauled by the railways.

SUMMARY

Earthwork is so vital in implementation of construction projects that construction technology related to earthmoving is receiving a lot of attention worldwide. Earthmoving equipment and plants are continuously being innovated to execute voluminous earthwork in less and less time. Trenchless (no-dig) technology is now so advanced that it is now possible to dig through roadirailway embankments without disrupting existing facilities. This technology can be used in digging under river beds for layingireplacing utilityiprotection pipes. Groundwater problem does not pose problems anymore, as there are several solutions to this problem. More serious problems exist in construction of marine structures. Dredging is routine work in some countries just for survival.

REVIEW QUESTIONS

1. What is meant by cohesion? What are the problems related to cohesive soils?

2. Explain how consolidation is different from compaction.

Dredging should receive adequate attention in India because beds of dam reservoirs, rivers, and canals are being silted up. Regarding earth filling for embankment formation, dredged materials could be conveniently used. Grading during embankment formation especially for highways can be accurately carried out using laser beams. Even setting out has now been made easy by replacing theodolites and dumpy levels with ‘total station’, which is a fully integrated instrument that captures all the data necessary for a three dimensional position fix and displays it on digital readout systems recorded at the press of a button. All these throw light on how onetime arduous earthwork has now been made easily manageable utilizing innovative technology.

3. What does site development work comprise of! 4. How grid lines are established at construction

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5 . Why mechanized earthwork in both excavation and filling is taken into consideration these days in project execution work ?

6. Name the operations involved in earthwork in excavation.

7. There are two ways of groundwater control. Name them.

8. What is the difference between temporary and permanent sheet piling?

9. What is a cofferdam? Name the materials used for building cofferdams.

10. Describe the electro-osmosis process of temporary groundwater exclusion.

11. Define a caisson. Describe briefly the four types of caissons.

12. Name the methods of trenchless technology. Describe how pipe jacking is carried out. 13. Describe how lasers and software could be

incorporated in the grading operation in order to achieve quick and accurate results.

14. What is the objective of dredging? 15. Describe the basic design of a dredger.

References

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