Complete Cofferdam (1)

Dept. of Building, Civil & Environmental Engineering

CONSTRUCTION OF A COFFERDAM

BLDG 6831 – CONSTRUCTION CONSTRUCTION PROCESS Professor – Dr. Dr. Tarek Zayed SUBMITTED BY:

Last Name Musugu Mekkha Dasari Ramganesh

First Name Aneesh Reddy Divya Gilbert Sujay

ID # 6598862 6488463 6373844 6409369

CONSTRUCTION OF A COFFER DAM

Table of Contents 1.

INTRODUCTION ................. ................. ................. .................. ................. .................. .................. ........... 5

2.

TYPES OF COFFERDAMS .................. ................. .................. ................. .................. ................. ............... 6 2.1 Braced Cofferdams .............................................................................................................................. 6 2.2 Earth-Type Cofferdams ....................................................................................................................... 6 2.3 Timber Crib Cofferdam ....................................................................................................................... 6 2.4 Double-Walled Cofferdam .................................................................................................................. 7 2.5 Cellular Cofferdam .............................................................................................................................. 7

3.

COFFERDAM DESIGN CONSIDERATIONS ................ .................. ................. .................. .................. ........ 8

4.

FORCES ACTING ON COFFERDAM .................. ................. ................. .................. .................. ................. 9

5.

EQUIPMENTS AND MATERIAL REQUIRED FOR INSTALLATION ................. ................. .................. ....... 13 5.1 Pile driving hammer .......................................................................................................................... 13 5.2 Cranes with clamshell buckets .......................................................................................................... 16 5.3 Concrete pump truck ........................................................................................................................ 18 5.4 Pumps for dewatering ....................................................................................................................... 21 5.5 Barge: ................................................................................................................................................ 22

6.

COFFERDAM COMPONENTS ................. ................. .................. ................. .................. ................. ....... 24

7.

GENERAL CONSTRUCTION METHOD................ .................. ................. .................. .................. ............ 30

8.

REMOVAL OF COFFERDAM ................ ................. .................. ................. .................. ................. .......... 33

9.

APPLICATION OF COFFER DAM ................ ................. .................. ................. .................. ................. .... 34 9.1 SAFETY REQUIREMENTS .................................................................................................................... 35 9.2 ADVANTAGES OF COFFERDAMS ....................................................................................................... 35 9.3 DISADVANTAGES OF COFFERDAMS .................................................................................................. 36

10.

MODES OF FAILURE ................. .................. ................. ................. .................. ................. ................ 36

10.1 FABRICATED TEES AND WYES ......................................................................................................... 36 10.2 SHEETS AND INTERLOCKS ............................................................................................................... 36 10.3 ENVIRONMENTAL CONDITIONS ...................................................................................................... 36 10.4 STABILITY ................. ................. .................. ................. ................. .................. .................. ............... 37

11.

CASE STUDY – ............................................................................................................................. 39

COFFERDAM CONSTRUCTION AND DEWATERING TAUNSA BARRAGE REHABILITATION PROJECT .................................................................................................................................................... 39 1


Table of Contents 1.

INTRODUCTION ................. ................. ................. .................. ................. .................. .................. ........... 5

2.

TYPES OF COFFERDAMS .................. ................. .................. ................. .................. ................. ............... 6 2.1 Braced Cofferdams .............................................................................................................................. 6 2.2 Earth-Type Cofferdams ....................................................................................................................... 6 2.3 Timber Crib Cofferdam ....................................................................................................................... 6 2.4 Double-Walled Cofferdam .................................................................................................................. 7 2.5 Cellular Cofferdam .............................................................................................................................. 7

3.

COFFERDAM DESIGN CONSIDERATIONS ................ .................. ................. .................. .................. ........ 8

4.

FORCES ACTING ON COFFERDAM .................. ................. ................. .................. .................. ................. 9

5.

EQUIPMENTS AND MATERIAL REQUIRED FOR INSTALLATION ................. ................. .................. ....... 13 5.1 Pile driving hammer .......................................................................................................................... 13 5.2 Cranes with clamshell buckets .......................................................................................................... 16 5.3 Concrete pump truck ........................................................................................................................ 18 5.4 Pumps for dewatering ....................................................................................................................... 21 5.5 Barge: ................................................................................................................................................ 22

6.

COFFERDAM COMPONENTS ................. ................. .................. ................. .................. ................. ....... 24

7.

GENERAL CONSTRUCTION METHOD................ .................. ................. .................. .................. ............ 30

8.

REMOVAL OF COFFERDAM ................ ................. .................. ................. .................. ................. .......... 33

9.

APPLICATION OF COFFER DAM ................ ................. .................. ................. .................. ................. .... 34 9.1 SAFETY REQUIREMENTS .................................................................................................................... 35 9.2 ADVANTAGES OF COFFERDAMS ....................................................................................................... 35 9.3 DISADVANTAGES OF COFFERDAMS .................................................................................................. 36

10.

MODES OF FAILURE ................. .................. ................. ................. .................. ................. ................ 36

10.1 FABRICATED TEES AND WYES ......................................................................................................... 36 10.2 SHEETS AND INTERLOCKS ............................................................................................................... 36 10.3 ENVIRONMENTAL CONDITIONS ...................................................................................................... 36 10.4 STABILITY ................. ................. .................. ................. ................. .................. .................. ............... 37

11.

CASE STUDY – ............................................................................................................................. 39

COFFERDAM CONSTRUCTION AND DEWATERING TAUNSA BARRAGE REHABILITATION PROJECT .................................................................................................................................................... 39 1


11.1 INTRODUCTION ............................................................................................................................... 39 11.2 DESCRIPTION OF PROJECT .............................................................................................................. 40 11.3 CONSTRUCTION PROCESS ............................................................................................................... 41 11.4 DEWATERING .................................................................................................................................. 43 11.5

EQUIPMENTS USED AND THEIR PRODUCTIVITY ANALYSIS ................................ ............... ................. .................. ....... 45

11.6 PRODUCTIVITY ANALYSIS ...................................................................................................... 47 11.6.1 PHRASE 1- STONE FILL LAYER AND EARTHFILL EMBANKMENT CONSTRUCTION .................... 47 11.6.2 PHRASE 2: SHEET PILE INSTALLATION ...................................................................................... 53 11.6.3 PHRASE 3: DEWATERING USING PUMPS: ................................................................................ 53 11.6.4 PHRASE 4: EXCAVATION USING BACKHOE: .............................................................................. 56 12.

CONCLUSION ................. ................. ................. .................. ................. .................. .................. ......... 58

13.

REFERENCES REFER ENCES .................. ........................... .................. .................. ................... ................... .................. .................. .................. .................. .................. .................. .................. ............. .... 60

Table of Figures Figure 1 Types of cofferdams. For use on land: ( a) cross-braced sheet piles; ( b) cast-in-place concrete cylinder; ( c) anchored sheet piles; ( d ) braced vertical piles with horizontal sheeting. For use in water: ( e) cross-braced sheet piles; ( f ) earth dam; ( g ) tied sheet piles; ( h) anchored sheet piles with earth berm; ( i) steel sheet-pile cellular cofferdam; ( j) rock-filled crib. ................................................................................ 7 Figure 2 ....................................................................................................................................................... 10 Figure 3 ....................................................................................................................................................... 10 Figure 4 ....................................................................................................................................................... 11 Figure 5 ....................................................................................................................................................... 11 Figure 6: Pile hammers ............................................................................................................................... 15 Figure 7: Crane with pile hammer .............................................................................................................. 15 Figure 8: Clamshell ..................................................................................................................................... 16 Figure 9: Trailer-mounted boom concrete pump ........................................................................................ 19 Figure 10 ..................................................................................................................................................... 20 Figure 11: Dewatering pumps ..................................................................................................................... 21 Figure 12 Figure 13: Steel sheet piling ...................................................................................................... 21 Figure 14: H-piles and/or wide flange beams ............................................................................................. 22 Figure 15: Traditional Sheet Pile Shapes .................................................................................................... 23 Figure 16: Typical Types of Interlocks ....................................................................................................... 24 Figure 17 Sheet pile .................................................................................................................................... 33 Figure 18: Taunsa Barrage (downstream face) ........................................................................................... 39 Figure 19: Cofferdam completed in parallel at the front and back of the enclosure ................. ................. . 40 Figure 20: Dump truck unloading stone at the nose of the cofferdam ........................................................ 42 2


Figure 21: Sheet piling in the cofferdams with simultaneous dewatering .................................................. 44 Figure 22: Shows different phrases ............................................................................................................. 46 Figure 23 ..................................................................................................................................................... 49 Figure 24 ..................................................................................................................................................... 56

Table 1 Quantities of material used for construction of cofferdams ........................................................... 43 Table 2: Type of pumps and their capacities which were made available at the site are ............................ 54

3


ABSTRACT Coffer dams are temporary enclosures to keep out water and soil so as to permit dewatering and construction of the permanent facility in the dry environment. The word "cofferdam" comes from "coffer" meaning box, in other words a dam in the shape of a box. Generally a cofferdam involves the interaction of structure, soil and water. In the 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. In our report we will be focusing on the construction process, equipment used for construction and the various types of coffer dams. And also we will be discussing a case study which describes the construction of a cofferdam that was built to facilitate the construction of sub weir and rehabilitation of Taunsa Barrage, which is situated on the huge river of the Indus valley known as the Indus River, in the province of Punjab, India. In this case study, the equipments used in the cofferdam construction are dump trucks, loaders, scrapers, vibratory pile driver, backhoe and dewatering pumps. The means and methods utilized to construct the coffer dam and their productivity analysis are clearly shown. To facilitate productivity calculations the project was divided into four phrases. Taunsa barrage was constructed in the year 1958 and currently supplying water for four main canals - two on the right of the bank and two on the left of the bank.

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1. INTRODUCTION

A cofferdam is a temporary construction method used in order to do construction in wet excavations. It is installed in the work area and water is pumped out to expose the bed of the body of water so that workers can construct structural supports, perform repairs and any other types of work using construction equipment. A coffer dam is also called as caisson in some parts of world.

Working inside a coffer dam can be dangerous if it is not installed properly or not safely pressurized. Various materials are used for its construction and its design must be compatible with weather conditions, waves, currents, construction equipment, construction methods, internal permanent structures and ground conditions. There are various types of cofferdams such as braced, earth type, timber crib, double walled sheet pile and cellular which are discussed below.

Generally, major loads imposed on cofferdams are hydrostatic forces of water and dynamic forces due to current and waves and heavy equipment is used for its construction such as pile drivers, cranes with clamshell buckets, concrete pumps trucks as well as pumps for dewatering are used in the construction process. The effective management of equipment on site as well as workers is an important step in cost control and maintaining efficient productivity.

“A cofferdam i s a temporary stru ctur e design ed to keep water and/or soil out of the excavation in which a bridge pier or other structure is built.”

- Standard Handbook of Heavy Construction

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2. TYPES OF COFFERDAMS The construction process for each type is different based on whether it is used on land or in water, as illustrated in figure 1. In general there are five types of coffer dam and they are as follow (Nemati, 2007): 

Braced



Earth-Type



Timber Crib



Double-Walled Sheet Pile



Cellular

2.1 Braced Cofferdams Braced cofferdam is formed from a single wall of sheet piling. It is constructed by driving sheet piles into the ground to form a box around the excavation site and then this “box” is braced on

the inside of it. Interior is dewatering using pumps. They are primarily used for bridge piers in shallow water around 30-35ft depth.

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

2.3 Timber Crib Cofferdam It is one of the kinds of cellular-type cofferdam. It is first constructed on land and then floated into required place. The lower portion of each cell matched with contour of river bed. It uses rock ballast and soil to decrease seepage and sink into place. It is also known as “Gravity Dam”. In general it consists of 12’ x 12’ cells. It is used in rapid currents or on Rocky River beds. It

should be properly designed to resist two lateral forces i.e tipping/overturning and sliding.

6


Figure 1 Types of cofferdams. For use on land: ( a) cross-braced sheet piles; (b) cast-in-place concrete cylinder; (c) anchored sheet piles; (d ) braced vertical piles with horizontal sheeting. For use in water: (e) cross-braced sheet piles; ( f ) earth dam; ( g ) tied sheet piles; ( h) anchored sheet piles with earth berm; (i) steel sheet-pile cellular cofferdam; ( j) rock-filled crib.

2.4 Double-Walled Cofferdam In this type of cofferdam, two-parallel rows of steel sheet piles are driven into the ground and tied together with anchors and wales then filled with soil. There are three principle types: 

Box: Consists of straight flush walls



Semicircular cells connected by diaphragms



Circular cells connected with tie-rods or diaphragms

2.5 Cellular Cofferdam There are two main types of cellular cofferdam they are circular and segmental. It can be used on a temporary or permanent basic. In this type of cofferdam force are resisted by the mass of the cofferdam. (Nemati, 2007)

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3. COFFERDAM DESIGN CONSIDERATIONS The following are some of the design considerations which should be checked before the construction and during the design of cofferdam. 

Scouring or undermining by rapidly flowing water



Stability against overturning or tilting



Upward forces on outside edge due to tilting



Stability against vertical shear



Effects of forces resulting from:



Ice, Wave, Water, Active Earth and Passive Earth Pressures

An important consideration in the design of cofferdams is the hydraulic analysis of seepage conditions and erosion of the bottom when in streams or rivers. Significant deformations of elements may occur at different stages of construction because of the typical construction of coffer dam under adverse conditions in a marine environment, thus it is difficult to maintain close tolerances. Provisions must be made for deviations in dimensions so that the finished structure may be constructed according to plan. Deconstruction of the cofferdam must be planned and executed with the same degree of care as its installation, on a stage-by-stage basis. The effect on permanent structure due to the removal of coffer dam must be considered. Due to this reason, sheet piles extending below the permanent structure are often cut off and left in place, because their removal may affect the foundation soils adjacent to the structure. (Nemati, 2007) Where the cofferdam structure can be built on a layer of impervious soil, the area within the cofferdam can be completely sealed off. Where the soils are pervious, the flow of water into the cofferdam cannot be completely stopped economically, and the water must be pumped out periodically and sometimes continuously.

8


A dewatered area can be completely surrounded by a cofferdam structure or by a combination of natural earth slopes and cofferdam structure. The type of construction is dependent upon the depth, soil conditions, fluctuations in the water level, availability of materials, working conditions desired inside the cofferdam, and whether the structure is located on land or in water. (Washington, 2013)

4. FORCES ACTING ON COFFERDAM A cofferdam involves the interaction of the structure, soil, and water. The loads imposed include the following: 

Hydrostatic pressure



Forces due to soil loads



Current forces on structure



Wave forces



Ice forces



Seismic loads



Accidental loads



Mooring forces



Scour

The loads imposed on the cofferdam structure by construction equipment and operations must also be considered during installation of the cofferdam a well as during construction of the structure itself. (Nemati, 2007) Hydrostatic pressure

Two factors must be considered they are the maximum probable height outside the cofferdam during construction and the water height inside the cofferdam during various stages of construction. The hydrostatic pressure for partially dewatered cofferdam is shown in figure 3.

9


2

F1 = wh1 /2

2

F2 = wh2 /2

Where , w = unit weight of water h1 = outside water height F1 = outside hydrostatic force F2 = inside hydrostatic force

Figure 2

If h1= 2h2 then F1 =4F2 and F3 =3/4 F1

Figure 3

Forces due to Soil Loads

The soils impose forces acts locally on the wall of the cofferdam and globally upon the structure as a whole. Local forces are main component of the lateral force on sheet-pile walls, causing bending in the sheets, bending in the wales, and axial compression in the struts. These forces are added to the hydrostatic forces. Active pressure and passive pressure due to soil load is shown in the figure 4.

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Figure 4 Current Forces on Structure

In a cofferdam, the current force consist not only the force acting on the normal projection of the cofferdam but also on the drag force acting along the sides. With flat sheet piles, the latter may be relatively small, whereas with z-piles it may be substantial, since the current will be forming eddies behind each indentation of profile, as shown in figure 5.

Figure 5 Wave forces

Waves acting on a cofferdam are usually due to local winds acting over a restricted fetch and hence are of short wavelength and limited to height. Waves can also be produced by passing boats and ships, especially in a restricted waterway.

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Ice forces

These are of two types, that is the force exerted by the expansion of a closed-in solidly frozenover area of water surface which is called as static ice force and the forces exerted by the moving ice on breakup which is called as dynamic ice force. (Nemati, 2007) Seismic Loads

In most of the projects, they are not considered in design of temporary structures. But for very large, important, and deep cofferdams in highly seismically active areas, seismic evaluation should be performed. Accidental loads

Accidental loads are the loads usually caused by construction equipment working alongside the cofferdam and impacting on it under the action of waves. Mooring forces

They are derived from two separate actions. The first is the impact of the barge and tugboats as they moor to the cofferdam or the waves are produced as they move the barges while moored. The other force is the wind pressure on the total sail area of the barge. Gale force wind is a common occurrence along most coasts and on large lakes. The combination of high wind and waves will cause major damage to the cofferdam and equipment if no preparation is made to accommodate those events. (Washington, 2013) Scour

Scour of the river bottom or seafloor along the cofferdam may take place due to river currents, tidal currents, or wave-induced currents. Some of the most serious and disastrous cases have occurred when these currents have acted concurrently.A very practical method of preventing scour is to deposit a blanket of crushed rock or heavy gravel around the cofferdam, either before or immediately after the cofferdam sheet piles are set. A more sophisticated method is to lay a mattress of filter fabric, covering it with rock to hold it in place. (Nemati, 2007)

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5. EQUIPMENTS AND MATERIAL REQUIRED FOR INSTALLATION

Equipment’s: 

Pile driving hammer -

Vibratory or Impact



Crane of sufficient size- clamshells and draglines



Concrete pumps trucks



Dewatering pumps



Barges may be required



Dozer, loader, backhoe, trucks be may required

Materials: 

Steel sheet piles are typically used



H-piles and/or wide-flange beams for wales and stringers

5.1 Pile driving hammer A pile driver is a mechanical device used to drive piles into soil to provide foundation support for buildings or other structures. The following are different types of pile driving hammers:    

Diesel hammer. Hydraulic hammer. Hydraulic press-in. Vibratory pile driver.

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In the above 4 types of pile driving hammer, vibratory pile driver is most commonly used for construction of cofferdam. Vibratory pile hammers contain a system of counter-rotating eccentric weights which are powered by hydraulic motors and designed in such a way those horizontal vibrations cancel out, while vertical vibrations are transmitted into the pile. The pile driving machine is lifted and positioned over the pile by means of an excavator or crane, and is fastened to the pile by a clamp as shown in figure 6iaction breaks friction resistance between pile surface and the soil, thus the force of gravity acting on hammer causes the pile to sink. Vibrating hammer typically weighs from 2 to 20 tons and they are used to drive bearing as well as sheet piles.

Productivity Factors: 

The type of piling being driven



The subsurface soil conditions



Power requirement



Type and size of the hammer



Length of the crane boom



The length of leads required (if they are used)



Type and size of the crane used.

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Figure 6: Pile hammers

Figure 7: Crane with pile hammer

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Productivity:

Operational efficiency for a pile driver typically range from 30 to 40 min per hour because of time required to move the crane and equipment to the location of each pile and set up everything for driving. Cycle time depends on cross-section of pile, its length and subsurface soil conditions. Productivity = Estimating driving rate * operating factor The time required to drive piles (A) = (no of piles *length of the pile) / (productivity*working hours per day) Total time = A+ setup time + demolition time (Schaufelberger, 1998)

5.2 Cranes with clamshell buckets A crane-shovel which is equipped with crane boom, clamshell bucket and accessories, fairlead assembly, and necessary cables is known as clamshell. Clamshell is capable of operating at, above, and below ground level and it can handle material ranging from soft to medium stiff soils. It is used to lift material vertically during construction of coffer dam. The clamshell is capable of excavating to great depths but lacks the positive digging action and precise lateral control as that of backhoe and shovel.

Figure 8: Clamshell 16


Productivity:

Clamshell has three capacities heaped capacity, plate line capacity/struck capacity and waterlevel capacity. Productivity factors: 

Class of materials.



Height of lift.



Angle of swing.



Bucket size.



Boom length.



Job and management conditions.



Disposal methods.



Size of hauling units.



Operator skills.



Equipment conditions.

There are two methods to determine clamshell productivity the y are: General output model:

Hourly output (cy/hr or m3/hr) = P = (3600 *Q * f * k* f1 * f2 * t)/CT Where k = bucket fill factor. P = productivity in cy/hr or m3/hr. Q = bucket capacity in loose cy or m3. f = earth volume change conversion factor. f1 = swing-depth factor (Table 18.4).

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f2 = job and management conditions. t = operating time factor. CT = cycle time in seconds (Table 18.2).

Productivity = Volume per cycle * Cycles per hour Productivity (cy/hr or m3/hr) = (3600 * Bucket capacity * Bucket fill factor * Job effiecency)/CT Where, CT= cycle time in sec, Bucket capacity in cy or m3

5.3 Concrete pump truck The concrete pump is a machine which is used for transferring liquid concrete by pumping. There are two types of concrete pumps. The first type of concrete pump is attached to a truck. It is known as a trailer-mounted boom concrete pump because it uses a remote-controlled articulating robotic arm to place concrete with pinpoint accuracy. Boom pumps are used on most of the larger construction projects as they are capable of pumping at very high volumes and because of the labor saving nature of the placing boom. They are a revolutionary alternative to truck-mounted concrete pumps. They are used in the construction process of a cofferdam. The second main type of concrete pump is either mounted on a truck and known as a truckmounted concrete pump and it is commonly referred to as a line pump or trailer-mounted concrete pump. They are commonly used for placing concrete applications such as swimming pools, sidewalks, and single family home concrete slabs and most ground slabs. Concrete pump process depends on the receipt of mixed concrete from the concrete batch plant to the concrete pump to pump the concrete into the formwork in the site. This operation has

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several factors affecting the productivity that must be taken into consideration from the hauling unit (Truck mixer) and the concrete pump, such as: 

Workability of the concrete.



Capacity of the pump.



Speed of the pumping



Crew skills.



Type of formwork.



Layout of the site.



Height at which concrete has to be pumped.

Figure 9: Trailer-mounted boom concrete pump

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Figure 10 To calculate Concrete pump production we have to image the process of pumping concrete to the final place could be column, slab, wall formwork or any shape that need to fill concrete I assume the process it will be as; 

RT 1 = Required time to pump maneuver for the first position.



RT2 = Required time to change the position of the pump * No. of Positions Change.

Where; No. of Positions Change depends on the site size and the boom length and the operation

skills are determining the no. of position change. 

RT3 = Required time to Truck mixer maneuver for the first position



RT4 = Required time to pump concrete from the truck

Where, CF4 =

Truck mixer volume

/ (Speed of Pump * Type of formwork factor)

RT5 = Required time to switch the mixing trucks * No. of Mixer Trucks Where; No. of Mixer Trucks =

Total Concrete required

Concrete pump Producti on (CCY / hr .) =

/ Truck mixer volume

Required Volume of Concrete

Where; Total Time = RT 1 + RT 2 + RT 3 + RT 4 + RT 5 20

/ Total Time


5.4 Pumps for dewatering Dewatering pumps are used to pump water from the interior of the cofferdam and it should be done in such a way as to preclude the possibility of water moving through uncured masonary or concrete. Pumping is done by placing sump outside the horizontal limits and below the elevation of the work being placed or as directed by the engineer. Pumping to dewater a cofferdam should not start untill any underwater concrete has sufficiently set to withstand the hydrostatic pressure generated by the pump.

Figure 11: Dewatering pumps

Figure 12

Figure 13: Steel sheet piling

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Figure 14: H-piles and/or wide flange beams

5.5 Barge: A barge is a flat-bottomed boat, built mainly for river and canal transport of heavy goods. Some barges are not self-propelled and need to be towed or pushed by towboats. If the cofferdam is constructed far away from land then the goods and equipment are required to be transported to the site of cofferdam construction. In barge is used to transport the required heavy goods and equipment to the site. As shown in figure 13. Properties of Steel Sheet Piling:

The following are the properties of steel sheet piles: 

Moderately watertight



High shear and bending strength



High interlock strength



Easy to install/remove, Reusable



Can be cantilevered but typically require additional structural member i.e. wales and cross bracing.

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Figure 15: Traditional Sheet Pile Shapes

Steel Sheet Pile Interlocks

There is no industry Standard for steel sheet pile interlocks. Interlocks should fulfill the following requirements: 

Provide relative water or earth-tight connections



Permit reasonable free sliding to connect sheets during installation



Provide minimum guaranteed pull strength



Allow minimum swing between locks in order to form a circle

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Figure 16: Typical Types of Interlocks

6. COFFERDAM COMPONENTS The following are 4 types of components in braced type cofferdam: 

Sheet piling



Bracing frame



Concrete seal



Bearing piles

The typical cofferdam, such as a bridge pier, consists of sheet piles set around a bracing frame and driven into the soil to sufficient depth to develop vertical and lateral support and to cut off the flow of soil or water. (Nemati, 2007)

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The structure inside may be founded directly on rock or firm soil or may require pile foundations. In the latter case, these generally extend well below the cofferdam. To dewater the cofferdam bottom must be stable and able to resist hydrostatic uplift.

Placement of an

underwater concrete seal course is the fastest and most common method to withstand uplift.

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SHEET PILE COFFERDAM CONSTRUCTION SEQUENCE:

For a typical cofferdam, such as for a bridge pier, the construction procedure follows the listed pattern. Step 1 -

Pre-dredge to remove soil or soft sediments and level the area of the cofferdam Step 2 -

Drive temporary support piles and temporarily erect bracing frame on the support piles.

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Step 3 -

Drive sheet piles to grade and ties are provided for sheet piles at the top as necessary.

Step 4 -

Excavate slightly below grade, while leaving the cofferdam full of water and drive bearing piles. Place rockfill as a leveling and support course.

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Step 5:

Figure: Tremie

Figure: concrete pouring using

Figure: Tremie concrete

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Step 6:

Check blocking between bracing and sheets and dewater

Step 7:

Remove sheet piles and bracing, as well as backfilling and construct new structure

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7. GENERAL CONSTRUCTION METHOD As we know, cofferdam is a kind of water tight construction which is designed to facilitate construction projects where the particular area is normally submerged. For the construction of a coffer cam we have various kinds of materials and equipment’s which enable us to perform the

work at a faster rate. (htt) Cofferdams are rarely installed as easily as they are planned and designed. You must expect and anticipate problems that will require redesign and innovative solutions. However, it is rewarding to solve the demanding construction and knowing it will help successfully complete the project. The construction of a coffer dam completely relies on following the exact process and sequence involved. And also the builder and designer should possess proper understanding of the project. In general, the cofferdams are limited to 60 foot long sheet piles because if these sheets are made longer than 60 foot then it would cause difficulties in transporting, handling, threading and manufacturing. The first step in construction is to place the wale system after the access is worked out. The wales are placed over a barge and floated to the position. Along with this to grip the wale system in place, guide piles and support frames are installed. When the barge floods partially and towed from under the suspended whale frame, then using cranes the wale frame is lowered to elevation. After that the wales are used as a guide to thread and drive the sheet piling. Normally at least two layers of wales are placed where the top and bottom layers will be perform as a stabilizing template to control the sheet piles. Generally in marine environment we will be observing some waves, current, and wind. So to guide the sheet piles a supporting template is used as it is almost impossible to maintain the vertical and horizontal alignment which is necessary to close the cofferdam and prevent the interlocks from splitting open. But if the sheet piles are not kept plumb, then the interlocks will split apart in tension or the closing pair can bind up due to compressive friction and refuse to be driven. During cofferdam installation a driving template is used. Usually the wale system is used as a driving template. The template wales should be marked with the proper location of every sheet pile pair interlock that touches the wale. Special care should be taken to ensure that the first pair

30


is set plumb in the proper location because it will be acting as a guide for the rest of the sheet piles. In final closure, it should never be made at a corner as this corner works in both directions. If either sheet wall line is out of plumb, the sheet interlock will probably split open. The other reason to be careful in initial alignment is that this will largely define the direction the piles will take as they continue to penetrate the ground. If the interlock is started off tight and out of line, it will likely split apart as it is being driven. This will damage the pile and may require very expensive and time consuming repair procedures. When the sheet piles are fully in place and driven to the top of the upper template, the template wales can be lowered, if needed. The pairs of sheet piles should be advanced in about five foot increments. With the sheets carefully driven and the wale in position, often the sheets are welded or bolted to the top wale to provide cofferdam stability during excavation operations. A crane and a clam bucket usually perform the excavation, although in some instances a backhoe can be effective. Excavation should be carried out along the sheet piles first, keeping a low hump in the middle. This allows the clam bucket to rest against the sheets and stays upright so it can stuff the bucket. If a depression is created in the middle of the excavation, the bucket will roll on its side and it will not be able to excavate the wedge of soil adjacent to the sheet piles. When the excavation is nearly complete, a steel beam spud is placed between the wales and the sheet pile alcoves. After the above process, tremie concreting is carried out so as to minimize the flowing concrete contact with the water. The method is to induce the fresh concrete under the previously placed concrete and pillow it up and out. The tremie placement is a continuous operation until completed, going 24 hours a day without interruption. Tremie pours usually involve large volumes of concrete, often several thousand cubic yards of concrete. When the concrete has cured enough to gain enough strength to withstand the dewatering forces (about two or three days), dewatering can begin. (Washington, 2013)

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DEWATERING:

In dewatering process, the pumping out of water from the interior of a cofferdam is carried out in such a manner that it prevents the possibility of water moving through uncured concrete. A proper sump is placed outside below the elevation of the work which is placed and the pumped water should be properly discharged according to the regulations. The most important aspect during dewatering is that the underwater concrete should set so that it can withstand hydrostatic pressure created by pumping. After the cofferdam is dewatered, the clean \up process can begin. The surface will be rough and undulating. There will be layers of mud, debris, and dead fish that must be cleaned up. Once the cleanup is done, the top of the tremie concrete will have about six inches of laitance. The laitance is a weak layer of nearly pure cement that has been washed to the surface of the concrete by the dynamics of the concrete tremie placement. While the cleanup and laitance removal is progressing, the cofferdam will continue to leak and require substantial pumping. The leakage water will be contaminated by the mud and debris in the cofferdam until all remedial work and cleanup is completed. All water removed from the cofferdam during this stage probably will have to be processed before returning the water to the river, lake, or bay. At this point, a safety precaution is inserted. No gas-powered machinery should ever be allowed inside a cofferdam. The danger for explosion and carbon monoxide poisoning is too great. Even the use of diesel powered equipment in the cofferdam should be kept to an absolute minimum. Whenever it is possible, engines outside the cofferdam should power all machinery. These actions will both reduce congestion in the cofferdam and provide for safer working conditions. (htt) (Washington, 2013)

32


8. REMOVAL OF COFFERDAM The contractor must remove all the parts of the cofferdam after the completion of required work. This shall be done in such a way as not to disturb or damage the permanent structure. Sheet piling used in the construction of cofferdam may be left in place with the approval of the Engineer, provided the pilling is cut off at elevations approved in advance by the Engineer and the cut off portions are removed from the site. (Washington, 2013) (htt)

Figure 17 Sheet pile

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9. APPLICATION OF COFFER DAM A nautical application of the term cofferdam is a watertight structure used for making repairs below the waterline of a vessel. The name also is applied to void tanks which protect the buoyancy of a vessel. Cofferdam are constructed to permit dewatering an area and facilitate the construction of foundations, bridge piers, dams, dry docks, and like structures in the open air. The following are some of its major applications: 

Hydroelectric Dam Construction – Cofferdams are used to divert water away from the shoreline of a river to allow for the foundations of a dam to be constructed. In this application, generally one half of the river width is enclosed by the cofferdam at a time to maintain overall flow.



Bridge Construction – Cofferdams are used to divert water away from bridge foundation positions, either on the shore or within the waterway.



Ship repair – Sometimes cofferdams are used to generate a “dry dock” condition for a ship in order for repairs to proceed. This generally occurs when the ship cannot be moved to an actual dry dock, and it can also be more cost effective in some cases.



Oil Rig and Dam Construction – This is the primary reason why coffer dams exist. They are quick to build and use welded steel and other metals; they provide a temporary and dry platform to work freely.



Sunken Vessel Recovery: Cofferdams can be used to expose a sunken vessel in shallow waters to allow for recovery and repair if appropriate.



Ship Recovery A very rarer use of a Coffer dams is to help in recovery missions for ships that have sunk in shallow water. They can be built quickly and aid removal in certain circumstances. In

34


the past coffer dams have helped recover ships such as the USS Maine, a ship which sunk in 1898 played an important part in Spanish-American history. By using a coffer dam to pull up this ship from the sea bed it helped give researchers an insight into the history of this boat.

9.1 SAFETY REQUIREMENTS In cofferdam construction, safety is a paramount concern, since workers will be exposed to the hazard of flooding and collapse. (Nemati, 2007)





good design proper construction



verification that the structure is being constructed as planned



monitoring the behavior of the cofferdam and surrounding area



provision of adequate access



light and ventilation, and



Attention to safe practices on the part of all workers and supervisors.

9.2 ADVANTAGES OF COFFERDAMS 

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

35


9.3 DISADVANTAGES OF COFFERDAMS 

Special equipment required



Relatively expensive



Typically very time consuming & tedious



If rushed, sheets can be driven out of locks or out of plumb



When in flowing water “log jams” may occur creating added stress on structure

10.

MODES OF FAILURE

The primary modes of failure of a cofferdam are as follows in Structural:

10.1 FABRICATED TEES AND WYES Numerous failures have involved welded connector piles. Such failures in welded tees normally occurred in the web of the main sheet pile, the web often rupturing on both sides of the tee stem and separating the tee into three pieces. Weakness in these tee members is attributed to improper welding of steel with high carbon content and laminations in the steel sheet piles used in fabricating the tees.

10.2 SHEETS AND INTERLOCKS Interlock failure has resulted primarily from hard driving through dense or excessively deep overburden, overburden containing boulders, or from attempting to drive sheets of the connecting arcs past distortions in previously filled main cells. Splicing new and used sheet piling of different manufacturers has resulted in unpredictably high localized stresses in the interlocks and in the webs of sheets with resulting failure.

10.3 ENVIRONMENTAL CONDITIONS Scour and other effects of river currents have contributed to a number of cofferdam failures. Where the overburden is susceptible to erosion, scour due to high velocity flow is a serious 36


problem. By removing the lateral support provided by the overburden interlock, stresses have increased. Where driving through the overburden was difficult, some sheets have not penetrated to rock or have been driven out of interlock. Continued scour exposed these deficiencies and resulted in loss of cell fill and subsequent failure. High water has contributed to several failures by raising the level of saturation in the cell fill thus increasing interlock stresses.

10.4 STABILITY Soil Mechanics:

Cofferdams built in accordance with current design practice have generally proved adequate as far as the soil mechanics aspects of the design are concerned. However, there is the exception of piping failures at cofferdam cells tying into existing structures or into high ground. In these cases, failures have resulted from loss of cell fill due to piping caused by inadequate provision for seepage control. Foundations:

A few cofferdam failures have occurred because of foundation failure well below the base of the cells. This mode of failure has been precipitated by faults, slip planes, or high uplift pressures not recognized as problems during design. Also, foundation failure has occurred because of excavations located too near the cofferdam cells which allowed stress relief and relaxation of the rock. Saturation of Cell Fill:

Saturation of the cell fill is associated with many failures.

The pressure of the water when

added to the lateral pressure of the cell fill increases the interlock stresses. The saturation of the fill in the connecting arc is a particularly potent danger because of the magnitude of the tension that can be created on the outstanding leg of a connector.

It should be noted that saturation can

be caused by means other than the common leakage through the interlocks, holes, splices, and filling by the hydraulic dredge method. Waves splashing over the top of the cells, leakage, or breaks in the discharge lines of unwatering pumps over the cells can quickly cause saturation of the fill.

37


Construction Practices:

A number of failures have occurred during construction of cofferdams which may have been attributable, in part, to construction practices. Unless the sheet piling is driven in overburden, the lateral stability of the cell is largely dependent on the support furnished by the template until fill is placed in the cell. If this support is inadequate or the filling operations impose severe loads on the sheet piles, local distortion or collapse may occur. The practice of driving sheet piles in pairs may be detrimental if the bedrock is uneven. Windows or split interlocks can occur with possible loss of cell fill and subsequent failure. Therefore, when piles are driven in pairs, the sheets should be seated in rock individually.

38


11. CASE STUDY –

COFFERDAM CONSTRUCTION AND DEWATERING TAUNSA BARRAGE REHABILITATION PROJECT

11.1 INTRODUCTION Taunsa Barrage is situated on the huge river of the Indus valley known as the Indus River, in the province of Punjab, India. The irrigation system has all the rivers interconnected through a series of link canals that facilitate inter-basin transfers. The irrigation process is carried out by diverting the water from the rivers through a sequence of barrages, releasing water into main canals and subsequently to enormous irrigation network of distributaries and minor channels.

Figure 18: Taunsa Barrage (downstream face) Figure 20 In the nineteenth century, the progress in irrigation system began and established good amount of existing barrages in the province of Punjab. The Taunsa barrage was constructed in the year 1958 and currently supplying water for four main canals - two on the right of the bank and two on the left of the bank. The details of the barrage are, it has 65 bays/gates each one is of 60ft wide and parted by 7ft thick piers. The total width of the structure is 4,346ft and it has a design capacity to pass a flood of 1,000,000 cusecs.. Consequently, a number of ventures were added to the upstream of the barrage which includes a huge reservoir at Tarbela and during the following

39


years the annual uttermost flood has been steadily reduced to about 650,000 cusecs. cusecs. (Nadeem, 2008)

11.2 DESCRIPTION OF PROJECT A coffer dam has been constructed for the rehabilitation of the Taunsa barrage situated over Indus River in the province of Punjab, India. This project was undertaken by Punjab Barrages Consultants (the Engineer) and Punjab Irrigation Department (the Employer) and was funded by World Bank. This project was performed during 2005 – 2008 by dividing the work into mechanical and civil works and was performed through three major contracts. The civil works contract was presented to Descon Engineering Limited (DEL) in Joint Venture with China Gazooba Corporation (CGGC) under international competitive bidding. The contractor, Descon Engineering Limited (DEL) and China Gazooba Corporation (CGGC) planned the construction of the entire project in two years. In which every year they have planned to construct one half of the weir and to rehabilitate one half of the bays of the old barrage. The rehabilitation work contained, replacing old weaker concrete from the chute and stilling basin floor. And due to this the cofferdams were constructed in one half of the barrage length on the upstream and downstream of the construction site.

Figure 19: Cofferdam completed in parallel at the front and back of the enclosure enc losure

40


To supply water for irrigation during construction of the sub weir the main structure of the barrage was kept in operation and because of this the sub weir was positioned at a distance of 925ft downstream of the existing barrage so that the silt load from the left and right pockets of the barrage can be released downstream. During the construction of one half of the sub weir, the other half of the barrage was kept open to pass the river discharge with a much greater capacity than the maximum observed flood of 300,000 cusecs for the non-monsoon period (Oct to June) and from July to Sep the monsoon period, no activity was performed inside the river channel. To safeguard against floods, the maximum flow of 300,000 cusecs for the spring season was used to estimate the height of cofferdams. And this in turn gave a capacity of half the barrage and the available river channel downstream was much more than the required capacity for the flood. Hence, it guaranteed satisfactory factor of safety for the diversion channel and the height of the cofferdam. To safeguard the activities of the project, continuous monitoring of the flow of river was carried out and to safeguard the current structure, construction activities were planned such that head across the barrage remains within 15 ft whereas the design head across the barrage is 24 ft. The main cost component of the project was sub weir estimated to be around $60 Million and other temporary works like for care and handling of water was estimated to be $14 Million. A detailed procurement plan for the required materials and equipment was organized to ensure that the materials are available at the site at the right time. (Nadeem, 2008)

11.3 CONSTRUCTION PROCESS The Construction of cofferdams is a critical activity and is considered critical when the construction of the project is carried out in the flowing river. It requires large volumes of the materials like stone and earth fill at site which was dumped in the river on the upstream and downstream for construction of the sub weir and also for the repairs of other structures inside the river. The main equipment’s used for the construction of cofferdam in flowing water was by heavy

machineries like dozers, dump trucks and excavators. The stone dump was constructed in advance of the earth fill embankment which was constructed in its shadow. The dumper dumped

41


the stone in reverse on top of the embankment and dozers moved it across the flowing water. The water washes away some stone and some was left behind at the toe of the embankment. The earth fill material was also dumped from the top as it settles down under the load of machinery. During the construction of the coffer dam high vigilance is required for monitoring day and night. And also consistent maintenance of the cofferdams was also required like placement of materials at the point of observed settlement to ensure that the required freeboard is not eroded and the cofferdams perform the function they are built for that is to provide safe working conditions. The construction of cofferdam was a continuous process so as to maintain a stable progress inside the river. During the process, continual supply of materials made the operation successful. For this purpose, during construction the stock piles of materials like stone and earth fill were maintained at the site and supplemented by direct supplies from the quarries. When the construction work approaches the center of the river channel, the velocity was constantly increasing and so is the scour at the nose of the cofferdams, thus during this time more materials is required to achieve the planned progress.

Figure 20: Dump truck unloading stone at the nose of the cofferdam

42


Careful planning was required to achieve the targets as there was very limited time was available for the construction of cofferdam. This activity was the most critical in the time plan as no other activity could have been carried out until the cofferdams complete. Table 1 Quantities of material used for construction of cofferdams Earth filling

11.46 M cft (324,394 m3)

Stone

100,000 cft (3000 m3)

Sheet piles -

Area

224,000 sft

Depth

35-40 ft

Sand bags

60,000 No.

11.4 DEWATERING After the construction of coffer dam, dewatering was the main operation to be carried out. It required immense pumping effort with proper planning in maintaining the water levels to required point. Dewatering mainly required surface water removal and lowering of the subsurface water levels. So the main equipment’s used for the dewatering process are large

number of tube wells along with pumps and screens in the substrata, based on the water level required for concreting in different parts of the compound. In the sub weir region, this process was mostly required and was considered very critical to construction work. Here, to analyze the impact of sheet piles and overall stability of the cofferdam section seepage modeling was carried out by using SEEP/W software. The quality of results of the model depends on the parameters used for the analyses like the permeability of the soil strata, boundary conditions, etc. However, once a realistic model is available, sensitivity analyses may be carried out to study the impact of different parameters. And the decision to install a sheet pile in the cofferdams was primarily based on the results of the model. (Nadeem, 2008)

43


Figure 21: Sheet piling in the cofferdams with simultaneous dewatering

With the use of ten new generator sets, there was continuous supply of electricity throughout day and night to maintain the pumping operation. And also standby units were provided at the site to minimize the risk of failure. At the starting phase of the work, they have planned to establish single enclosure for construction of sub weir. But after proper analysis of the flow conditions across the barrage, the results suggested that it was required to construct four enclosures of 700 ft width each instead of one single enclosure of 2800ft length. This subdivision resulted in an early start of construction of sub weir and it assisted in rational phasing of dewatering for each enclosure in a separate sequence. In order to lower the level of water table to require level, it required 50-60 wells in each enclosure of 700 ft. The availability of pumps and generators was critical to achieve dewatering of the site for construction, so there was extra capacity to make sure a continous operation of the project.

44


The total generation capacity required for the pumps in the sub weir area was more than 3 MW for which 8 generator sets of 375 KVA were provided. And for every two generators required for dewatering, a standby generator was also provided to ensure uninterrupted power supply for dewatering operation. In the dewatering process, the pumped out water to be disposed in the surrounding water bodies was considered a major task which required detailed planning. The specially designed disposal mains of steel and flexible hose pipes were used to stream line the site and the pumped water disposal point had to be clear of the coffer dam toe to safeguard against erosion of the cofferdam.

11.5 EQUIPMENTS USED AND THEIR PRODUCTIVITY ANALYSIS EQUIPMENT USED:

Construction of cofferdam was carried out using heavy equipment’s such as 

Dozers.



Loaders and Dump trucks.



Backhoe.



Vibrating pile hammer.



Dewatering pumps- tube wells along with pumps and screens in the substrata.

Weight of the above mentioned machinery was around 10-15 tons and can move freely on newly constructed embankment. First stone dump was constructed followed by earthfill dump. To construct stone embankment, the dumper is used to dump stone in reverse on top of the embankment and dozer is used to move material across flowing water. KEY POINTS: 

In the project, labors were working 24 hr per day. There are two shifts, day shift and night shift.



During the execution of the project, job and management conditions are considered as excellent hence f 2 = 0.84 (Table 2.1) 45




Conversion factors are applied wherever necessary.



Operating factor of 50 min/ hr is considered.



In both earthfill and stonefill same dozer and truck type are assumed.



Dewatering is a hammock activity. Both sheet pile, excavation and dewatering are planned simultaneously.



Productivity calculations are done using General Output Model (GOM).

Duration = Quantity/Productivity.

The project is divided into 4 phrases, detail productivity analysis and duration calculations for each phrase are shown below:

Figure 22: Shows different phrases

46


11.6 PRODUCTIVITY ANALYSIS 11.6.1 PHRASE 1- STONE FILL LAYER AND EARTHFILL EMBANKMENT CONSTRUCTION

Loader is used to fill the truck at the stone quarry and dump truck is used to dump material at the embankment. First the stone dump was constructed then earthfill embankment was constructed in its shadow. The dumper dumped the stone in reverse on top of the embankment and dozers moved it across the flowing water. Similar process is adopted for construction of earthfill embankment. The water washes away some stone and some was left behind at the toe of the embankment. Both stonefill and earthfill layers are compacted due to the load of machinery which are moving on top of them. Stone size is governed by velocity of water flow which was observed to be 8-10 fps in river Indus. Thus stone size used in construction was greater than 100lb weight with one face cut to minimize the rolling of stone under its own weight. When water starts flowing along the cofferdam, it tends to undermine the stone by removing sand from underneath the stone. The stone launches itself to curtail further removal and provides stability to the bed. Earthfill material was clayey slit- 98% passing #200 sieve. The earthfill was provided to hold the water from flowing across the cofferdam. The earthfill used was fine grained soil (mainly clayey silt) with sufficiently low permeability value of 10 -3 to 10-4 cm/s that can hold the water from flowing across the cofferdam. The stability of the embankment was ensured through providing a slope flatter than the angle of repose of the soil. Clayey slit was available in the surrounding area in ample quantities. The earthfill material was borrowed from different surrounding areas but for this project it is assumed that earthfill material was borrowed from dump yard located at 20 km from the construction site. The truck and loader type for earth fill are same as those used in stonefill. (Nadeem, 2008).

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11.6.1.1 STONEFILL LAYER: Dump Truck details:

Name of the dump truck: SITOM MONSTER 4x2 dump trucks 15 ton: Standard Model Engine Horsepower: 210HP Diesel Regular loading capacity-10000kg~20000kg (10-20 metric ton) Max. loading capacity- 25000kg (25 metric ton) Cargo Capacity = 20.216 LCY*0.67 = 13.545 BCY Transmission type: Manual Place of origin: Hubei China (Mainland) Max speed: 85km/hr and economical speed was 60km/hr Stone Quarry was at 50-60 km distance. 60 km distance was considered because continuous material supply was of highest priority and considering least or avera ge can affect it. (Alibaba, 1999) Productivity calculations: Wheel loader:

Articulate wheel Loader of 4 cu yd heaped capacity Rock – loose materials - 33 sec cycle time (min) (notes table 8.9) K = (80+95)/2 = 0.875, f =0.67, f 2= 0.84, t = 50/60 = 0.833 Job and management are considered excellent = 0.84 General Output Method (GOM):

Loaders capacity P = 60*4*0.67*0.875*0.84*0.833/(33/60) = 179 BCY/hr Truck producitivity:

48


Load time = Haul unit capacity/loader productivity at 10 0% efficiency Load time = Number of bucket loads * excavators cycle time Load time = 13.545 BCY /179 BCY/hr = 4.54 min Cycle time = Load time + hauling time + dump time + return time Assuming the average site condition the spot, maneuver and dump time for rear dump = 1.1min Total cycle time of truck = 4.54min + 1.1 min+ 60 + 43 = 108.64 min Hauling distance = 60km/ (60km/hr) = 60 min, Return distance (empty) = 60km/ (85km/ hr) = 43min Productivity of truck = truck capacity/ cycle time = 13.545/ (108.64/60) = 7.48 BCY/hr

Figure 23

Number of trucks = productivity of loader/ productivity of truck = 179/7.48 = 24 trucks It is mentioned in the project that the rate of dumping required for such construction was around 20-25 dumps in one hour to keep it moving forward at the required phase.

49


And by calculation we got 24 trucks which are used at the job site to deliver the stone. Total stonefill quantity used = 37037.037CCY Time required to dump the stonefill material = (37037.037*0.77) /(7.48*24) = 158.86/24 = 7 days (labour are assumed to be working 24 hr /day i.e 2 shifts per day ) Dozer productivity:

844H Wheel dozer is used to push the material because the concentrated wheel load will provide compaction and kneading action to the ground surface and good for long distance, these two features are required to satisfy the project criteria. Name: 844H Wheel dozer Features: Operating weight = 156120 lb Blade Capacity = 16.1 m3-30.7 m3 (21.1 yd3-40.2 yd3) Direct Drive Forward 1 2 3 Average

Speed (mph) 4.5 7.9 14 8.8

Direct Drive Backward 1 2 3

(caterpillar) Productivity Calculation: General output model:

P = (60 *Q * f * k* f2 * t)/CTc Where; k = blade capacity correction factor (Given Graph) = 1 (zero grade) Q = blade capacity in loose cy or m3 = 20 LCY f = earth volume change conversion factor = 0.67

50

Speed (mph) 4.9 8.8 15.4 9.7


f2 = job and manage. Conditions = 0.84 t = operating time factor = 50/60 = 0.833 CTc = cycle time corrected for grades in minutes= to + (d/V1) + (d/V2) V1= hauling speeds = 8.8 mph or 236 m/min V2 = returning speeds = 9.7 mph or 261m/min to = fixed time (loading, turning, dumping, and raising & lowering bucket) Operating conditions direct drive transmission, hence dozer fixed c ycle time to= 0.10min (Table 4-3) d = distance for hauling and returning = 250 m CTc = 0.10 + (250/236) + (250/261) = 2.12 min P = (60*20*0.67*1*0.84*0.833)/2.12 = 265.37 BCY/hr Since the material is required for dozer to push and unloading whole material at the site from dump truck takes 7 days. Thus the dozer should operate throughout 7 days period.

11.6.1.2 EARTHFILL: Articulate wheel Loader details:

Articulate wheel Loader of 4 cu yd heaped capacity - 33 sec cycle time (min) (notes table 8.9) K = (80+100) / (2*100) = 0.9, f =0.79, f 2= 0.84, t = 50/60 = 0.833 Job and management are considered excellent = 0.84 General Output Method (GOM):

P = (60*4*0.9*0.79*0.84*0.833) / (33/60) = 217 BCY/hr Productivity of dump truck:

Name of the dump truck: SITOM MONSTER 4x2 dump trucks 15 ton: Standard Model 51


Cargo Capacity = 20.216 LCY*0.79 = 15.97 BCY Load time = Haul unit capacity/loader productivity at 100% efficiency Load time = Number of bucket loads * excavators cycle time Load time = 15.97 BCY /217 BCY/hr = 4.42 min Cycle time = Load time + hauling time + dump time + return time Assuming the average site condition the spot, maneuver and dump time for rear dump = 1.1min Total cycle time of truck = 4.42 min + 1.1 min+ 20 + 7.5 = 23.02 min Hauling distance = 10km/ (60km/hr) = 10 min, Return distance (empty) = 10km/ (85km/ hr) = 7.5 min Productivity of truck = truck capacity/ cycle time = 15.97 / (23.02/60) = 41.63 BCY/hr Number of trucks = productivity of loader/ productivity of truck = 217/41.63 = 6 trucks The quantity of Clayey silt = 42,4444.44 cu yd or 11.46 million cu ft Duration = (42,4444.44 * 1.11)/(41.63*6) = 1886.19/24 = 79 days Sine both stonefill and earthfill layers where construction simultaneously the total d uration for this operation = 86 days or 3 months Dozer productivity:

Name: 844H Wheel dozer P = (60*20*0.79*1*0.84*0.833)/2.12 = 312.89BCY/hr

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11.6.2 PHRASE 2: SHEET PILE INSTALLATION

Sheet Piles were installed to cut seepage rate across the earthfill and improve its stability. The length of the sheet pile is 40ft (EL 427- EL 387). The flow nets showed that the sheet piles are useful in cutting down the seepage by about 35% of its original value.

Productivity Calculations:

Type: Crane with vibrating hammer of 100 ft/hr driving rate is used and operational efficiency = 40 min/hr Then productivity = 100 ft/hr * (40/60) = 66.66 ft/ hr The time required to drive 40ft long 1120 piles is (1120*40ft/pile)/(66.66 ft/hr*24hr/day) = 28 days or steel h-pile of 40ft length around 2.5 piles/hour (table 18.2 construction equipment management john E. Schaufelberger, prentice hall) Duration = (1120/2.5)* 60 = 23 days Therefore it takes around a month to complete installation of sheet piles.

11.6.3 PHRASE 3: DEWATERING USING PUMPS: Dewatering is a hammock activity. The total area is divided into 4 enclosures. Dewatering is done along with sheet pilling and excavation. For dewatering total 196 pumps are used and the combined capacity = 220 cu sec (cs). The details of each pump are shown in the table below:

53


Table 2: Type of pumps and their capacities which were made available at the site are S.NO.

PUMP TYPE

CAPACITY

Cs 1.

2.

3.

NO. OF

NO. OF

TOTAL

PUMPs

PUMPS

CAPACTITY

Estimated

Provided

Cs

60

30

Submersible

0.5

pumps

1.0

120

128

128

Centrifugal

0.5

68

90

45

pumps

1.0

Mud pumps

2.0

7

Total

8

5

10

196

290

220

Productivity calculation:

Size of the single enclosure = 700*9843*40= 275.6 M cu ft or 7804122.9 m3. The total amount of water in the single enclose = (7804122.9*1000kg/m3)/3.785 = 2061.85 M gallon. The combined capacity of all pumps used in the site = 220 cu sec or 98742.86 Gallon/min. Total Duration of dewatering = (2061.85 M*4)/ (98742.86*60*24) = 60 days or 2 months.

54


Graph 1: Dewatering in single enclosure

55


11.6.4 PHRASE 4: EXCAVATION USING BACKHOE:

Excavation is done after dewatering and to facilitate construction of sub weir.

Type: Crawler mounted Backhoe

Figure 24

Features

Bucket capacity Engine(water cooling) Pear part Rctary radivs Min clearance Perfomance patameter Traveling Speed(Low/High) Swing speed Climb capacity Max traction force Operation range Max excavation height Max excavation depth Max unceding height Max excavation radius

0.45 (m3) 2225(mm) 420(mm) 2.2/4 (km/h) 0-14(rpm) 70 (%) 75(kN) 7965(mm) 4910 (mm) 5585(mm) 7510(mm)

(Weiku, 2011)

56


Productivity calculations: General output model:

P = (3600*Q*f*k*f 1*f 2*t)/CT Substrata soil = medium sand to fine sand k = bucket fill factor = (95+110)/2 = 102.5 (Table 8.4). Q = bucket capacity = 0.45 LCM (0.59 LCY) f = earth volume change conversion factor= 0.89 f 1 = swing depth factor = 1 f2 = job and management conditions = 0.84 t = operating time factor = 50/60 = 0.833 CT = cycle time in seconds = 14sec (Table 8.5)

Max excavation height = 7.965m Loading height is assumed to be 3m meters Then (3/7.965)*100 = 38% <60% and > 30% Max excavation depth = 4.910 m Then (2.4/4.910)*100 = 49% <60% and > 30% Depth-swing factor: f1 = 1.0 because it fulfills the two depth and angle of swing constraints. P = productivity = (3600*0.45*0.89*1.025*1*0.84*0.833)/14 = 73.86 BCM/hr Duration = total volume of excavation / productivity = (30*700*39370)/ (73.86*60*24) = 3 months

57


12.

CONCLUSION

In this case study, the coffer dam was constructed on the Indus River for rehabilitation of Taunsa Barrage. The construction process was carried out with proper initial analysis of the site for construction and dewatering processes. They have carried out 24 hours vigilance over the coffer dam during and after construction process which enabled them to detect faults or failures in the coffer dam. Overall construction period of cofferdam is 6 months because overlap between activity duration. SUMMARY TABLE

DESCRIPTION

EQUIPMENT

PRODUCTIVITY

DURATION

Phrase 1 Dump truck

7.48 BCY/hr*24

Stone fill

= 179.52 BCY/hr Loader

179 BCY/hr

Dozer

265.37 BCY/hr

Dump truck

41.63 BCY/hr*6

3 months

= 249.78 BCY/hr Earthfill

Loader

217 BCY/hr

Dozer

312.89BCY/hr

Phrase 2 Sheet Pilling

Crane mounted Pile

66.66 ft/ hr

driven Vibratory

1 month

hammer Phrase 3 Dewatering

Submersible,

220 cu sec or

centrifugal and mud

98742.86 Gallon/min

2 months

pumps Phrase 4 Excavation

Backhoe

73.86 BCM/hr 58

3 months


Every cofferdam is unique and requires thorough analysis. The designer must take into account a large number of parameters. The design must be compatible with the weather conditions, waves, currents, construction equipment, construction methods, internal permanent structures, and ground conditions. Comparable cost studies should be analyzed to determine if the cofferdam method is favored over other techniques, such as precast or caisson construction. Often the cofferdam designer must work closely with the project design engineer to arrive at a mutually satisfactory procedure.

59

Complete Cofferdam (1)

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