CASE STUDY

COFFERDAM CONSTRUCTION AND DEWATERING TAUNSA BARRAGE

REHABILITATION PROJECT

INTRODUCTION

Taunsa Barrage is located on the river of Indus river in the region of Punjab, India. The irrigation system

is connected to all the rivers through a series, this process is done by diverting the river water in sequence

to barrages then releasing the water into main canals and eventually to the network where water is

distributed to small channels. The taunsa barrage was built in 1958 in such a way that four main canals

were constructed with two canals on either side of the barrage. The barrage consist of 65 gates which

were divided with a thickness of 7 feet piers with a total width of the structure constituting to 4,346 feet

and was designed in a way that 1,000,000 cusecs of flood can pass through the barrage.

Figure: Taunsa Barrage

PROJECT DESCRIPTION

The civil work contractor for the project was Descon Engineering Limited (DEL) in association with

China Gazooba Corporation (CGGC) and they were selected under international competitive bidding. The

process of rehabilitation was planned in a way such that in first year they construct half of the weir and

rehabilitate half bays of the barrage. This rehabilitation work consist of strengthening the old concrete at

chute and basin floor, for this the cofferdam was built on the upstream and downstream of the restoration

site.

Figure: Early stage of Construction

Figure: Construction of Cofferdam at front and back of the enclosure

It was constructed in such a way that the barrier across the barrage could resist to a greater maximum

observed flood of 300,000 cusecs for non-monsoon and monsoon seasons. Thus taking this kind of

precautions made all satisfactory factor of safety to divert and also the height of the cofferdam. There was

also a periodic monitoring of the rise of water level and flow of river, which made them to build the head

across the barrage within 15 ft and design head being at 24 ft across the barrage.

The major cost region of the project was estimated to be at $60 million and other works like maintenance

of dam and securing of water was estimated to be around $ 14 million. Rest all schedules and planning

was done to get all those materials and equipment’s to the site at right time.

CONSTRUCTION PROCESS OF COFFERDAM

In the initial stage of the construction materials like stone and earth fill was brought to the site to dump in

the upstream and downstream of the river in large quantities to construct the sub weir. In this kind of

construction the major equipment’s used are excavators, dozers and dumpers. These dumpers dump the

stone on top of the embankment whereas dozers move across the flowing water and also the earth fill

material was dumped from top so that it settles down. But during the built up of the cofferdam it should

be monitored keenly time to time thus to safe guard the area as well as to place the materials over the

settled area.

The major advantage while constructing the cofferdam was the continual supply of the materials like

stone and earth fill which were maintained at the site with a scheduled supply. As the construction work

approached the center of the river channel there was a constant rise in the velocity which required more

materials timely to make sure the progress was going in a planned way. This is because of having limited

time to build the cofferdam, activity of constructing the cofferdam became a major priority and no other

activity is carried out till the completion of the cofferdam.

Figure: Dumper unloading stones

Material quantities for constructing a cofferdam:

Stone - 100,000 cft (3000m3)

Sheet piles - 224,000 sft (Area), 35 – 40 ft (Depth)

Sand bags - 60,000 No

Earth filling - 11.46 M cft (324,394 m3)

DEWATERING

In the process of construction of coffer dam, the next step is dewatering. Dewatering method requires a

lot of pumping of water with certain measures like maintain the water levels to a desired point. The main

equipment’s used for dewatering are tube wells, pumps and screens for concreting at required different

areas of the compound.

Sheet piles analysis and overall stability is calculated in a software called SEEP/W for cofferdam section

seepage. Thus a realistic model is available where for different parameters sensitivity analyses is done.

Eventually based on these results a sheet pile is installed.

In the primary phase of work, single enclosure for construction of sub weir has been planned. After a

keen analysis for the flow conditions across the barrage it was decided to construct a four 700 ft wide

enclosures instead of having a one single enclosure of 2800 ft length. Subsequently, to lower down the

water table 50 to 60 tube wells were required in each enclosure of 700 ft. Being large portioned area in

spite of having pumps and generators there was extra capacity provided for keeping the project to run

swiftly.

Generation of power supply being a challenged task, the pumps required 3 MW for which eight

generators of sets 375 kVA was provided. As two generators required for dewatering there was stand by

generator provided thus making sure that there is no interruption of power supply. Thus the pumped water

was directed to the surrounding water bodies.

Figure: Construction of Sub – Weir

Figure: Cofferdam with sheet piles during dewatering

EQUIPMENTS USED

Heavy equipment’s were used during the construction of the cofferdams, such as

Loaders and Dump trucks

Dozers

Backhoe

Dewatering pumps – tube wells along with pumps and screens

Vibrating pile hammer

Each machinery used constitute a weight around 10 – 15 tons which can move freely on newly

constructed embankment. Usually stone dump was built then was followed by earth fill dump. The stone

is dump in reverse on top of the embankment by the dumper during the construction.