Types of Temporary Dams: What is a Cofferdam?
A cofferdam is a temporary dam erected to exclude water and soil from specific areas. It is built to permit damming, diversion or dewatering and construction of a permanent structure in a dry area.
The dam should have walls that prevent water from entering the building site. Therefore the height of the wall has to be more than the extreme water level and must also be waterproof. The dam is favored in areas where the construction site is large and with a reasonable depth of dry soil bed.
Definition of the 3-Ds
- Dewatering: It is the removal of surface and underground water mostly on a building site. Dewatering occurs before excavation of the subsurface for cellar space, foundation repairs, and shoring.
- Diversion: It is to change the direction of water flow. It is done to allow a project to proceed.
- Damming: It is the creation of a barrier that holds back water. The wall is used to maintain the water level, storage of collected water and prevention of water from crossing safety thresholds.
Types of Cofferdams
There are two main classifications temporary dams:
This type of dam is applicable in environments where the place is very small and has a depth of 6m. The barrier finds application in bridge construction. It has a single row of cantilever sheet pile which is usually heavily braced. The joints in the sheet piles are properly sealed. These temporary dams are suitable for moderate water flow velocity.
Timber and wooden sheets are erected on the perimeter of the construction area. Steel sheets placed at an equal distance to each other are driven into the river bed on the inside of the structure. Putting sandbags on the inner walls can enhance the firmness of the dam.
Braced cofferdams find application when it is hard to drive piles into the waterbed. Two piles supported by wooden cribs are driven into the bed. The wooden cribs are placed alternatively to form pocket filled with soil and stones.
The preparation of the wooden framework is done on the ground and then floated to the cofferdam construction site. The sand and other loose material layers covering the impervious bed are dredged out.
These temporary dams are employed when a large area is to be enclosed. The two walls help to improve the stability of the dam. A double wall dam can hold water up to a height of 12m and its height above of the water level.
It has two parallel vertical sheet piling walls packed with soil between the spaces to help prevent leakage.
The dam is suitable for use in areas where the water table level is less than 3m and has a low velocity. Construction occurs after the dewatering of the site. The structure utilizes locally available materials such as even soil, fine sand or clay.
The dam’s height is usually one meter more than the maximum water level. It helps to prevent water from flowing to the other side even when there are waves.
The slope is pitched with rubble stones to stop water action from scoring the embankment. To resist water spillage; even sheet piles get placed at the center. After the setup is complete water from the main site is pumped out, and construction begins.
These cofferdams are suitable for large-scale dewatering where the max water depth is 21m especially when building marine structures. There are two types of cellular temporary dams based on the shapes, circular and diaphragm type.
This kind of dam has circular arcs on the outer and inner sides. Straight diaphragm walls connect the arches. A unique material known as the Y-element enables the connection between the diaphragm walls and the arches.
The design allows for the interconnection of steel sheet piles to allow for bending. Non-permeable materials like clay fill the walls of the dam to reduce leakage due to the increase in the membrane weight. The barrier is beneficial since increasing the effective length happens by lengthening the diaphragm.
It consists of a main cellular cell with a large diameter, which is interconnected by smaller cell arcs. The connecting cells walls are perpendicular to those of the main large diameter circular cell. T-piles join the segmented cells to the main cell.
The dam is self-sustaining since filling of each cell is independent. The cells are more stable and more expensive than those of the diaphragm type. However, it is limited to withstand lateral pressure caused by high heads due to interlocking tension.
It is a common type of temporary dams constructed using rock fill around the dewatering site. Engineer prefers to build it where rocks are readily available. Since rocks are very permeable fillings, an impervious membrane of soil is used to prevent seeping.
To protect the dam from wave action the impermeable layer has riprap. Only a height of 3M is possible and the slope of 1:1.5.
Dam-it Dams Portable Cofferdams
Unlike the above types of portable dams, Dam-it Dams cofferdams don’t require construction before use. They are pre-fabricated tubes made of a geo-textile material.
The cofferdams use onsite water to fill inner dual tubes, which allows the dam to inflate evenly and unrolls to form a strong barrier. They are available for height of 1′ to 12′, widths of 2′ to 23′ and unlimited lengths.
Benefits of the Modern Dam-it Dams Cofferdam
Working over water is very expensive and challenging than working on dry land. The easiest way to perform construction jobs in water is to minimize the workload over water and do much of the work on dry land. The following are some advantages of these temporary dams.
- Provides a safe working environment
- Allows excavation and building of structures in poor environments
- It is reusable
- Does not require the use of specialized equipment for deployment
- Leave a small footprint on the environment unlike other cofferdams
- There is easy installation and removal
Application of Temporary Dams
- Transport engineering: It can be used for the construction of support walls, ground water retention, bridge pier, tunnel work and ramps.
- Civil Engineering: Cofferdams can be to assist in the construction of foundations, basements and underground car parking.
- Port construction: Used in docks and jetty works.
- Water engineering: Suitable in construction of culverts, weirs scour and flood protection walls and securing embankment construction.