Braced cofferdams are the sheeting and bracing systems to support walls of an excavation or to exclude water from an excavation. It is mostly used in urban areas. They are frequently used in the construction of bridge piers and abutments,
The various components of braced cofferdams are:
Laggings – Horizontal timber planks placed by hand as the excavation proceeds.
Sheet piling – Timber, steel or concrete sheet piles installed vertically by driving.
Soldier beams – Vertical beams of timber or steel for taking reaction from laggings. They are not required when sheet piles are used.
Wales – Horizontal beams for transferring reaction from laggings or sheeting to the struts.
Struts – Compression member for carrying reaction from one side to other side of excavation.
Braced cofferdams are classified into three general types. They are:
- Braced Coffer Dams With Wales and Soldier Beams
- Braced Cofferdams With Wales and Without Sheet Piles
- Braced Cofferdams With Sheet Piles
Braced Coffer Dams with wales and soldier beams – They are used for small excavation because no heavy equipment is required for construction. They are suitable for depth of excavation up to 4.5 m as shown in Fig.1.
Braced Cofferdams With wales and without sheet piles – They are constructed of soldier beams driven prior to any excavation. Laggings are placed between the soldier beams as the excavation proceeds. Wales and struts are installed as soon as the excavation reaches the level of each tier. This construction is used in deep excavation. The soldier beams provide stability of the system & prevent bottom heave. Soldier beams are sometimes left in place and used as support of the permanent wall as shown in Fig.2.
Braced Cofferdams With sheet piles Sheet Piles – The sheet piles are driven prior to excavation to the proposed depth as shown in Fig.3. As excavation progresses wales and struts are installed. The excavation is continued and another set of wales and struts are installed.
This type has the following advantages.
- There will be no danger of bottom heave.
- There will be no danger of loss of ground in granular soils.
- The sheet pilings serve as cut-off wall.
- The cost of de-watering is reduced.
- The phenomena such as piping, boiling and blow up of soil is prevented.
Sheet pile cofferdams are generally made just large enough to permit the construction operation within. Therefore, the physical dimensions of the structure govern the shape in plan. However, when conditions permit circular cofferdam may be advantageous. In circular cofferdams the wales acts as compression rings requiring no struts, thus eliminating the cost of struts and also provide large clear working space within the cofferdam.