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Cofferdams are temporary structures that are built to keep the water away from the execution site. This enables construction to be carried out on dry surfaces.
A braced or strutted cofferdam is a temporary enclosure consisting of vertical or horizontal sheeting with internal struts. It is probably the most common type of all cofferdams because it is relatively economical when utilized for excluding shallow water, earth, or both. While the procedures of design of braced cofferdams in water are relatively simple, the design of such cofferdams in open cut requires a thorough understanding of the principles of earth pressure theories and their limitations.
Uses of Braced Cofferdam
Its uses range from supports for shallow trench excavations to bracing systems for multi-level basement construction where adjacent ground must be protected from collapse or settlement damages. This is almost invariably the case when excavation is required in urban areas. In localities where the adjacent ground need not be protected, the excavation is generally made with sloping sides without the use of such cofferdams.
Because of their relative economy, braced cofferdams are frequently used in the construction of bridge piers and abutments. Generally, the braced cofferdam is more economical than cellular cofferdams and caissons up to the depth of 40 ft of water. Even when caissons are sunk to greater depths, the upper portion of the bridge piers may still be constructed inside a braced cofferdam. In such a case, the upper portion of the caisson is provided with connections to braced cofferdams.
The construction of braced cofferdams for large and deep excavations, particularly in soft soils, is difficult and expensive. Therefore, the engineer should investigate other possible schemes of excavation before it is adopted.
Common types of Braced Cofferdams
A braced cofferdam consists of three or four principal components:
- Lagging: Horizontal timber planks placed by hand as the excavation proceeds.
- Sheet piling: May be timber, concrete, or steel and is installed vertically by driving.
- Soldier Beams: Vertical beams (timber or steel) for taking reactions from the lagging. Soldier beams are not required in the case where sheet piling is used.
- Wales: Horizontal beams for transferring the reaction from the lagging or the sheeting to the struts.
- Struts: Compression member for carrying the reactions from one side to the other side of the excavation.
According to the arrangement of the components, the braced cofferdams may be classified into three general types.
The type of bracing as shown in figure 1, is the most useful in small scale excavation because it does not require heavy construction equipment. When the excavation is advanced to a depth equal to one length of the soldier beam, lagging, soldier beams and struts are placed. Wedges are driven at one end of the strut to tighten up the system. Additional sets of lagging, soldier beams and struts are installed as excavation progresses.
The second type (figure 2) of bracing consists of driving soldier beams prior to any excavation. Lagging is placed between soldier beams as excavation progresses. Wales and struts are installed as soon as the excavation reaches the level of each tier. This type of bracing is suitable for deep excavations. The soldier beams driven in the soil afford some stability of the system and also offer some resistance against the heave of the bottom of the excavation. Soldier beams are sometimes left in place and used as support of the permanent wall as shown in alternate section B-B.
If condition permits driving of sheet piling, type 3, may be used advantageously. The sheet piling is driven to the desired depth prior to excavation. This type has the following advantages:
- In soft soil where the danger of bottom heave is critical, sheet piling driven to a great depth offers considerable resistance against such danger.
- In purely granular soils where loss of ground would occur due to excavation prior to placement of lagging, the sheet piling eliminates such a danger.
- In excavation of granular soil below water level, the sheet piling serves as a cut-off wall and therefore reduces the amount of de-watering cost as well as reduces the danger of piping, boil, or blow-out.
Cofferdams are generally made just large enough to permit the operation of the construction within. Therefore, the shape in plan is governed by the physical dimensions of the substructure. However, when conditions permit, circular cofferdams may be advantageous. In a circular cofferdam, the wales act as compression rings requiring no internal strutting, thus eliminating the cost of struts and also providing large clear working space within the cofferdam.
Foundation Design by Wayne C. Teng