Braced Cofferdam – Uses and Types

Uses of Braced Cofferdam

What is a Cofferdam? A cofferdam is a temporary structure built to keep water away from the construction site, allowing for work to be carried out on a dry surface.

What is a Braced or Strutted Cofferdam? A braced or strutted cofferdam is a common type of cofferdam made up of vertical or horizontal sheeting with internal struts. It is used to exclude shallow water, earth, or both and is relatively economical in these situations.

Uses of Braced Cofferdam Braced cofferdams have a wide range of uses, from supporting shallow trench excavations to bracing systems for multi-level basement construction in urban areas. They are also frequently used in the construction of bridge piers and abutments, as they are more economical than cellular cofferdams and caissons in depths up to 40 ft of water.

Considerations for Braced Cofferdam Construction The construction of braced cofferdams for large and deep excavations in soft soils is challenging and expensive, making it important for engineers to consider alternative excavation schemes before adopting this method.

Components of Braced Cofferdam

1. Lagging: Horizontal timber planks used as the excavation progresses
2. Sheet piling: Vertical sheets made of timber, concrete or steel installed by driving
3. Soldier Beams: Vertical beams made of timber or steel for taking reactions from the lagging
4. Wales: Horizontal beams for transferring reactions from lagging or sheeting to struts
5. Struts: Compression members for carrying reactions from one side to the other

Types of Braced Cofferdam

Braced Cofferdam can be classified into three general types based on the arrangement of components. The type of bracing, as shown in figure 1, is commonly used for small scale excavation as it does not require heavy construction equipment. The process involves installing lagging, soldier beams, and struts when the excavation reaches a depth equal to one length of the soldier beam. Wedges are driven to tighten up the system, and additional sets of components are installed as the excavation progresses.

Type 2 Braced Cofferdam

The second type of braced cofferdam, as shown in figure 2, consists of the following components:

1. Soldier Beams
2. Lagging
3. Wales
4. Struts

In this type of bracing, the soldier beams are driven into the soil prior to any excavation. The lagging is placed between the soldier beams as excavation progresses. Wales and struts are installed once the excavation reaches the level of each tier.

This type of bracing is suitable for deep excavations as the soldier beams driven into the soil provide stability to the system and resistance against the heave of the bottom of the excavation. The soldier beams may also be used as support for the permanent wall, as shown in alternate section B-B.

Advantages of Type 3 Bracing using Sheet Piling

1. Resistance against Bottom Heave in Soft Soil: Sheet piling driven to a great depth offers considerable resistance against bottom heave in soft soil, which is a critical concern.
2. Prevention of Ground Loss in Purely Granular Soils: In purely granular soils, the sheet piling eliminates the danger of loss of ground due to excavation prior to placement of lagging.
3. Reduction of De-Watering Cost and Danger of Piping, Boil, or Blow-Out: In excavation of granular soil below water level, the sheet piling serves as a cut-off wall, reducing the amount of de-watering cost and the danger of piping, boil, or blow-out.

Advantages of Circular Cofferdams Circular cofferdams offer some advantages over other shapes in terms of construction cost and working space. These advantages are due to the unique design of circular cofferdams where the wales serve as compression rings, eliminating the need for internal strutting. This results in lower construction costs and provides a large clear working space within the cofferdam.

Determining the Shape of Cofferdams The shape of cofferdams is primarily determined by the physical dimensions of the substructure and the construction operations required. However, when conditions permit, circular cofferdams may be preferred due to their advantages. The size of the cofferdam is usually just large enough to allow for the construction operations to be carried out efficiently.