Counterbracing Retaining Structure Method in Excavation Support: Pile and Sheet Pile Approaches

Counterbracing Method Using Piles

Before starting excavation and retaining wall construction, concrete or steel piles are installed at regular intervals—usually spaced 2 to 3 meters apart—on opposite sides of the excavation area. The length of the piles is typically 1.25 to 1.35 times the excavation depth.

After pile installation, excavation proceeds in stages. If the soil is prone to collapse, planking or shotcrete lining is applied between piles as excavation advances.

When the excavation reaches a sufficient depth, horizontal waler beams are installed across the piles, and compression struts are placed between opposing piles. This process—excavation, installing horizontal walers, and compression struts—is repeated at different depths until reaching the excavation base. Note that for practical reasons, the minimum vertical spacing between compression struts should be at least 3 meters.

Counterbracing Method Using Continuous Sheet Piles

In this method, before excavation starts, instead of installing individual piles, continuous sheet piling is driven on both sides of the excavation.

This approach eliminates the need for planking or shotcrete to prevent soil collapse. However, the installation of horizontal walers and compression struts follows the same procedure as with piles.

Advantages and Limitations

Counterbracing is highly advantageous for excavations with small widths, offering high speed and low cost. It is especially useful for channel construction projects.

However, for excavations wider than about 10 meters or very deep excavations, the compression struts and braces installed at multiple levels may become cumbersome and complicate construction activities.