## BRACED EXCAVATION

Braced excavations consist of sheet piles driven into the soil to form the sides of an excavation (Figure) such as in the construction of bridge piers, abutments, and basements. As excavation proceeds within the area enclosed by the sheet piles, struts are added to keep the sheet piles in place.

**Braced excavations.**

The top struts are installed, followed by others at lower depths. The wall displacements before the top struts are installed are usually very small but get larger as the excavation gets deeper. The largest wall displacement occurs at the base (bottom) of the excavation (Figure ). Wall displacements are inconsistent with all the established earth pressure theories.

The critical design elements in a braced excavation are the loads on the struts, which are usually different because of different lateral loads at different depths, the time between excavations, and the installation procedure. Failure of a single strut can be catastrophic because it can lead to the collapse of the whole system. The analysis for the forces and defl ection in braced excavation should ideally consider the construction sequence, and numerical methods such as the fi nite element method are preferred. Semi-empirical methods are often used for shallow braced excavations and in the preliminary design of deep braced excavations. The finite element method is beyond the scope of this book. We will only discuss a semi-empirical method.

Lateral stress distributions for use in the semi-empirical method are approximations from field measurements of strut loads in different types of soil. The lateral stress distributions used for coarsegrained and fi ne-grained soils are shown in Figure. These lateral stress distributions are not real but average approximate stress distributions to estimate the maximum strut load. The real lateral stress distributions are strongly affected by arching action, as discussed in Section .

The lateral stress distribution for coarse-grained soils (Figure ) was extrapolated from strut loads measured for dense sand adjacent to the excavation. The appropriate value of friction angle is f'p, but because we cannot rely on dilation, the design friction angle should be f'cs. For fine-grained soils, a total stress analysis is used, and the lateral stress distribution depends on the stability number, gHo/su (Peck, 1969). If the stability number is less than 4, the stress state of the soil adjacent to the excavation can be assumed to be elastic, and the recommended lateral stress distribution is depicted in Figurec. However, if the stability number is greater than or equal to 4, the stress state of the soil adjacent to the bottom of the excavation is expected to be plastic, and the recommended lateral stress distribution is depicted in Figure 15.26d.