Clay Anisotropy and Braced Wall Behavior

A technique is presented which modifies the basal heave analysis procedures for braced excavations to account for the strength anisotropy exhibited by most natural, soft to medium clays. In the case of a braced wall, the clay on the passive side of the wall can develop a much lower strength than the clay on the active side, if the clay is strongly anisotropic. Since the existence of a lower strength zone is not considered by conventional basal heave analysis procedures, these techniques theoretically lead to an overestimate of basal stability. Finite element analyses are used to demonstrate the effects of anisotropy on the behavior of braced excavations. Analysis results show that if the soil is presumed to be isotropic when it is truly anisotropic the following trends develop: (1) The basal heave safety factor may be from 10 to 50 percent larger than the actual one, depending on the degree of anisotropy; (2) lateral wall movements and ground surface settlements can be larger than expected; and (3) unusually high loads can develop on the lower struts in cases of marginal stability.