Mechanisms of load transfer and arching for braced excavations in clay

This paper presents a detailed interpretation of the evolution of stresses around a braced excavation in a deep layer of soft clay. Excavation support is provided by a diaphragm wall and multiple levels of rigid cross-lot bracing. Undrained shearing of the clay is represented by an advanced effective stress soil model that simulates important features of behavior including anisotropic stress-strain-strength relationships, small strain nonlinearity, and hysteretic response upon load reversal. The results provide new insight for explaining the development of lateral earth pressures for braced excavations and give a quantitative illustration of conceptual load transfer mechanisms and soil arching discussed previously in the literature. Reversals in the direction of shearing occur when the upper retained soil is squeezed against the bracing by deep-seated incremental movements in the soil mass, and arching of stresses below the lowest level of bracing. These mechanisms apply for a wide range of soil profiles when the wall is not keyed into an underlying bearing layer. Field measurements from an instrumented project in Taiwan lend credibility to the stress paths predicted in these numerical experiments.