Design of braced excavations to limit ground movements

The authors have developed a new approach to the estimation of ground movements around braced excavations retaining thick deposits of soft clay, incorporating actual stress–strain data and the undrained shear strength profile of the soil on site. The method is based on the assumption of a plastic deformation mechanism local to a braced excavation, and which avoids any slippage on shear surfaces. Mobilised shear stresses beneath and around the face are found from an equilibrium calculation derived from the deformation mechanism by virtual work. The strains required to mobilise these stresses are finally entered into the deformation mechanism to predict boundary displacements. The outcome is a prediction based on simple calculations that otherwise would have called for elaborate constitutive modelling and finite element analysis. The capability of this mobilisable strength design (MSD) method in calculating the magnitude of wall displacements is demonstrated through the back-analysis of a previously published case history of a braced excavation at a soft clay site in Singapore. The relative effectiveness of heave-reducing piles in controlling ground movements is then demonstrated using the MSD method.