Energy Conservation as a Principle Underlying Mobilizable Strength Design for Deep Excavations

Finite-element analyses (FEA) and case histories of deep excavations in soft clay are used to validate a decision-making tool based on an extended mobilizable strength design (MSD) method that permits the designer an extremely simple method of predicting ground displacements during an undrained excavation. This newly extended MSD approach accommodates a number of issues that are important in underground construction between in situ walls, including alternative base heave mechanisms suitable either for wide excavations in relatively shallow soft clay strata or narrow excavations in relatively deep soft strata, the influence of support system stiffness in relation to the sequence of propping of the wall, and the capability of dealing with stratified ground. In addition, a simplified MSD framework is proposed for analyzing a database of 110 deep excavation case histories worldwide. The approach examines the governing factors controlling deformation in deep excavations and offers simple guidelines for designing support structures for deep excavations. These developments should make it possible for a design engineer to make informed decisions on the relationship between prop spacing and ground movements or the influence of wall stiffness and on the need for and influence of a jet-grouted base slab, for example, prior to conducting project-specific FEA.