Deep excavations frequently cause problems, and sometimes trigger catastrophic collapses, especially in soft clay. In principle, these problems are well understood, but designers may fall between the two stools of naive empiricism and overelaborate finite element analysis (FEA). A new approach, Mobilizable Strength Design (MSD), has been developed to bridge this gap. MSD specifies deformation mechanisms tailored to each stage of construction. Each stage is analysed for energy balance, with incremental subsidence creating a drop of potential energy which must equal the work done deforming the soil and the support system. Incremental deformations are summed, while soil non-linearity is allowed for. The non-linear response of a representative shear stress-strain test is required, but estimates can be based on routine soil characterisation. It is demonstrated that MSD back-analyses not only fit FEA results for soft clay within ± 30%, but also fit the soil-structure deformation data of 110 field studies within a factor of 1.4. Finally, a new set of dimensionless groups is defined to characterise deep excavations in clay without the need for any analysis at all. These are used to chart the maximum wall displacements taken from the field database, and an elementary formula is proposed which predicts these 110 maximum displacements within a factor of 2.9.

Guidelines are deduced for designers. In particular, it is shown that wall stiffness within the typical range of sheet-piles, secant piles and diaphragm walls has little or no effect on wall deformations.