Mobilisable Strength Design for Deep Excavations

Geotechnical engineers often rely on simple, closed-form solutions to estimate capacity of foundations and apply safety factors to crudely prevent excessive deformation. Alternatively, finite element method (FEM) provides a more rigorous analysis but requires significant effort and a detailed knowledge of the soil constitutive relationships, which may not be available. Hence, a new rational design method is warranted, suitable for hand (or pocket calculator) calculation of serviceability deformations. The mobilisable strength design (MSD) method uses a combination of theory of elasticity and theory of plasticity to satisfy these criteria and proposes a strain field derived from idealised soil behaviour enabling prediction of boundary displacements. Of particular interest to geotechnical engineers are flexible retaining walls supporting deep excavations in urban areas.
The MSD method has been developed for cantilever retaining walls, braced excavations and narrow excavations, but involves some limiting assumptions. These include: an assumed "plastic" deformation mechanism normal to the ground at the free surface, perfectly rigid props for braced excavations and undrained conditions, limiting the analysis to short term loading in clays. This research aims to provide extensions to the MSD method. Initially, altering the assumed deformation mechanism to satisfy the boundary conditions at the soil surface, and extending the solution to encompass prop deformations. The stress distribution along the retaining wall can then be calculated enabling prop forces and calculation of wall bending moments for more efficient wall design. The effects of passive piles and surcharge on the ground surface will be investigated. Finally, new MSD solutions will be attempted for analysis of drained conditions and dynamic loading. FEM analyses will be carried
out to compare with the new solutions.