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

Abstract

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.

Ground movements due to deep excavations in Shanghai: Design charts

“…Figure 7 shows a summary of geotechnical parameters for a site in Shanghai . MSD analysis of a given excavation can be carried out incrementally, with characteristic soil parameters changing accordingly (Lam & Bolton, 2011). The characteristic depth for soil properties is regarded here, however, as the mid-depth of the completed excavation.…”

“…Lam & Bolton (2011) suggested a definition for the wavelength based on assessment of the degree of wall end fixity. In either case, the MSD analysis of a given excavation must proceed incrementally as the wavelength  reduces stage by stage as new supports are fixed.…”

“…According to Lam & Bolton (2011) the wall bulging deflection (w max ) can be related to the average shear strain ( average ) in the adjacent soil mass by equation (22): (22) Lam & Bolton (2011) define a displacement factor  which is modified in this paper to  * using  M=2 as the deformation parameter. Rearranging equation (8) we get: *…”

“…This was modified by Lam & Bolton (2011) to include narrow excavations. Their analysis was based on the conservation of energy in the deforming mechanism, taken stage by stage.…”

“…A dedicated MSD analysis as described in Lam & Bolton (2011) can be used to make site specific predictions of wall bulge. Here, however, MSD concepts will be used simply to derive dimensionless groups for the purposes of charting field monitoring data.…”