Optimization of design of supported excavations in multi-layer strata

In this paper, the authors present their robust geotechnical design (RGD) methodology for the design of braced excavations in multi-layer strata with a mix of sand and clay layers. The essence of RGD is to derive an optimal design through a careful adjustment of the design parameters so that the response of the braced excavation system is insensitive to the variation of uncertain soil parameters (called noise factors). Within the RGD framework, the effect of the uncertainties of soil parameters on the variation of the system response is evaluated using first order second moment (FOSM) method in conjunction with the finite element method (FEM). Furthermore, the design robustness is sought along with the cost efficiency and safety. Thus, the RGD methodology involves a multi-objective optimization, in which robustness and cost are treated as the objectives and the safety requirement is treated as a constraint. As cost and robustness are conflicting objectives in a braced excavation design, such optimization often leads to a Pareto front. Finally, through the use of the knee point concept, the most preferred design that meets the safety requirement and strikes a balance between the two objectives (cost and robustness) is identified on the Pareto front. The significance of the RGD methodology is illustrated with a braced excavation design example in multi-layer strata.