Robust design optimization applied to braced excavations

Design of an excavation support system must satisfy the minimum factors of safety for stability requirements and the wall deformation and/or ground settlement requirements. In this paper, the authors present an application of robust geotechnical design (RGD) method on the design of braced excavations. 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 "noise factors" such as uncertain soil parameters, model errors, and construction variation. The robust design of a diaphragm-wall-supported excavation requires an optimal selection of the design parameters such as the length (L) and thickness (t) of the wall, the vertical spacing of the struts (S), and the stiffness (EA) of the strut. Within the RGD framework, the effect of uncertainties in the noise factors on the variation of the system response is evaluated. Furthermore, the design robustness is sought along with the cost efficiency and safety. Thus, the RGD methodology involves a multi-objective optimization. As cost and robustness are conflicting objectives, such optimization usually leads to a Pareto front, which offers a tradeoff that can aid in making an informed decision.