Stress paths in deep excavations under undrained conditions and its influence on deformation analysis

The objective of this study is to examine the stress state of soils during deep excavation, in relation to the determination of appropriate soil parameters for deformation analysis of a deep excavation case using the finite element method. Two well documented case histories of a deep excavation were utilized for the validation of the analysis procedure and the selection of soil stiffness parameters. Results from the Hardening Soil model showed that the out-of-plane stress has significance influences to the direction of soil effective stress path. In addition, most of the soil inside and outside excavation zone is in the elastic behavior. Even though the effective stress path of soils adjacent to the diaphragm wall have undergone yield, but the characteristics of those soils are still dominated by the elastic behavior. Hence, the unloading/reloading parameters are predominant in a deformation analysis of an excavation case. When the undrained shear strength and unloading/reloading modulus were precisely specified, even the Mohr-Coulomb model could obtain good prediction of the wall deflections. Moreover, a hypothetical case was employed to investigate the performance of the computed ground surface settlements. The result showed that the computed ground settlement from Mohr-Coulomb model was close to the result from the Hardening Soil Small model if the layer of soft soil is deep enough and a layer of small strain stiffness zone is introduced at bottom of the model geometry.