Geotechnical design of a secant piled underground pump station in liquefiable ground

Abstract

The geotechnical design of underground structures with a high importance level, in a dense urban area, in liquefiable ground, and adjacent to existing structures that are sensitive to settlement is challenging because multiple model inputs need to be considered, many of which act concurrently. The new structure should consider each of these factors in both short and long term conditions, and in various combinations. This paper presents the design process for an underground pump station located in central Wellington adjacent to existing sensitive structures in liquefiable ground. The overall stability of the proposed construction sequence was assessed, and resulting actions on the structural elements of the shaft for both short-term and long-term conditions were evaluated in the 3D finite element modelling (FEM) software, Plaxis 3D. The FEM based technique included a settlement assessment for the sensitive adjacent structures and provided structural actions for the design of the structural elements, in addition to assessing performance in a design level earthquake and subsequent liquefaction. The secant pile shaft provides very little deformation and settlement outside of the pile group, is able to resist very high loads from the surrounding structures, produces lower vibration during construction, and good watertightness considering high in situ groundwater levels.