Observed performance of a 30.2 m deep-large basement excavation in Hangzhou soft clay

Abstract This paper firstly performed an extensive investigation of a 30.2 m basement excavation (9310 m2 in plan) in Hangzhou soft clay. The 52 m-deep diaphragm walls and six levels of concrete struts were adopted as retaining structures to reduce the deformations caused by excavation with the bottom-up method. To comprehensively study the characteristics of this large-scale excavation, the behaviors of diaphragm walls, struts, columns, the maximum wall deflection rate, ground surface settlement and utility pipelines were focused and investigated during the whole excavation process. Based on the analyses of measured data, the following major conclusions were obtained: (i) The wall and the ground experienced a maximum displacement increment in stage 6 which was the coupled product of the “creep effect” of Hangzhou soft clay and the “depth effect” of excavation. (ii) The soil excavated in deep can produce greater stress release than the soil excavated in shallow. However, the cast of base slabs can constrain the inward wall movements and the followed horizontal ground movements successfully. (iii) Benefit from the integrity of 52-m-deep diaphragm walls and six levels of concrete struts, more-uniform movements of the column in the vertical direction were produced. (iv) The ground surface and the utility pipelines experienced more time-dependent settlement due to the much longer exposure duration time in stage 4. (v) In this project, deflections near the excavation middle were up to 3.5 times of those near corners, implying that for a large excavation not adopting the zoned excavation technique, the behaviors of corner-stiffening are significant. (vi) A general relation between the excavation area and wall deflection was also presented based on this and 16 similar cases.