Deformation characteristics of ultra-deep circular shaft in soft soil: A case study

Abstract

A circular shaft is often used to access a working well for deep underground space utilization. As the depth of underground space increases, the excavation depth of the shaft increases. In this study, the deformation characteristics of a circular shaft with a depth of 56.3 m were presented and analysed. The main monitoring contents included: (1) wall deflection; (2) vertical wall movement; (3) horizontal soil movement; (4) vertical surface movement; and (5) basal heave. Horizontally, the maximum wall deflection was only 7.7 mm. Compared with the wall deflection data collected for another 29 circular excavations, the ratio of maximum wall deflection to excavation depth of this shaft was smaller due to a smaller ratio of diameter to excavation depth. The wall deflection underwent two stages of deformation: the first stage was mainly circumferential compression caused by the mutual extrusion of joints between walls, and the second stage was typical vertical deflection deformation. The horizontal soil movement outside the shaft was greater than the wall deflection and the deep soil caused great horizontal movement because of dewatering at confined water layers. Vertically, a basal heave of 203.8 mm occurred in the pit centre near the bottom. Meanwhile, the shaft was uplifted over time and showed 3 stages of vertical movement. The surface outside the shaft exhibited settlement and uplift deformation at different locations due to different effects. The basal heave caused by excavation was the dominant factor, driving the vertical movement of the shaft as well as the surrounding surface. The correlation between the wall deflection and the surface settlement outside the shaft was weak.

Keywords

• Circular shaft
• Site monitoring
• Deflection
• Vertical deformation
• Soft soil