Basal heave stability of supported circular excavations in clay

Abstract

Deep circular shafts are commonly used for example to construct access shafts for transit tunnels, pump stations for wastewater tunnels, and launch shafts for tunnel boring machines. Since the earth pressures acting on a circular shaft subjects the shaft support to ring compression, the reinforcement in the structural elements can be reduced and the need for internal support is eliminated, thereby speeding up excavation. The design of circular support systems for excavations in clays involves assessing the ground stresses and ground movements, the capacity of the structural elements, and the basal heave stability. This study focused on assessing the basal heave stability of diaphragm wall supported circular excavations in clays using the finite element method. The analyses have shown that the basal heave factor of safety is dependent on the undrained shear strength of the clay, the geometrical properties of the excavation system and the thickness of the soil stratum. Based on these results, a simplified method is proposed for assessing the basal heave factor of safety for axisymmetric supported excavations in clay.