Deep Excavation in Soft Normally Consolidated Clay

This paper presents the case histories of deep excavation for the basements of two projects using 11 retaining walls supported by stabilizing berms. Most of the case studies reported in the literature are confined to the use of stabilizing berms to support retaining walls for deep excavation in firm overconsolidated clays but not soft normally consolidated clays. This paper reviews the performance of 11 retaining walls supported by stabilizing berms for deep excavation in soft normally consolidated clays. Only four of these 11 stabilizing berm-supported retaining walls performed satisfactorily while the other seven performed dismally. The larger horizontal deflection of retaining walls embedded in soft normally consolidated clays as compared to overconsolidated clays is caused by creep of soft normally consolidated clays. The effect of creep of soft normally consolidated clays in deep excavation is not well understood. Based on the case study of an excavation of 8.3 m (27.2 ft) deep, an empirical relationship between the retaining wall deflection due to creep of soft clays and the total thickness of soft clays is established. The present study highlights the need to check the slope stability of the stabilizing berms in soft normally consolidated clays during excavation. This has never been thought to be necessary for stabilizing berms supporting retaining walls in overconsolidated clays for deep basement construction.