Undrained End Bearing Capacity of an Improved Soil Berm in an Excavation

Abstract

For a wide excavation in soft soil, the excavation can be stabilized by an embedded improved soil berm to increase wall stability and control soil movement. An embedded stiff berm essentially behaves like a horizontal pile subjected to a load applied by the retaining wall and derives its resistance to horizontal movement from both end bearing and interfacial shear resistance on the top and bottom of the berm. This resistance helps to restrain the wall from moving inwards to the excavated side. However, to date, there is no known reported literature on the determination of the undrained ultimate bearing capacity of such a berm, especially for the unit end bearing, qb. In this paper, the undrained end bearing of an improved berm under a plane strain condition was determined. The undrained end bearing capacity was first derived using a solution from a proposed upper bound analysis based on observations from centrifuge tests and then modified, taking on the basis of an equivalent finite element analyses. The proposed end bearing capacity factor Nc lies between the upper bound and lower bound solutions. The solution showed that the undrained end bearing capacity is not a constant but decreases during the excavation process. Furthermore, it was shown that the existence of an improved soil berm will provide an additional pressure relative to the passive pressure to control the wall displacement.