A three-dimensional mechanism for global stability of slurry trench in frictional soils

To construct urban metro stations, the concrete diaphragm walls are widely used as the support of deep foundation. During the installation of concrete diaphragm wall, the global instability of slurry trench may occur due to the insufficient internal support. The instability originates at a shallow depth blow ground surface and propagates to the ground surface. This paper develops a three-dimensional mechanism for analysing the trench global instability. In the mechanism, the instability is initiated by the movement of a rigid block with conical discontinuity surfaces. The trench stability is analysed using the kinematic approach of limit analysis and the upper bound solution of the safety factor is obtained. The optimal mechanism is achieved through minimising the safety factor. The critical depth of instability and the influence zone on the ground surface are identified. With the higher frictional angle, the critical failure depth becomes smaller and the volume of the failure zone decreases. The drop of slurry level significantly increases the risk of trench instability especially for cohesionless soils. The mechanism is validated through analysing the slurry trench instability during the construction of a subway station in Suzhou, China. The analysis result shows a close agreement with the field measurements.