Passive Earth Pressures on Retaining Walls for Pit-in-Pit Excavations

The technique of pit-in-pit excavations is increasingly used in China to provide an efficient solution of maximizing the utilization of underground space while minimizing the amount of solid waste due to excavations. One of the critical steps toward the successful implementation of the pit-in-pit excavation technique is to rationally estimate the passive earth pressure, which is typically done using approaches such as numerical modeling, field monitoring, or traditional Coulomb or Rankine earth pressure theories. This paper presents a simplified approach for computing passive earth pressures for pit-in-pit excavations. A trapezoidal-shaped failure wedge is formed between two levels of retaining walls. A complete symmetric soil arch is used to describe the stress field of soils in the rectangular zone, whereas a half arch with one base acting on the wall and another base acting on the inclined shear surface is proposed for soils in the triangular zone. The soil arching effect is explicitly considered to derive the earth pressure for both cohesionless and cohesive soils at any intermediate passive state. A parametric study is conducted to demonstrate that the mobilized passive earth pressure increases nonlinearly with the magnitude of wall movement and soil shear strength parameters. The allowable spacing between two walls is also defined to produce a design chart. In the end, the proposed model is assessed against experimental measurements from model-scale tests.