Stability of Circular Tunnels in Clay with an Overlay of Sand

Abstract

In the present study, numerical solutions have been developed to determine the internal compressive normal pressure required to be offered by the support system for maintaining the stability of long, circular tunnels located in clay with an overlay of sand. The compressive normal pressure is assumed to be uniform on the periphery of the tunnel, which can be offered by means of lining. The solutions were obtained on the basis of a finite-element lower-bound limit analysis and second-order cone programming. For different angles of internal friction of sand, the solutions have been obtained by varying the ratio of unit weight of the upper sand layer relative to that of the lower clay layer. The magnitude of required support pressure is found to be decreasing continuously with an increase in the internal friction angle of the overlying sandy soil for a given cover of the tunnel. The increase or decrease in the magnitude of support pressure required in the layered soil medium as compared to the case of the homogenous clay layer with properties same as that of the lower clay layer depends on the combined influence of the unit weight ratio, the thickness of both layers, and the angle of internal friction of the overlying sand layer. The parametric design charts are developed, which can be utilized by practicing engineers for determining the thickness and stiffness of lining for circular tunnels excavated in the clay underlying sand layer.