Evaluating closely spaced twin-tunnelling-induced shear stiffness change by subsurface settlement in clayey ground

Abstract

Cases of closely spaced twin tunnels demonstrate that ground settlement above the 2nd tunnel, referred to as T2 constructed after the first tunnel (T1), exhibits pronounced differences from that above T1. The differences can be attributed to the disturbance induced by T1 construction. Based upon closed-form plasticity solutions for an unloading cylindrical cavity, this study employs subsurface settlement troughs to determine the shear modulus change during twin-tunnelling. Observations of 1 g model tests for twin-tunnelling in overconsolidated kaolin clay are interpreted from literature. After T1 construction, for positions with different radial distances but a constant angle with respect to the vertical line above the tunnel center, the graph of radial displacement/tunnel radius versus tunnel radius/radial distance exhibits good linearity. Further analyses demonstrate that the shear stiffness decreases as the angle increases. In comparison, during T2 construction, the shear modulus of soils close to T1 degrades, while the other side increases. In light of the observations of combined axial–torsional tests, the tendency of shear modulus change can be attributed to the rotation of principal stress axes. The findings can explain the differential settlements over closely spaced twin tunnels in clayey ground.