Tunnel Interaction in Low-Permeability, Squeezing Ground

Tunnelling through squeezing ground may induce deformations and stress redistributions in a large area, thereby affecting the later construction of neighbouring underground structures. In general, the tunnelling-induced stress redistributions cause the surrounding ground to experience plastic yielding, which may be unfavourable for a later constructed tunnel. In low-permeability, water-bearing ground, however, tunnel excavation additionally induces pore pressure relief over an extensive area, wherein the effective stresses and the undrained shear strength increase, thereby generating more favourable conditions for later works – an effect analogous to advance drainage. Considering the example of two parallel tunnels, constructed consecutively with a shielded tunnel boring machine (TBM), the present paper numerically evaluates the effect of the first on the later excavated, second tunnel in the cases of dry and water-bearing, squeezing ground. The investigations demonstrate radically different effects. In dry ground, the interaction between the two tunnels is minor. Conversely, in low-permeability water-bearing ground, the consolidation preceding the second tunnel excavation leads to a remarkable reduction (even up to 90%) in the rock pressure developing on the TBM shield, and thus also in the required thrust force to overcome shield skin friction. 1. INTRODUCTION The term "squeezing" in tunnelling refers to the phenomenon of large rock deformations or, equivalently, the development of large rock pressures on the tunnel support that hinders said deformations. Although squeezing may manifest itself rapidly upon tunnel excavation, in most cases it is markedly time-dependent due to rheological processes (creep) or consolidation processes in the case of saturated, low-permeability ground (Barla 2001; Anagnostou and Kovári 2005; Anagnostou 2007). The tunnelling-induced deformations and stress redistributions in squeezing ground may be substantial and affect an extensive area, and thus also other existing or planned underground structures within it. In the case of a twin tunnel, for example, the excavation of the 1st tube may affect the stress field at the location of the subsequently excavated 2nd tube, and hence the rock deformations during its construction and the rock pressure on its support. And vice versa, the later construction of the 2nd tube may affect the rock pressure acting upon the lining of the 1st tube.