Investigation on the Cause of Damages of a Deep Tunnel

Abstract

A study is presented on the damages that developed in the segmented precasted concrete liner of a deep tunnel excavated by means of a tunnel boring machine (TBM). Various possible causes are considered i.e., the pore-water pressure acting on the liner; the excessive mountain pressure; the anisotropic in situ stress field; and the incomplete filling of the rock-lining gap. The in situ investigation, carried out to quantitatively assess the level and the extent of the damages, ruled out the pore pressure as the main source. In fact, minor water leakages were observed at the fractured sections. The remaining alternatives were analyzed through a series of nonlinear finite-element calculations in the plane strain regime. They account for the strain softening behavior of the lining and for the lack of tensile stresses at the rock-lining interface. The results give insight into the rock-liner interaction and show that the observed damages are likely to depend on the incomplete filling of the gap. On this basis some conclusions are drawn on the stresses developing within the liner and on the procedures suitable for its structural rehabilitation.