Influence of tunnel failure on the existing large-diameter tunnel in improved soil surround

Abstract

In this paper, the effect of failure of a temporary opening on the stability of nearby existing large diameter tunnel in cement-treated soils is discussed. Firstly, the centrifuge model set-up and procedure for studying the interaction of large diameter parallel tunnels in improved soil surround were developed. The stability of existing first tunnel, stress-redistribution around the first tunnel and influence zone due to the failure of second tunnel were studied. The results of the centrifuge tests were then used to develop a numerical model in a geotechnical finite element software package GeoFEA version 9 for parametric studies. The dominant failure mode of improved soil surround, stress redistribution in surrounding soil as well as the surface deformations obtained from the finite element analysis were found to be similar to the observations from centrifuge tests. The influence of failure of second tunnel was evident when the spacing between the two tunnels was kept as 2.3 times the tunnel diameter even for the stronger and thicker first tunnel. On the other hand, the influence of failure of second tunnel was found to be negligible when the spacing between the tunnels was increased to 3 times the tunnel diameter, for both the stronger and weaker first tunnel. The distortion in the first tunnel was observed as asymmetric ovalization for centre-to-centre spacing of 2.3 diameters or less, but was absent for spacing of 3.0 diameters with cover-to-diameter ratio of 1.25. Larger spacing is necessary for deeper tunnels to eliminate the interaction effects due to failure of nearby tunnel.