Ground loss due to circular tunnel deformation in sands

Considerable ground settlement can occur during tunnel construction. Shape of the settlement trough during tunnel deformation closely follows a normal distribution curve. However, most analytical solutions only approximately represent this profile because of the assumption of uniform radial displacement field around the tunnel during its deformation which presumably exists only at shallow depths. For large diameter tunnels and when non-uniform grout pressure is used, tunnel axis is prone to distortion making these solutions unreliable. This paper examines experimental results of tunnel collapse in small-scale centrifuge environment. Tunnel walls were represented by pressurised rubber membrane. Cover to diameter ratio of the tunnel and relative density of the sand were the only variables in the tests. All tests were conducted on the large diameter centrifuge at IIT Bombay. The results show that deformation above the crown and width of settlement trough, both increase near to the ground surface. The results are compared to data available in the literature and relationship between point of inflection and tunnel depth is derived.