PREDICTION OF GROUND SUBSIDENCE DUE TO SHALLOW TUNNELLING IN SOFT GROUND

Abstract

Metro underground tunnels are built usually with a low overburden and close to the surface and beneath populated areas, which causes the consequences of failure. Assessment of tunneling on above ground structures has therefore become a very important and a crucial issue in metro cities. In the case of shallow bored tunneling, the soil tends to move inside the tunnel due to release of in-situ stresses that inevitably causes ground movement, which reflects at the ground surface in the form of a subsidence trough. Any structure within this zone of subsidence is likely to experience some movement; in some cases even resulting in distress in the structure. In the present study, the problem of ground subsidence due to shallow tunneling has been investigated with main emphasis on simulation of tunnel excavation process and tunnel-structure interaction has. Empirical rules for prediction of ground movements due to tunneling can lead to reasonably good estimation of ground subsidence profiles but only in Greenfield condition, in situations with similar past experience and with average ground conditions. However presence of surface structure modifies the subsidence trough and an appropriate tunnel-structure interaction analysis becomes necessary for realistic assessment. In the present study an investigation has been made for predicting the ground response due to tunneling by using the Finite Element Method as it can deal with factors like soil non-linearity, anisotropy, dilatancy, excavation simulation and tunnelstructure interaction etc. Elliptical Convergence Method based on prescribed displacement approach of linear finite element analysis has been used for prediction of ground subsidence due to tunneling. A parametric study has been carried out within the framework of elasto-plastic material behavior to have an insight into the effect of: elastic properties, and diameter and depth of tunnel on the subsidence trough. The Drucker-Prager yield criterion with non-associative flow theory of plasticity has been adopted. An attempt has been made to study the behavior of soil mass with respect to stresses developed for the case of a single tunnel in green field condition as well as for twin tunnels with surface structure