A method of estimating surface settlement above tunnels constructed in soft ground

A technique suitable for the analysis of lined tunnels constructed in soft soil is described. This technique permits consideration of the soil-lining interaction, lining weight, and plastic failure within the soil. Particular attention is given to the simulation of loss of ground including the annular void created by the difference between the tunnelling machine and lining diameters. The analysis is used in a parametric study to identify potentially significant factors affecting the prediction of surface settlement due to tunnel construction. Consideration is given to the effects of elastic modulus; the distance between the tunnel and the underlying rigid base stratum; plastic failure; initial stresses; ground loss (including the annular void); and the injection of clay grout into the tailpiece void. The results of this analysis indicate that elastic anisotropy, ground loss, and clay grout injection are all important factors that are not usually considered in analyses. In particular, a parameter called the gap is defined in the paper and is used in an attempt to incorporate practical tunnelling experience into the calculation of surface settlement. Furthermore, it is shown that in predicting surface settlements underestimation of the true elastic modulus below the tunnel may decrease the predicted surface settlement and lead to overestimation of the invert heave. Underestimation of the modulus above the tunnel may also decrease the predicted surface settlement for situations where there is limited ground loss and extensive plastic failure.