AN EVALUATION OF SIMPLIFIED TECHNIQUES FOR ESTIMATING 3-DIMENSIONAL UNDRAINED GROUND MOVEMENTS DUE TO TUNNELING IN SOFT SOILS

A number of simplified approaches have been used by various researchers to estimate the three-dimensional stress changes and ground deformations due to shallow tunnels in soft soils. The effectiveness of these simplified analyses, such as (i) axisymmetric analysis, (ii) longitudinal plane strain analysis, and (iii) empirical cumulative probability distribution approach, has been examined by comparison of results with those from a full three-dimensional elastoplastic finite-element analysis. For tunnels located at shallow depths, axisymmetric analyses are generally found to be unable to predict the correct magnitude of displacement around a tunnel heading. However, the trend of the three-dimensional deformation behaviour near the tunnel heading can be reasonably approximated by the average of the normalized displacement curves predicted by the two axisymmetric assumptions as described in this paper. The three-dimensional distribution of the displacement near the ground surface, on the other hand, can be approximately predicted by the cumulative probability approach, provided that the three empirical parameters required by the equations are reasonably estimated. In this paper, these parameters were estimated from the result of a two-dimensional plane strain cross-sectional analysis. Finally, it was found that modelling of three-dimensional deformations by a longitudinal plane strain analysis did not give good results for the cases examined.