| A systematic, semi-theoretical method for predicting settlement due to tunnelling in cohesive-frictional soils is presented. With the method, the soil deformation at the tunnel crown can be calculated. Using several case histories, the validity of the method for various ground conditions is examined. In addition, the relationship between the crown deformation and surface settlement is identified, as well as the relationship between the width of the surface settlement trough and the depth of burial of the tunnel.; The method used to estimate the crown displacement is the gap parameter method. The gap parameter method takes into account the settlement near the crown of the tunnel due to (i) the three-dimensional, elasto-plastic deformation at the tunnel face, (ii) the geometry of the shield and lining system, and (iii) the effect of workmanship. Originally, the method was developed to compute the crown deformation of tunnels in clays (undrained condition). Cohesive-frictional soils were accommodated by modifying an existing closed form solution to calculate the unrestricted, plane strain crown deformation using effective stress strength and deformation properties. The limitations of the closed form solution were investigated using finite element analyses for both the undrained and effective stress conditions. Comparison of the calculated crown displacements and observed displacements reported in several case histories show reasonable agreement. The range of surface settlement to crown displacement ratio is shown to be remarkably narrow considering the diversity of the ground conditions (ranging from soft clay to dense sand). In addition, a reasonable or conservative estimate of the trough width can be made based on the ratio of depth to tunnel radius. |
