Predictions of ground deformations in shallow tunnels in clay

Twenty-eight tunnels are used to evaluate predictions from an analytical solution for shallow tunnels in saturated ground. The solution assumes plane strain conditions at any cross-section perpendicular to the tunnel axis and poroelastic behavior of the ground and elastic behavior of the liner. Stresses and deformations are obtained with this method for short- and long-term conditions anywhere in the continuum. Of particular interest to this study are the short-term surface settlements at the ground surface. Comparisons between predictions and observations from actual tunnels show good agreement, generally within 15% difference. The analyses show that: (1) most of the ground movements are caused by the gap parameter, which is a measure of the three-dimensional deformations at the tunnel face, the physical gap between the liner and the perimeter of the excavation, and of the workmanship; (2) most of the ground deformations take place within a distance of three to four radii around the tunnel; (3) the bottom boundary of zero vertical deformation should be placed at a distance of two tunnel diameters below the tunnel center line, or at the location of a stiff soil layer, whichever comes first; (4) the horizontal movements around the tunnel are relatively smaller than the vertical movements; and (5) the analytical solution tends to underpredict the maximum soil deformations and overestimate the settlement trough; however, with an appropriate estimation of the gap parameter and small soil yield, the differences between predictions and observations are small.