The interaction between yielding supports and squeezing ground

In this paper we investigate the interaction between yielding supports and squeezing ground by means of spatial numerical analyses that take into account the stress history of the ground. We also present design nomograms which enable the rapid assessment of yielding supports. The idea behind yielding supports is that squeezing pressure will decrease by allowing the ground to deform. When estimating the amount of deformation required, one normally considers the characteristic line of the ground, i.e. the relationship between the ground pressure and the radial displacement of the tunnel wall under plane strain conditions. The computation of the characteristic line assumes a monotonic decrease of radial stress at the excavation boundary, while the actual tunnel excavation and subsequent support installation involve a temporary complete radial unloading of the tunnel wall. This difference, in combination with the stress path dependency of the ground behavior, is responsible for the fact that the results obtained by spatial analysis are not only quantitatively, but also qualitatively different from those obtained by plane strain analysis. More specifically, the relationship between ground pressure and deformation at the final state prevailing far behind the face is not unique, but depends on the support characteristics, because these affect the stress history of the ground surrounding the tunnel. The yield pressure of the support, i.e. its resistance during the deformation phase, therefore proves to be an extremely important parameter. The higher the yield pressure of the support, the lower will be the final ground pressure. A targeted reduction in ground pressure can be achieved not only by installing a support that is able to accommodate a larger deformation (which is a well-known principle), but also by selecting a support that yields at a higher pressure.