The Effect of Tunnel Advance Rate on the Surface Settlements

We investigate numerically the evolution of surface settlement during continuous tunnel excavation by formulating and solving the poro-elasticity equations in a frame of reference, which is fixed to the moving face. In tunnelling through low-permeability soils, consolidation processes take place in general simultaneously with the spatial stress redistribution in the vicinity of the advancing face. Apart from the borderline case of highly permeable ground or very slow excavation, the excavation advance rate must be taken into consideration. One can readily verify by a dimensional analysis that the time-development of settlement is governed by the ratio of advance rate to ground permeability. The higher the advance rate, the lower the surface settlements and the angular distortion will be.