Method to evaluate the long-term surface movements by tunneling in London clay

Long-term ground movements occur after tunnel construction in clayey soils due to dissipation of excess pore pressure generated during construction and a new seepage state that depends on the drainage conditions of the tunnel. A methodology was proposed in the past to evaluate the long-term surface settlements for tunnels in London clay. This paper extends the previous work by proposing a new method to estimate both consolidation-induced vertical and horizontal displacements, as well as consolidation-induced peak horizontal strains. In this method, a new relative soil-lining permeability index, which was modified from the earlier work, is proposed by assuming a radial seepage flow into a tunnel. Using this index along with tunnel geometry and soil properties, a series of equations was derived to better facilitate the calculation of the long-term ground displacements and strains compared to the normalized charts adopted in the previous evaluation method. The applicability of the proposed method is tested against two case histories in London.