The effects of excavating twin tunnels during cyclic loading on the progressive settlement of existing footing

Abstract

Tunnels may be constructed in a pair undercrossing existing geotechnical structures exposed to cyclic loading. Such construction changes the stress state in the ground in different ways regarding the tunnelling sequence and would lead to large settlement or even failure to those structures under cyclic loading. Large physical model tests were carried out to examine the impact of excavating twin tunnels during cyclic loading on the progressive settlement of footings. The vertical soil stress, footing settlement and ground deformation were analysed regarding different construction sequences and spacings between twin tunnels. The results indicate that previous cyclic loading history affects the alteration of soil stress caused by tunnelling activity. Compared with excavating twin tunnels successively, excavating the tunnels in sequence leads to greater settlement caused by the subsequent cyclic loading. The change in the relative density of soil is invalid to explain the enlargement of the settlement. Finite element analysis was performed to investigate the mechanism of the effects of excavating twin tunnels during cyclic loading on the footing settlement. It was found that the increase of footing settlement in the subsequent cyclic loading and the greater settlement in the sequential tunnelling case are attributed to the greater stress ratio (q/p) in the ground.