A simplified two-stage method to estimate the settlement and bending moment of upper tunnel considering the interaction of undercrossing twin tunnels

With the expansion of existing networks of underground system, many metro tunnels are excavated in a pair undercrossing existing tunnels. The new excavations will affect the stress distribution and settlement behavior of the existing tunnels, particularly when the clearance is small. Generally, the construction of the second undercrossing tunnel induces more settlement in the upper tunnels compared to that caused by excavating the first one, due to the interaction between the new excavations. The degree of interaction makes it difficult to predict the settlement of existing tunnels. In this paper, a simplified two-stage analysis method considering the interaction between the undercrossing twin tunnels is proposed to estimate the induced response of existing upper tunnel. In the first stage, the greenfield settlement caused by the first excavation is estimated using Peck’s empirical function, and that caused by the second excavation is modified with a modification factor to consider the influence of the first tunnel. In the second stage, the additional stress calculated using the greenfield displacement is imposed on the existing tunnel resting on Pasternak foundation model. The calculated settlement profile and bending moment of the existing tunnel using the two-stage analysis method are compared with those obtained from finite element analysis and historical field monitored data. The effects of the characteristic parameter of Pasternak foundation model, the spacing between the new twin tunnels, and the modification factor of greenfield settlement, on the settlement profile and bending moment of the existing tunnel are also investigated.