۱- Introduction

۲- Analytical solution

۳- Verification

۴- Parametric analysis

۵- Case study

۶- Conclusions

References

Abstract

This paper proposed an analytical method to study the mechanical behaviours of the existing tunnel due to new tunnelling below. The coefficient of subgrade reaction of the existing tunnel can be considered as a variation instead of a constant. The resultant deflections of the existing tunnel can be calculated by a superimposed method. The proposed method is verified by the finite element method. The influences of the input parameters on the deflection of the existing tunnel are studied. Based on a below-crossing project in Beijing, the results obtained by our method match well with the monitoring data.

Introduction

With the increase of subways constructed in densely built-urban areas, the cases of a new tunnel excavated below the existing tunnel are commonly encountered. The key issues of such adjacent tunnelling project are to guarantee the safety and serviceability of the existing tunnel during new tunnel construction. Due to the inherent complexities of the soil-tunnel interactions, it is a great challenge to study the mechanical behaviours of the existing tunnel. The additional movements of existing tunnel induced by adjacent tunnelling have been reported by some researchers using the field measurements [1–۷]. The numerical analyses [8–۱۲] and physical model tests [13–۱۶] are also used as powerful tools to gain insights into the mechanical responses of the existing tunnel. The analytical method, using the theory of beam on elastic foundation, provides a simple and efficient way to study the interactions between the new and existing tunnels. Some studies [17–۲۰] have been performed to investigate the responses of above-crossing tunnelling on the existing tunnel. It is noted these methods are unavailable for the cases of tunnelling below the existing tunnel. The coefficient of subgrade reaction at and adjacent to the intersection of the existing and new tunnels decrease due to excavation. For a specific case of tunnelling below the existing tunnel without clearance, Liu et al. [21] proposed a superposition method to analyse the mechanical behaviours of the existing tunnel. To address the general issue of new tunnelling below existing tunnel with clearance, a superposition method based on the theory of beam on elastic foundation are adopted to study the mechanical responses of the existing tunnel. The major contribution of this research is that the coefficient of subgrade reaction along the existing tunnel can be considered as a function instead of a constant. The results obtained by our method are in agreement with those by numerical simulation. Parametric analyses are also performed to study the soil-tunnel interactions on the mechanical behaviours of the existing tunnel. Moreover, the mechanical responses of the existing twin tunnels due to new twin tunnels excavation in Beijing are investigated. The results of the deflections of existing twin tunnels obtained by the proposed method are in accordance with the on-site monitoring data.