Analysis of internal forces in tunnel lining for twin piggy back tunnel geometry

Abstract

Constructing metro tunnels in urban areas offers a sustainable solution to increasing traffic demands while supporting socio-economic development. In the design phase, metro tunnels are typically arranged as two closely spaced, parallel tunnels to optimize operational efficiency. However, during construction, mechanical interactions between the tunnels can alter the internal forces acting on their linings. This study uses the finite element method to analyze internal forces in the linings of two vertically aligned, parallel tunnels. The analysis focuses on normal forces and bending moments, taking into account the construction sequence.Results show that the sequence of construction significantly influences internal force distribution, with the first tunnel being affected by the excavation of the second. The study also examines the impact of volume loss, tunnel depth, and spacing. Across these scenarios, the findings consistently indicate that the tunnel constructed first—whether upper or lower—is subjected to increased normal forces and bending moments due to the excavation of the second tunne