Three-Dimensional Modelling of Twin Tunnelling with Different Skew Angles in Two-Layered Soils

Research has been carried out to study the effects of new tunnelling on an existing adjacent tunnel to ensure the safety and serviceability of tunnels. Prior studies on twin-tunnel interaction have mostly centred on simplifying perpendicularly crossing tunnelling in a single-layered soil stratum. New tunnel excavation beneath an existing tunnel at different skew angles in two-layered strata can lead to different patterns of stress redistribution and adverse impacts on the existing tunnel. In this paper, results of three-dimensional centrifuge and numerical modelling carried out to study the twin-tunnel interaction with varying advancing orientations and layered soils will be reported. The influence of new tunnel excavation on an existing tunnel was simulated in-flight by controlling both the tunnel weight and volume losses. An advanced hypoplastic constitutive model that can capture stress-, path-, and strain-dependency of soil behaviour is utilised for numerical back-analyses and parametric studies. Cases investigated include twin-tunnel interaction at three different skew angles (30°, 60°, 90°) in a uniform sand layer and at skew angle of 90° in two-layered sand with different relative densities and thicknesses. Distinct load redistribution patterns will be presented to explain deformation mechanisms of the existing tunnel at different tunnel advancing skew angles to highlight the effects of tunnelling orientation. The results of perpendicularly crossing tunnelling in two-layered sand will also be reported and compared to reveal the influence of layered soil. The findings and new insights can help engineers better estimate advancing tunnelling effects on existing tunnels and enhance the safety of tunnel construction.