Numerical Analysis of the Vertical Crown Displacements in Triple Adjacent Tunnels with Rock Bolts and Pipe Roofings

Abstract

This study aimed to investigate the effects of installing pipe roofings and rock bolts before bench cuts during the excavation of a tunnel. The limited space available during excavation resulted in the formation of triple adjacent tunnels. To solve the issue of narrow spacing between the tunnels, middle posts were added for greater stiffness, and pipe roofings were installed to prevent collapse in tunnel sections with shallow overburden where the rock weathering was significant. PLAXIS 3D 2018, a finite element analysis program, was used to simulate the wall rock displacement during the bench cuts with pipe roofings and rock bolts installed. In addition, the difference between the presence and absence of bias pressure was studied. It was found that, in the absence of bias pressure, the tensile and compressive forces were symmetric from side to side. However, under bias pressure, the tensile force remained unchanged. Moreover, the compressive force under bias pressure was three times greater than without bias pressure and was concentrated at the sidewall of the eastbound tunnel and the crown of the mass rapid transit (MRT) tunnel. The results may be helpful for the design and construction of multiple adjacent tunnels in the future.