Comparison of rectangular and circular bored twin tunnels in weak ground

Abstract

The recent innovation of a rectangular tunnel boring machine (TBM), and its use in the Hongzhuan Road tunnel underpass by the China Railway Engineering Group (CREG), has revitalized shallow depth soft soil tunneling. This paper presents the findings of a numerical study using PLAXIS to determine the surface settlements and moments produced in tunnel linings for circular and rectangular twin tunnels. The effects of the relative positions of twin tunnels, critical distances, volume losses, depths of burial, and tunnel sizes for both circular and rectangular tunnels are the key parameters of this investigation. The results indicate that rectangular tunnels are suitable for shallow depths in weak ground as they have lesser settlement compared with circular tunnels. This is crucial for tunneling beneath important structures such as railway lines and existing roads. However, the maximum bending moment produced in the rectangular tunnel lining is higher than that for circular tunnels. The use of rectangular TBMs is an unconventional method in modern day tunneling; however, the analysis in this project recommends that tunnel industry engineers consider this method for shallow depth weak ground tunneling.