Effects of rock pillar width on the excavation behavior of parallel tunnels

Most of the road tunnels were designed as a twin tunnel dual-carriage system with one traffic direction in each tunnel. However, for some long road tunnels, three-parallel-tube tunnel was required to meet the needs of construction and operation. The main aim of this paper is to numerically investigate the effects of rock pillar width on the behavior of three-parallel-tube and four-parallel-tube tunnels during excavation. The results showed that the interaction effects were related not only to the geological condition and tunnel cross-section, but also the rock pillar widths. As the pillar width reduces, the interaction effects becomes more evident. When the rock pillar width was less than two and a half times the radii of the main and pilot tunnels in adverse geology section, grouting for rock mass was required to improve the stability of the tunnel. For twin-tube tunnel, if the rock pillar width near the portal section were too narrow then the central-pillar-gallery tunnel may be constructed to reduce this effect.