Effects of tunnelling on existing support systems of intersecting tunnels in the Sydney region

Full three-dimensional (3D) elasto-plastic finite element analyses are conducted to investigate the effects of tunnelling on existing tunnel support systems, i.e. shotcrete lining and rockbolts, of orthogonally intersecting tunnels in the Sydney region with a high horizontal regional stress regime. It is found that the zone of influence extends about 2.5 tunnel diameters (2.5D) from the intersection. During driving of the new tunnel, the existing support system in the zone of influence is affected according to the progression of the new tunnel face to/from the existing tunnel and the effects are not significant when the new tunnel face is located 4.5D from the intersection. The approach of the new tunnel face towards the existing tunnel causes the existing shotcrete lining on the approach side of the existing tunnel to undergo tensile cracking, while neither further compressive failures nor further tensile cracking is observed when the new tunnel is driven away from the existing tunnel. It is suggested that temporary reinforcement should be installed inside the zone of influence during approach and local thickening of the shotcrete lining of the existing tunnel is necessary in locations immediately adjacent to the intersection.