Progressive deformation and mechanical behaviour of intersecting tunnels in soft ground

Stress concentrations and large ground deformations are sources of major concern during the construction of tunnel intersections. Such problems are even more critical in urban areas with buildings affected by the induced progressive ground movements. Therefore, particular considerations have to be taken into account for such projects. This paper deals with the effects of the underground openings on primary tunnels (tunnel intersection) using detailed three-dimensional finite-element models on a case study – Hakim tunnel project, Tehran, Iran. The effects of different stages of construction of intersections on stress distribution and ground deformation are evaluated. Moreover, the effective trailing distance of different faces of the tunnel from the centre of the intersection and the effect of tunnel drilling on internal forces of the shotcrete lining are discussed. The results showed that the values of the surface settlement remain almost unchanged at a distance approximately twice the equivalent diameter of the child tunnel from the intersection centreline. Also, the increase in the circumferential bending moments near the intersection is non-significant compared with the changes in the longitudinal bending moment. It was found that the circumferential and longitudinal bending moments next to the intersection can increase by up to 250 and 350%, respectively.