Failure modes of highway tunnel with voids behind the lining roof

Abstract

Small-scale (1:30) model tests were performed to investigate the failure modes of highway tunnels that have single voids with different scales immediately above the lining roof. The tunnels employed have horseshoe-shaped cross-sections which match those typically used to carry two lanes of traffic. In the absence of a void, the mechanical behavior of the tunnel lining is directly related to the ratio of the horizontal to vertical stress, λ. Failure occurs when the compressive stress experienced is high, and the eccentricity is small. Furthermore, it is the shoulders and knees of the tunnel lining that are most susceptible to compressive failure. In the presence of a void, the sign of the bending moment can become reversed, and failure occurs due to bending-induced tension. The intrados of the roof lining is prone to moment-induced tensile stress. It experiences less thrust as well as higher eccentricity which is completely different from the behavior without a void. In other words, the void converts the tendency of the tunnel lining to undergo thrust-induced compressive failure into a tendency to undergo moment-induced tensile failure. When the void angle (angle subtended by the void at the center of the tunnel) is larger than 50°, the failure mode of the tunnel lining is entirely dependent on the void, rather than the earth pressure conditions. That is, the locations and sequence of formation of the cracks formed are almost identical, regardless of the value of λ. Understanding the differences in these failure modes (i.e. with and without voids), should assist in making tunnel maintenance much more efficient. The tunnel lining’s most vulnerable positions can be made less vulnerable by careful choice of the tunnel’s structural parameters, and other supporting measures can be implemented to ensure the safety of the tunnel.