Experimental studies on effect of voids behind tunnel linings on progressive failure process of tunnel structures

The voids behind the tunnel linings seriously affect the interaction between the linings and the surrounding soils, generally in the form of cracking, spalling or even failure, directly affecting the safety of tunnel operation. The experimental investigation is conducted to examine the effect of the twin voids between the surrounding soils and the tunnel linings at the crown and spandrel on the progressive failure process of tunnel linings, the thrust forces and the bending moments in the tunnel linings. The results show that: (1) The stress state of tunnel linings is greatly affected by the presence of twin voids, which can induce the shortage of load carrying capacity of tunnel structures. (2) As the loading increases, the cracks are noted to propagate firstly at the inner fiber of the tunnel invert, secondly at the inner fiber of the linings between the twin voids, thirdly at the outer fiber of the lining on the right side of the void located at the spandrel diagonally above the tunnel, fourth at the inner fiber of the linings at the left side of the void at the crown, then at the outer fiber of the lining at the left arch spring, finally at the outer fiber of the linings at the right haunch. (3) The changes in void sizes significantly affect the distribution of internal force of the linings. The increase in void sizes results in a decrease in the magnitude of thrust forces in the linings between the twin voids, while the magnitude of bending moments between the twin voids increases as the void sizes increase, which then causes significant damage to the tunnel linings between the twin voids. (4) The failure process of tunnel linings is greatly affected by the location and quantity of voids. The propagation laws of lining cracks are even more complicated under the influence of twin voids. These studies can provide reference for preventing and repairing lining cracks influenced by the multiple voids behind the tunnel linings.