DEFORMATION DAMAGE OF SHALLOW TUNNELS AND CAVE-INDUCED GROUND COLLAPSE IN ARCHEOZOIC GNEISSIC GRANITES-THE JINING TUNNEL AS CASE EXAMPLE

Shallow tunnels in ancient granites are often in tensile stress state and have the challenges of instability in vault faces and local ground collapse. This paper takes the Jining Tunnel as one example and carries out the engineering geological survey, laboratory tests and surrounding rock stability simulation. It analyzes the stability of the surrounding rock from three aspects of granite weathering zoning, rock structure controlling and rock mass qualifying. Surrounded by slightly-moderaltely weathered granites, most of the tunnel is located below the groundwater level. The rock joints are extensively filled with mud. Deformation and damage in forms of block down and cave was often occur. The differential deformation and uneven settlement occurred at the junction of the paleo-weathering granite crusts with the upper Tertiary mudstone. Four collapse pits are emerged due to collapse of caves in shallow depths. The stress monitoring results show that the contact pressure of the top arch is less than the gravity stress, and the lateral pressure is smaller, which is unfavorable to the vault stability. In order to ensure the safety of personnel and machinery equipment, supporting measures with strengthening are undertaken, which prove to be of positive engineering significance.