Failure mechanism analysis for tunnel construction crossing the water-rich dense fracture zones: A case study

For deep tunnel crossing the fracture zones with water-rich strongly weathered granite, water inrush and collapse disasters occur frequently. Based on a high-risk tunnel under construction, this research tests and analyzes the engineering properties and the spatial distribution characteristics of unfavorable geological structures through the innovative comprehensive unfavorable geological exploration technology composed of UAV with lidar-high power geophysical prospecting technology-TSP advances geological prediction test. Then the space-time evolution law of tunnel vault settlement is analyzed and the exploration results of unfavorable geology is checked out based on the tunnel field test. Then, the fluid-solid coupling numerical calculation model for deep-buried tunnel crossing the densely distributed fracture zones with rich water by three benches seven steps method is established. It is found, under the influence of coupling disturbance of multiple fracture zones, the plastic zone area of the surrounding rock increases sharply, which is 4-5 times of the plastic zone area in the stable tunnel sections. And the pore water pressure in the surrounding rock will fluctuate more significantly when crossing the fracture zone with the larger mileage stake number (the larger the serial number), the pore water in the surrounding rock will flow more rapidly, and high-pressure pore water in the rock mass at the far end is more likely to converges into the tunnel arch foot and the lower part. Compared with other excavation parts, tunnel upper bench and tunnel face will become the main water gushing positions during tunnel construction, which are the best positions for the tunnel directional advance drainage. Advance small conduits grouting shall be carried out at the tunnel vault, and secondary grouting shall be carried out at the tunnel vault, and accurate grouting shall be carried out at the tunnel arch foot and the lower position, which can effectively reduce the occurrence of tunnel disasters such as tunnel collapse, mud inrush and water inrush.