Stability Analysis And Control Of Rock Mass During Subsea Tunneling In Unfavorable Geological Conditions

With the fast construction pace of transportation infrastructures in China, underwater tunnels have become an effective means for crossing rivers, lakes and oceans and have received intensive attention in academia and industries. Compared with mountainous tunnels, the undersea tunnels are technically more challenging due to the difficulty in field exploration, the impacts of high hydro-static and hydro-dynamic pressures, the difficulty in drainage design and water infiltration prevention, and the poor stability of rock mass due to the low arching effects in water. The stability control of rock mass in heavily weathered zones is the most critical task of undersea tunneling and is a key technique for undersea tunneling. This dissertation focuses on the stability analysis and control of heavily weathered rock mass during undersea tunneling with the first undersea tunnel, Xiamen Xiang’an tunnel, in China as a test-bed. Using theoretical analysis, numerical simulation, laboratory testing, and field instrumentation, the research is conducted in the following five major parts:1. The mechanism of the water infiltration during subsea tunneling is explored related to the water infiltration along the interface of different substrata, the infiltration within the fill material within the weak zones, the infiltration due to the over deformation of surrounding ground, and the infiltration due to the local ground failure. Based on theoretical analysis and numerical simulation, the impacts of groundwater on excavation stability during subsea tunneling are analyzed. A fluid-solid coupled model is developed for analyzing the instability modes, the instability mechanism, and the major impacting factors of the rock mass stability. The deformation, stress distribution, and plastic zone of rock mass during excavation are examined. It finds that the seepage has a large impact on the rock mass deformation, especially during the excavation of pilot pits within both sides of the tunnel. Pre-excavation grouting can significantly reduce the permeability of rock mass and improve the rock mass strength and modulus. As such, it is an effective way for excavation stability control during undersea tunneling.2. Based on the characteristics of the heavily weathered rock mass around Xiang’an tunnel, this research develops a complete set of grouting technology including determining different grouting methods and techniques for different ground conditions, selecting suitable grouting materials and composition of the materials, determining the grouting parameters for different ground conditions, developing new grouting machines and developing standards for grouting effect evaluation. The grouting technology is successfully applied to the heavily weathered F1 zones in Xiang’an tunnel.3. The field exploration technology with a combination long and short distance exploration methods is proposed for identifying the unfavorable geotechnical conditions. Based on the study of the relationship among tunnel crown deformation, ground surface deformation and ground cracking using the instrumentation data, the limiting value of seabed deformation is determined to prevent seabed cracking and water infiltration during tunneling. Based on the optimization of excavation sequence and excavation support design of double-sided pilot excavation and CRD excavation, the stability control technique is developed for large excavation of undersea tunnels in heavily weathered rock mass. The water infiltration prevention and emergency mitigation measures are developed for construction safety.4. Based on the water prevention and drainage design method, the relationship between the permeability of grouted rock mass, the permeability of tunnel liner, the flow rate of water infiltration, and the pressure on the tunnel liner is explored. The soil and water pressure distribution between the initial and finial tunnel liner is determined. The design guidelines for the liner of Xiang’an tunnel in unfavorable geotechnical conditions are proposed for preventing excavation instability and maintaining a safe and effective operation environment.