Study On The Soil Disturbance Mechanism Of Shield Tunnelling In Sandy Cobble Stratum

It has developed into a trend that using shield tunnel method to build the city metro tunnel. The geological engineering condition of strata that shield passes through is an important factor which affecting the safety of shield tunnel construction. China is a vast country with many complex geological conditions, and shield tunnel construction will face many technical problems that need to be solved when passing through these special and complex geological conditions. Sandy cobble stratum is a typical mechanical instability stratum and has characteristics like large interface between particles, no cohesion, point to point in force transmission under anhydrous condition and sensitive in ground disturbance response. The equilibrium state of the original stratum can easily be destroyed by shield tunnel construction and produces larger soil disturbance and ground loss. Under the latter effects such as groundwater, ground repeated loading, the ground loss will gradually develop to the ground surface, causing the surface to collapse.This thesis takes the city metro tunnel using shield tunnel method passes through sandy cobble stratum as engineering background, combining with the "Long-term safety and prediction method of underground structures under complex environments" and "Research on the soil disturbance mechanism of shield tunnelling in sandy cobble stratum" separately supported by National Basic Research Program of China (973 Program) and Innovation Foundation of Southwest Jiaotong University. Theoretical analysis, laboratory test, numerical simulation and other research methods are synthetically adopted to discuss the soil disturbance mechanism and soil disturbance characteristics of shield tunnelling in sandy cobble stratum. The main results obtained are as follows:1、The soil disturbance mechanism of shield tunnelling and formation reasons of delayed settlement in sandy cobble stratum is analyzed; A two-dimensional discrete element analysis model is established and obtains the soil disturbance characteristics of the ground at the excavation face and surrounding the tunnel, and on this basis the soil disturbance partition of shield tunnelling in sandy cobble stratum is proposed. The development process and influencing factors of delayed settlement is analyzed from the meso-level, and control measures are proposed to reduce the delayed settlement.2、A shield tunnelling test system, including 0800mm Earth-Pressure-Balance shield machine, shield starting device, segment lining structure and other ancillary equipments, has been developed. Combined with the laboratory tests, the physical and mechanical parameters, particle grading curve, sediment improvement and ground reinforcement solutions of the test stratum are obtained. Formulas of shield tunnelling parameters that suitable for the sandy cobble stratum are established according to the sandy cobble stratum properties.3、Laboratory test using the Φ800mm EPB shield tunnelling test system is carried out to analyze the soil disturbance characteristics of the shield tunnelling at different stages from the stratum horizontal and vertical section, and obtains the deformation characteristics and disturbance range at different buried depths. The change characteristics of the tunnelling parameters and ground deformation dynamic response after taking measures as sediment improvement and ground reinforcement solutions are analyzed and the relationship between tunnelling parameters at different stages are also discussed. Some important tunnelling parameters have been comparative analyzed and establish the mathematical model of different tunnelling parameters.4、EPB shield tunnelling analysis model, including the earth cabin, shield cutters, screw conveyor and tunnelling soil bin, is established using the particle discrete element method (PFC), and the whole process of shield tunnelling in sandy cobble stratum is simulated. The disturbance range, settlement morphology and ground movement characteristics are analyzed from the perspective of soil particle displacement, contact and ground stress, and establish the relationship of tunnelling parameters such as shield advancing speed, cutter rotation speed and unearthed rate.With the further opening the authority of city metro construction, more and more cities will join a new wave of metro construction and it will also face more complex technical challenges. The results of this thesis could provide reference on the impact of shield tunnelling on the soil disturbance, especially on the discrete stratum like the sandy cobble stratum, and protect the shield tunnelling safety and reduce the impact on the surrounding environment.