Study On Tunnel Face Stability And Grouting Reinforcement For Tunnels Overlying Water-rich Sand Stratum

With the prosperity and rapid development of China's economy,the population increases significantly and a large number of people flowed into the big cities.And the constructions of metros are necessary and urgently needed for the further developments of big cities.In the construction process of metros for coastal cities,the water-rich and weaken sand stratums are often encountered.The stratum have many disadvantages such as loose sands,low cohesive force,high porosity,large permeability coefficient and water-bearing with pressures.The unstability and damage of tunnel faces are caused easily by the excavation disturbance of tunneling,meanwhile,the water and sand gushing are induced correspondingly.The engineering accidents such as ground subsidence and tunnel collapse are caused if the management measures are improper,which will delay the construction process and lead to the terrible social influence.Therefore,the research on the stability of tunnel faces and the pre-grouting reinforcement techniques for water-rich and weaken sand stratums are the key scientific and urgent problems in the construction of underground engineering.In this research,the field investigations,laboratory tests,theoretical analyses,numerical simulations and field tests are combined to study the failure forms of tunnel faces and the key pre-grouting reinforcement techniques in water-rich and weakened sand stratum of Qingdao metroNo.2 line(the section of Pi-Miao).A series of significant research results have been achieved systematically,which are reflected in the following aspects:(1)Based on the summary of typical collapse cases of tunnels in Qingdao metro,the physical and mechanical property parameters of soils for water-rich and weakened sand stratum is studied systematically in laboratory.The effects of various water contents on the variation characteristics of mechanical property of silty clay aquifers were studied emphatically.In comparison with the physical and mechanical properties of sands and soils of other tunnels in China,the property differences caused by different sedimentary environments were concluded.And the unique engineering characteristics of water-rich and weakened sand stratum for Qingdao metro was studied and summarized.(2)According to the mechanism of water and sand gushing,the sources of sands,water,channels and hydrodynamic force of water-rich and weakened sand stratum were analyzed in the excavation sections.The objective conditions of sand and water gushing accidents were studied.The hydrodynamic theory was applied to the seepage process in water-rich and weakened sand stratum,the related models were established and the theoretical formulas were analyzed.Based on this,the happen criterion for sand and water gushing and the relationship between grain sizes and seepage velocity were studied.Moreover,the disaster evolution mechanism of water and sand gushing for Qingdao metro water-rich and weakened sand stratum were deduced,which can evaluate the possibility of water and sand gushing reasonably.(3)Considering the influence of overlying water-rich sand layers on the stability of tunnel face,the soil fracture structures of excavation face was constructed under the limit states.The upper limit of tunnel face stability was studied,the safety factors of tunnel excavation were established with the method of strength reduction,and the fracture ranges and fracture modes of the critical soils were obtained.Based on the theoretical analysis and calculation,the influence of cohesion of soils,internal friction angles,the thickness of the aquiclude,the water and soil pressure of the flowing sands and the excavation height on the stability of the tunnel face under the condition of water-rich and flowing sands.(4)In order to further validate the correctness of the the oreticaldeduction,many types of numerical calculation models were established.The deformation or instability of tunnel faces and the stress variation characteristics of surrounding rocks were obtained with different influence factors such as the thickness of the flowing sand layer,the safe distance between the sand layer and the tunnel,the friction angle of the aquifer and the cohesive force,etc.Also,the effects and sensitive indexes of various factors on the stability of tunnel faces and the influence degree of all factors on tunnel excavation was achieved.(5)By analyzing the stress and deformation variation characteristics of tunnel vault-hance-faces in the process of excavation,the release coefficients of displacement and stress were introduced,which can explore and characterize the influence of the surrounding rock and the tunnel face in excavation process.The deformation ordersand rates of the surrounding rocks and the tunnel faces were analyzed and studied,and the spatial deformation characteristics of the tunnels are explored.Based on the grouting reinforcement parameters and the deformation characteristics of the surrounding rocks in the flowing sand layers,the numerical simulation was analyzed and compared,and the excavation methods especially for grouting reconstruction in the flowing sand layers were optimized.The stability of surrounding rock was ensured,and the various sections of tunnels were successfully passed through.Therefore,the excellent effects have been achieved and has a certain guiding significance.