Study On Engineering Properties And Stability Of Tunnel Excavation Face Of Water-rich Tertiary Weakly Cemented Conglomerate

With the continuous development of China's transportation industry,the scale and area of the project construction are expanding,and the excavation depth is increasing.More and more tertiary rock mass between soil and rock has become an unavoidable focus and difficulty in tunnel engineering design and construction in the process of tunnel construction in central and Western China.Because of the special structure,low strength and poor cementation of the tertiary semi diagenesis,it has low self-bearing capacity and is sensitive to excavation disturbance.After excavation of tertiary semi diagenetic cavern,its self-stability is poor and the time of self-stability is short.And most of them are buried deep,causing them to often be below the groundwater level.The influence of groundwater on rock mass cannot be ignored.Collapse and large deformation are easy to occur in the process of excavation,and the stress of surrounding rock is complex.As a result,the safety of the project is poor,the progress is slow and the investment is increased dramatically,which causes many difficulties to the design and construction.Nowadays,the research on the tertiary semi diagenesis is mainly focused on the tertiary sandstone and mudstone,and there is little knowledge of the tertiary conglomerate and few related research results.Therefore,the study on the engineering properties and the stability of tunnel excavation face of tertiary weakly cemented conglomerate can fill in the blank of relevant aspects,and has double value for the development of engineering and theory.Based on the Zhongtiaoshan tunnel project of Menghua railway,this paper studies the engineering properties and the stability of the tunnel excavation face of the tertiary weakly cemented conglomerate.Through scene investigation,laboratory test,microscopic observation,theoretical analysis,numerical calculation and other methods,the internal mechanism and root cause of deformation and failure of tertiary weakly cemented conglomerate are revealed,the understanding of such stratum is improved,and the effective evaluation and analysis methods for its engineering properties and the stability of tunnel excavation face are mastered.The main achievements of this paper are as follows.(1)The particle size distribution in classical geotechnical mechanics analysis and fractal theory are combined to analyze the particle composition of the original and remolded samples of weakly cemented conglomerate through screening test.Then fractal theory is used to further study the fractal characteristics of the original and remolded weakly cemented conglomerate particle composition.From the aspect of particle grading,the undisturbed samples of weakly cemented conglomerate belong to well graded soil with excellent engineering properties;the remolded samples with less fine-grained content have good grading.With the increase of fine-grained content,the grain grading of remolded samples changes from good to poor.By using the method of fractal geometry,it is further analyzed that the particle distribution of weakly cemented conglomerate mostly shows the dual fractal dimension characteristics,and the fractal dimension value of fine particles in the constituent particles is significantly smaller than that of coarse particles,which reveals that the content of coarse particles in conglomerate is relatively small under the natural separation condition.The weakly cemented conglomerate with one-fold fractal dimension of particle distribution has the best gradation,and it has the highest shear strength and the best engineering properties according to the shear test.(2)The strength characteristics of weakly cemented conglomerate are systematically studied by large-scale shear test in laboratory.According to the shear test results,under the same cementation degree,the strength of weakly cemented conglomerate decreases with the increase of fine-grained content,and the internal friction angle also decreases.For the unconsolidated conglomerate,the relationship between the grain size fractal dimension value and the tangent value of the internal friction angle is negative index;the strength of the cementation degree is the basic reason for determining the strength of the tertiary weakly cemented conglomerate.Under the same particle composition,the different cementation degree causes the internal friction angle of the unconsolidated conglomerate in Zhongtiaoshan tunnel to decrease by 41.36% compared with the weakly cemented conglomerate.(3)From the X-ray diffraction test and thin section identification test of weakly cemented conglomerate,it can be seen that calcite and quartz are the main components of the framework of weakly cemented conglomerate particles,clay minerals act as cement between conglomerate particles,and the joint action of various minerals determines the strength characteristics of weakly cemented conglomerate from a macro perspective.The micro structure characteristics of weakly cemented conglomerate after shear failure are analyzed.Under the low normal stress,the conglomerate particles and structures are not obviously damaged,and the conglomerate mainly slides along the macro cracks between the particles.Under the high normal stress,the conglomerate is denser,and the particles around the shear plane will mostly produce cracks or even fracture decomposition.Due to the tertiary rock often buried deep,the ground stress is higher.the disturbed weakly cemented conglomerate particle structure in tunnel excavation is closer to the conglomerate particle state after shear failure under high normal stress.(4)Based on the upper-bound limit analysis,the failure model of tunnel excavation face in weakly cemented conglomerate is established,and the influence of pore water pressure on the failure of tunnel excavation face is considered in the model.For weakly cemented conglomerate with relatively good integrity,the excavation face of three-step excavation method has enough self-stability.Because the cohesion of unconsolidated conglomerate is zero,its excavation face cannot be stable without advance reinforcement measures.The water head height has an important influence on the stability of the excavation face of weakly cemented conglomerate tunnel.In order to reduce the height of water head and improve the self-stability of the tunnel face,it is necessary to design reasonable drainage and pressure relief measures.(5)FLAC3D is used to simulate the excavation of weakly cemented conglomerate in Zhongtiaoshan tunnel.Based on the catastrophe theory and strength reduction method,the stability of the excavation surface of weakly cemented conglomerate and unconsolidated conglomerate is discussed.The stability safety factor of the excavation face of the undisturbed weakly cemented conglomerate tunnel is 3.79,which has sufficient safety reserve,and its arch bottom position is the instability control point.The safety factor of the excavation face of the unconsolidated conglomerate tunnel is lower than 1,which does not have self-stability,indicateing that the degree of cementation fundamentally determines the stability of the excavation face of the conglomerate.Under the same cementation degree,the safety factor of the excavation face of unconsolidated conglomerate decreases with the increase of the fractal dimension of grain size,indicating that the fractal characteristics of grain size also have significant influence on the stability of conglomerate tunnel face under the same cementation degree.Finally,the results of numerical calculation and limit analysis are compared to verify the accuracy of limit analysis method.