Study On Mechanism Of Water And Sand Gushing In Tunnel Face Of Water-rich Sanded Dolomite Stratum And Its Treatment Countermeasures

Water-rich sandy dolomite formation is an unfavorable geological body caused by karst,which is very likely to induce sudden water and sand surges during the construction of tunnels and has caused a lot of economic losses.At present,the sudden water surge sand mechanism in the water-rich sand dolomite stratum has not been fully explored,and there is no suitable method of calculating the thickness of anti-burst body,but also the lack of effective countermeasures against sudden water surge sand.In order to serve the engineering construction.This paper relies on the Jixin Tunnel Project of the Chengdu-Kunming railway.Using field research,indoor tests,similar model tests and numerical simulations,this paper summarizes the composition of the disaster-causing system of water-rich sandy dolomite,reveals the causes of dolomite sandification,explores the physical and mechanical differences between sandy dolomite and unsanded dolomite,summarizes several water and sand inrush mechanisms in water-rich sandy dolomite formation,constructs a formula for calculating the thickness of anti-burst body in water-rich sandy dolomite formation,and finally proposes countermeasures against water and sand inrush in water-rich sandy dolomite formation.The main contents are as follows:(1)Through the research of several water and sand inrush accidents occurred in the tunnel and related geological data,combined with physical and geological observation means,the disaster-causing system of water-rich sandy dolomite is summarized and analyzed,and the characteristics of water storage structure and anti-burst body of disaster-causing system in water-rich sandy dolomite stratum are clarified.(2)Through XRD diffraction experiments,electron microscope tests,sectioning experiments and water quality tests,it reveals that the dolomite sandification is a dissolution phenomenon after hydroclastic action of dolomite with cataclastic and anisometric microlith-micrite structure.With the help of physical and mechanical tests,the physical and mechanical properties of sandy dolomite and unsanded fractured dolomite are found to be vastly different.The unsanded fractured dolomite is affected by joints and fissures as soft rock,while the sandy dolomite loses its rock matrix and has porous and permeable characteristics.(3)Through the simulation system of water and sand inrush developed for water-rich sandy dolomite strata,the stress field,water inrush and deterioration of face during tunnel excavation under different sandy dolomite ratios are monitored,and the different modes of inrush are summarized.Among them,the inrush mode of unsanded dolomite stratum is gradual inrush mode,which is caused by water inrush due to preventing the development of cracks in the anti-burst body.With the appearance of sandy dolomite bands,the inrush mode is gradually changed to the inrush induced by seepage failure in the sanded position.(4)Five main factors affecting the thickness of the anti-outburst body were investigated,including the proportion of sandy dolomite area,the GSI value of unsanded surrounding rock,the buried depth,the water pressure of the water storage structure and the size of the water storage structure.The order of the influence degree on the thickness of anti-burst body is as follows: proportion of sandy dolomite area > GSI value of unsanded surrounding rock > buried depth of tunnel > water pressure of water storage structure > size of water storage structure.Finally,the calculation formula of anti-burst body thickness under the influence of five factors is obtained through multivariate nonlinear fitting.(5)According to the characteristics of the disaster-causing system of water-rich sandy dolomite formation and the inrush mechanism of sandy dolomite formation,four construction control principles of advance prediction,drainage and pressure reduction,precise grouting and agile control are proposed.According to the four principles,the corresponding construction countermeasures are constructed.