Study On Mechanism Of Water Inrush In Muddy Limestone Karst Tunnel After Wetting And Drying Cycles

Tunnel water inrush and some other engineering disasters often encounter when tunnels are constructed in mountainous and karst areas with complicated topographic and geological conditions.In addition,Influenced by seasonal rainfalls,the surrounding rock of deep buried muddy limestone karst tunnels is usually located in a typical wetting and drying circulation environment.The cementation strength of fine mineral particles in the tunnel surrounding rock were weakened and then easily dissolved in the water due to the alternation of wetting and drying,resulting in the increase of porosity,the strength reduction and the obvious of the rock mass.With the accumulation of deterioration effect,water inrush disaster in deep buried karst tunnel may occur.Therefore,it is of great significance to revealing the water bursting mechanism and prevention and control of underground engineering through investigate the deterioration features and the disaster-causing mechanism of karst tunnel under the alternation of wetting and drying circumstances.Incorporating with the National Basic Research 973 Program of China(Grant No.2013CB036003)and Natural Science Foundation of China(Grant No.51734009)and taken Liupan Mountains karst tunnel as engineering background,the deterioration features and water bursting mechanism of tunnel surrounding rock under different ground stress state and hydraulic pressure are studied by the combining method of physical simulation experiment,theoretical and numerical simulation.The main research findings and conclusions are as follows:(1)Firstly,the muddy limestone of Liupan mountain tunnel was treated with different cycles of wetting and drying(N= 0,3,6,8,10,12),then the variation characteristics of the physical property(mass,density,longitudinal wave velocity and pore structure)and mechanical parameters(peak strength,elastic modulus,peak value)of the rock samples were studied.The results demonstrate that the permeability coefficient increases with the increase of wetting-drying cycles,and the pore size changes from micro-pore to large-scale pore size,and the strength parameters of the specimen deteriorate obviously.(2)Based on the geological conditions of Liupan mountain Tunnel and the water inrush disaster test system for karst tunnel,6 large-scale physical simulation experiment with different location conditions and water-resisting layer were carried out to investigate the effect of alternation of wetting and drying on the deformation of surround rocks.With the increase of hydraulic pressure,the ratio of mid-span deflection to total mid-span deflection increases exponentially,which is caused by the strength deterioration of water-resisting strata,and the ratio increases obviously when the hydraulic pressure is higher.It indicates that the failure of water-resisting strata is not only due to the increasing hydraulic pressure,the strength deterioration result from the alternation of wetting and drying is similarly noticeable.(3)Laboratory physical model tests revealed the evolution characteristics of water gushing in tunnel surrounding rock with different thickness of water-resisting strata under the action of dry-wet alternation,and further analyzed the evolution characteristics of seepage field,displacement field,stress field the tunnel surrounding rock in the process,as well as the failure mode of surrounding rock.The test results show that water-resisting strata presents different failure modes when the thickness of water-resisting strata is different,such as “central fracture mode” and “integral extrusion mode”;With the increase of wetting-drying cycles,both the convergence of the roof to floor and wall to wall increases accordingly,and the process shows a trend of slow fluctuations and then increase rapidly;Combined with the test results,the variation law of the minimum safe thickness of the waterproof layer under the action of “constant amplitude dry-wet alternation” and “incremental dry-wet alternation” was revealed.(4)Based on the above research results,by analyzing the deterioration effect of water on the mechanical properties of rock mass and taking discrete element program(UDEC)as the basic framework,a coupled method for calculating rock mass softening considering water infiltration and wet-dry alternation is proposed.The seepage flow field,main stress distribution and displacement variation of surrounding rock of different ground stress level tunnel were investigated,and the stability mechanism of surrounding rock of karst tunnel under the alternation of dry and wet was revealed.