Risk Assessment And Disaster Process Simulation Of Water Inrush In Karst Shallow Tunnel Under River

The construction of tunnels in karst areas is often faced with geological disasters such as water inrush,especially the construction of shallow buried tunnel under river,which is a very challenging work.Due to the in situ fault fracture zone,high weathering degree of rock mass,and the development of joints and fissures in the tunnel site,the possibility of river water intrusion into the tunnel is greatly increased.Therefore,the risk level of water inrush disaster is firstly evaluated from the macro perspective,and the risk level evaluation system of water inrush disaster in the early construction period of tunnel is developed.Finally,for the output high-risk tunnel section,the water inrush process is simulated to comprehensively determine the risk level.In this paper,firstly,from the aspect of macro risk assessment,according to the geological conditions and tunnel characteristics of the karst shallow tunnel under river,the attribute recognition model of water inrush risk assessment is established,and the immediate evaluation of hazard level is realized through autonomous programming.For the tunnel section with a higher risk level,the PFC3D software is used to establish the numerical model of fractured rock mass in tunnel fault zone,and simulate the erosion type water inrush in karst area.At the same time,taking the ratio of coarse and fine particles in the sample as the influencing factor,the time-varying characteristics of total particle loss mass,the contact relationship between particles,porosity and permeability coefficient were described,and the risk of water and mud inrush was evaluated by the relationship between the above parameters and time.The results are stated as follows:(1)According to the geological conditions of the engineering area and the characteristics of tunnel,10 typical disaster-causing factors affecting the water inrush,such as formation lithology,modified strata inclination,lithological contact zones,fault zone width,fault property,fissure development degree,surface water flow,surface catchment area,buried depth of tunnel,construction disturbance degree,are selected as the evaluation indexes,and an attribute recognition model for risk assessment of water inrush in karst shallow tunnel under river is established.The evaluation system is applied to the risk assessment of water inrush in Qinling water conveyance tunnel under-crossing Gaochuan river section and Yuelongmen tunnel under-crossing Gaochuan river section,it is found that the risk assessment results are basically consistent with the actual situation,which verifies the rationality and feasibility of the evaluation model.(2)Based on the VBA programming tool,the risk assessment model of water inrush in tunnel is programmed and designed as a user-friendly GUI interface.Four typical cases are selected to verify the feasibility of the system.Compared with the actual tunnel excavation,the evaluation result of this system is acceptable.To a certain extent,the accident of water inrush could be avoided,and it has great economic value.(3)In view of the high-risk tunnel section,the numerical simulation of water inrush should be carried out.The karst shallow tunnel under-crossing river is prone to collapse water inrush disaster in fault zone.The PFC3D particle flow program was used to carry out meso-numerical simulation of the water inrush phenomenon of fractured rock samples with the growth of hydraulic gradient,which revealed that the variation rule of a series of meso-parameters of samples in the seepage process.The whole process of water inrush can be divided into three stages:slow seepage,abrupt seepage,and steady seepage.In the slow seepage stage,the mass loss,porosity and permeability coefficient of fractured rock mass increase slowly,and the number of intergranular contact decreases slowly.In the abrupt seepage stage,the mass loss,porosity and permeability coefficient increase sharply,and the number of intergranular contact decreases sharply.In the steady seepage stage,the mass loss,porosity,permeability coefficient and the number of contact between particles remain basically unchanged.(4)In order to assess the risk of water inrush in fault zones with different gradations,three groups of fractured rock models with coarse and fine material ratio as variables were established to compare the development process of water inrush under the same seepage conditions.The results show that the content of fine particles in fractured rock mass directly affects the law of migration and loss.The more the content of fine particles is,the less the coarse particles will be,and the thinner the force chain network composed of skeleton particles will be,which leads to serious loss of fine particles under seepage pressure.That is to say,the larger the mass loss is,the faster the porosity and permeability increase,and the higher the mass loss rate of particles.Fault fracture zone with similar grading has higher risk of water and mud inrush,and vice versa safety.