| Water inrush and mud inrush are the main forms of groundwater disasters that endanger the construction and operation safety of karst tunnels.The natural geological conditions of the Qinghai-Tibet Plateau are complex.At present,there is still considerable controversy on the understanding of karst morphology,development scale,spatial distribution and karst dynamic conditions in the plateau,which will inevitably affect the analysis and judgment of the hydrogeological characteristics of karst tunnels in the plateau.Taking a traffic tunnel in southeastern Tibet as the research object,this paper studies the basic characteristics of plateau karst development in the region through regional geological data analysis,on-site hydrogeological investigation,drilling exploration and hydrogeological tests,groundwater chemical testing and analysis,and numerical simulation technology.,tunnel hydrogeological conditions,generalized the karst hydrogeological model of the study area,established a numerical model of the regional seepage field,simulated and analyzed the influence of tunnel construction on the seepage field,the water inflow of the tunnel during construction,and the water inrush mode of the karst section of the tunnel.And the theoretical analysis method is used to predict the water inflow of the tunnel,and the main methods suitable for the prediction of the water inflow in each section are summarized,and the reasons for the difference of the water inflow predicted by different methods are compared and analyzed,and the corresponding recommended value of the water inflow prediction is given.The main research results are as follows:(1)The karst development in the study area is dominated by tertiary paleokarst;The surface karst is not obvious,and there are no large depressions,sinkholes and karst troughs;Underground karst is relatively developed,mainly in Tertiary paleo-karst caves,dissolution fissures,and dissolution and fracture zones;Affected by the "drive" of the erosion base level in different periods,the karst in the region has significant vertical zoning and horizontal differences;Vertically,from top to bottom,with the regional main planing plane,the denudation plane and the localized Gongjue Basin erosion base plane successively,three layers of strong dissolution zones corresponding to the elevation are developed;In the horizontal direction,affected by the long-term relative decline and local erosion of the Gongjue Basin on the east side of the limestone strip,the karst in the limestone strip is strong in the east and weak in the west.In addition,under the influence of fault rupture,a deep dissolution and fracture zone develops near the fault in the area.(2)Rainfall and ice and snow melting on the high platform formed by the flattening on the north side of the study area are the main sources of groundwater recharge in the karst section of the tunnel.However,due to the influence of the surface karst shape and scale,groundwater recharge is mainly distributed and not large-scale.Concentrated infiltration recharge conditions.According to the depth of runoff circulation,groundwater runoff in the study area can be divided into three categories: local surface runoff,shallow runoff system,and deep runoff: Local surface runoff flow is the least;Shallow runoff migrates southward and northward along regional fault zones and karst interior space,and discharges in Maqu and Naqu respectively,and the discharge water flow is small;The deep runoff migrates from north to south and discharges in the form of hot springs,with the largest amount of water.The surface runoff mainly affects the tunnel at the entrance and exit of the tunnel,the shallow runoff has a greater impact on the limestone section of the tunnel,and the deep runoff has less impact on the tunnel.(3)The simulation results show that: The general flow direction of groundwater in the limestone in the middle of the tunnel is from northwest to southeast,and there is local surface or shallow runoff in the transverse ditch on the north side;The groundwater in the east Gongjue Formation is mainly discharged from west to east;The groundwater in the Triassic-Jurassic clastic rock on the west side flows from west to east,some of it flows into the surface trench,and some of it is recharged to the groundwater in the limestone as exogenous water.(4)According to the numerical simulation of the change process of water inrush in different sections of tunnel excavation,combined with the karst development characteristics of each section and the depth of the tunnel,the water inrush mode of the karst section of the tunnel is analyzed and summarized: Mixed recharge of groundwater and rainfall—synchronized water inrush pattern,mainly distributed in places with strong karst development or shallow buried sections of tunnels;Groundwater recharge-straight inrush water inrush mode is mainly distributed in weak karst development,large tunnel burial depth,and tunnel passing through aquifers;The mixed recharge of groundwater and rainfall-hysteretic water inrush mode is mainly distributed in the deep-buried section of the tunnel with strong karst development.(5)The main reasons for the differences in the prediction results of water inflow in various tunnels are analyzed and summarized: First,due to the strong uplift and freezing weathering of the Qinghai-Tibet Plateau,the development of surface karst and underground karst shows a certain mismatch.Therefore,the values of hydrogeological parameters(such as infiltration coefficient)based on the characteristics of surface karst development are often low,resulting in low prediction results of the infiltration coefficient method.Second,due to the strong uplift of the plateau and the development of underground structures(faults)karst,the groundwater level in the karst area is low,and there are further and deeper regional circulation and discharge pathways.Limited by the scope of the investigation,the runoff modulus taken is low.,resulting in the overall low water inflow predicted by the runoff modulus method.Third,the groundwater dynamics method is mostly based on the value of the groundwater level line at the borehole,and the dynamic changes of groundwater in the karst area and the continuous recharge of groundwater recharge sources are not considered in the calculation of the water inflow in the tunnel,resulting in the prediction of the groundwater dynamic method.Water volume is low.Fourth,the numerical simulation method cannot simulate the regional deep-circulating groundwater flow because the simulation software itself cannot set the function of far-field discharge and is limited by the scale of the model.Therefore,the model fails to take into account the regional deep runoff,which leads to the increase of water richness in the regional shallow runoff,resulting in a larger result of tunnel water inflow. |
PDF Full Text Request