Study On The Identification Of Complex Karst Water System And Its Application In Tunnel Water Disaster Prediction

With the development of western China,a large number of infrastructure projects such as transportation and water conservancy will inevitably cross the karst areas in southwest China,especially the areas with strong karst development.Tunnel is an important part of linear engineering in mountainous areas.Water inrush disaster in tunnel construction has become one of the common and dangerous hazards in tunnel construction in karst areas in south China.The structural characteristics and water cycle conditions of karst water system in south China are complex,which are mainly shown as follows:(1)There are frequent conversion between surface water and groundwater in the karst water system,(2)the recharge of karst groundwater has a double recharge mode of surface infiltration and point concentrated irrigation,(3)karst aquifer medium highly heterogeneous,and(4)the dynamic change of karst spring discharge makes it difficult to identify the structure and conditions of the karst water system.The water inrush disaster of karst tunnels has the characteristics of “strong suddenness,large water inflow,and high harmfulness”,which makes tunnel projects in karst areas often become the “stuck neck”of linear projects.In view of the problems encountered in tunnel engineering design and construction in karst strongly developed areas in southwest China,this article through the analysis of the development regularities of karst,identification of karst water system space structure and the conditions of the water cycle,and through the high resolution of karst springs flow monitoring for karst water system input-output response characteristic parameters,finally provided the scientific basis for karst mountain tunnel project line selection.This article takes Daqingshan Tunnel in Furong Town area of Zhangjihuai Highspeed Railway as the research object,and mainly includes the following five research contents:1)Combined with karst topography and geomorphology investigation and rock mineralogy analysis,the karst development law and its controlling factors in Furong Town are found out,and the possibility of zhangjihuai High-speed railway station in Furong Town is scientifically discussed from the perspective of karst hydrogeology.The karst development and groundwater runoff discharge conditions are controlled by Songbai syncline in the study area,and the karst development is stronger from the two wings to the core of the syncline,and the karst slope zone from the Daqingshan watershed to the core of the syncline is characterized by multi-stage karst platform,which is jointly controlled by the transverse fractures and along the stratum strike.Karst is strongly developed in synclinal core,and there are super large underground river systems with parallel axis,such as Kebidong and Guanfangwan(catchwater area is 200km~2 and 30km~2respectively).The synclinal wing and its raised end are middle and lower Cambrian strata with strong to medium karst development.There are karst water systems such as Lanhua Cave,Daiye Cave and Xiniu Cave in the area,and the catchwater area is less than 10km~2.Therefore,the Zhangjihuai high-speed railway line should avoid the karst strongly developed area in the core of Songbai synclinal in Daqingshan Area,and choose to pass through Daqingshan from the wing to the raised end,which makes it possible for the Zhangjihuai high-speed railway to set up a station in Furong Town.2)Combinined with karst hydrogeological survey and multiple groundwater tracer test,the boundary of karst water system,water circulation characteristics and spatial distribution characteristics of karst pipes in Xiniudong,Lanhuadong and Daiye caves were investigated.The karst water system of Xiniu Cave and Daiye Cave is mainly composed of a single karst pipeline system,while the karst groundwater system of Lanhua Cave is a complex karst groundwater system composed of multiple karst pipelines.The replenishment area of the Xiniu cave system is 2.34km~2,the entrance elevation of karst conduit is 690 m,the exit elevation is 650 m,and the average hydraulic gradient is 6.15%.The replenishment area of the Daiye Cave system is 3.74km~2,the entrance elevation of the karst conduit is 700 m,the exit elevation is 660 m,and the average hydraulic gradient is 3.39%.The Lanhua Cave system can be divided into an upstream section and a downstream section from the underground river skylight,the downstream section is a pressureless conduit with a measured length of 1600 m and an average hydraulic gradient of 2.81%.The Lanhua Cave system can be divided into the Zhangchaowan,Kongjiawan and Zehao Subsystems: the replenishment area of the Zhangchaowan subsystem is2.28km~2,the conduit inlet elevation is 630 m,the outlet elevation is 545 m,and the average hydraulic gradient is 5.37%;The replenishment area of the Kongjiawan subsystem is3.15km~2,the inlet elevation of the conduit is 620 m,the outlet elevation is 545 m,and the average hydraulic gradient is 14.6%;The replenishment area of the Zehao subsystem is3km~2,the conduit inlet elevation is 620 m,the outlet elevation is 531 m,and the average hydraulic gradient is 20.69%.3)Based on the long-term observation of rainfall and karst spring flow dynamics with high resolution,the rainfall and flow response characteristics of karst groundwater systems in Lanhuadong and Daiyedong are obtained,and the lag and delay law of karst spring flow response with different rainfall types and the recharge and discharge law of different karst pipeline systems are discussed.The storage capacity of the Lanhua Cave karst groundwater system is stronger than that of the Daiye Cave karst groundwater system,which is reflected in the lagging and longer delay time of the output(spring flow)of the Lanhua cave system to external input(rainfall),and a smaller variation in groundwater temperature.Lanhua Cave and Daiye Cave are both unclosed karst groundwater systems.When the total rainfall of the Lanhua Cave system is less than 25 mm,the infiltration coefficient of single rainfall varies within the range of 0.2?0.72;the total rainfall is 25?62.5mm,and the rainfall infiltration The infiltration coefficient varies greatly,most of which are changed in the range of 0.05 to0.5;when the total rainfall exceeds 62.5mm,the infiltration coefficient of a single rainfall is maintained at about 0.2.When the total rainfall of Daoyedong system is less than 70 mm,the infiltration coefficient fluctuates in the range of 0.2 to 0.8,and the infiltration coefficient fluctuates around 0.2 when the rainfall is greater than 70 mm.According to the analysis of flow attenuation of Taiye Cave,the release water of the pipeline medium is about 10%,and the release water of the dissolved fissure medium accounts for 8%-20%,which has both water conduction and water storage properties.The water of the fissure medium accounts for more than 70%,and it is the main water storage medium of karst groundwater.Under the condition of light rain,the surface infiltration recharge is the main recharge mode of the system,the fissure medium controls the recharge and drainage process of the karst groundwater system,and the spring discharge decline has only one decline stage.In the case of heavy rain,concentrated recharge and surface infiltration recharge are the main water replenishment methods of groundwater system.Pipes,dissolution fissures and fissures jointly control the water replenishment and drainage processes of the system,and the decline of spring flow has multiple decline stages.4)On the basis of rainfall infiltration method,the surface infiltration and point concentrated injection are incorporated into the calculation model of water inflow.On the basis of Modflow-CFP module,the surface-groundwater coupling model of karst pipefissure medium and laminar turbulent flow is improved,and the water inflow of tunnel is predicted respectively.The improved rainfall infiltration coefficient method fully considers the lag and delay effect of karst groundwater system,and introduces logarithmic normal distribution curve to distribute the water gushing process under the determined total amount of water gushing,so as to predict the water gushing in tunnel.When pipe water inrush occurs in Daiye Cave system,the peak flow of tunnel water inrush is 4.84m3/s under extreme rainfall condition of 1.5% frequency,and the peak flow is 2.41m3/s under rainfall recurrence period of 5 years(20%),and the duration of flood peak is 35 h.Modflow-CFP module was used to establish the coupling model of karst pipe-fissure system and laminarturbulent flow,and fully consider the dual recharge mechanism of karst water system,such as point-like concentration and plane infiltration,to predict the pipe-type water inwelling process through the Daiye Cave system.The peak water inflow of the tunnel is5.26m3/s in extreme rainfall of 1.5% frequency,and 2.39m3/s in rainfall return period of5 years(20%).5)On the basis of investigating the structural characteristics and the law of rechargerunoff-discharge of karst water system pipelines in Daqingshan area,the improved rainfall infiltration coefficient method and fissure-pipeline coupling model are comprehensively considered to predict the water inflow of different crossing schemes in Daqingshan tunnel.The maximum peak water inflow of Daqingshan Tunnel A scheme(Daiye cave scheme)is 5.26m3/s,the maximum peak water inflow of Daqingshan Tunnel B scheme(Lanhua Cave scheme)is 16.76m3/s,the maximum peak water inflow of Daqingshan Tunnel C scheme(watershed scheme)is 3.58m3/s,and the maximum peak water inflow of optimization scheme is 0.93m3/s.From the perspective of avoiding karst water damage to the maximum extent,the optimized scheme(Yongshun tunnel + Lanhua tunnel)has the least risk of karst water damage,and the optimized scheme is recommended as the final scheme.Through construction verification,the optimized scheme intersects the branch pipelines of the underground river system of Lanhua Cave on the plane,and the elevation of the bottom plate of the tunnel is 30-50 m higher than the water level of the underground underground river,avoiding the problem of karst water disaster.The characteristics and innovations of this paper are as follows: 1.By studying the structure identification of karst groundwater system and the response characteristics of high resolution rainfular-karst hydrological process,"atmospheric rainfall infiltration coefficient method" and "fissure-pipeline coupling numerical simulation method" are improved,so as to improve the prediction accuracy of karst tunnel water inflow.2.Developed the hydrological monitoring system suitable for karst basin,established a complete set of "investigation-experiment-monitoring-forecast" method system suitable for tunnel water damage prediction,and applied it to practical engineering construction.3.Through the hydrologic dynamic response characteristics under different rainfall conditions,the system recharge law under different rainfall conditions of piped karst small watershed is obtained,and the process of water filling and water release in water-bearing medium is identified,which provides a new idea and technology for the theoretical study of karst water flow system.