Study On Water Gushing Tendency Of Dacun Tunnel

In the process of tunnel excavation,a series of geological disaster problems may be caused.Among them,the problem of tunnel water inrush is very prominent,and there is often the saying of’ten tunnels and nine leakages’.However,the possibility of large-scale karst water inrush is higher in karst areas.Tunnel construction drainage will cause secondary disasters such as land subsidence,karst collapse and groundwater resource depletion in karst areas.In view of the seriousness of the problem of tunnel water inrush,this paper takes the actual project of Dacun Tunnel as the research object,and systematically studies the groundwater occurrence in the tunnel site area of Dacun Tunnel,the development of karst water on both sides of the tunnel line,the tendency of tunnel water inrush and the environmental impact of construction drainage,so as to provide theoretical basis for the smooth construction,operation and environmental protection of the tunnel.As one of the control projects of Mile-Yuxi Expressway,Dacun Tunnel mainly passes through Sinian strata.As a typical’old’stratum,it has experienced a long-term and turbulent geodynamic environment.The rock mass in the area has a high degree of fragmentation,fracture,joint and fissure development,complex geological structure conditions and abundant groundwater storage space.On the basis of extensive collection and review of regional geological data and hydrogeological data,based on field engineering geological survey and hydrogeological survey,the groundwater environment（hydrogeological conditions）of Dacun tunnel was analyzed and studied by means of geochemical analysis,cluster analysis and quantitative estimation of tunnel water inflow.The main conclusions are as follows:1.The Qilu Lake Basin crossed by the Dacun Tunnel belongs to the Yangtze quasi-platform of the first-order tectonic unit of the Chinese mainland,and is located on the side of the second-order tectonic unit of the Yangtze quasi-platform-the eastern Yunnan fold belt.The basin was formed in the first episode of the Himalayan movement in the middle Eocene-Oligocene,and its source power came from the subduction of the Indian plate to the Eurasian plate.The multi-stage and large-scale tectonic movements since the Paleozoic have had a profound impact on the Sinian strata in the study area.The complex geological history has caused the fragmentation of the Sinian rock mass in the study area,and the Sinian system in the study area and its surroundings was also covered by the?-J rock cover.The large-scale exposure of the Sinian system is caused by late denudation,and the denudation unloading effect makes the hidden damage left by the tectonic movement released and further strengthened.The fault block of Sinian strata and the fragmentation of rock mass reduce the possibility of large-scale karst（water）system development.Especially when the karst rock is connected with the non-karst rock,the dolomite may be more inclined to develop the dissolution fissure water rather than the large-scale pipeline aquifer.The possibility of large-scale karst water inrush during tunnel excavation is low.2.In terms of geomorphology,the Qilu Lake crossed by the Miyu Expressway is an inland lake without surface discharge,while the Qilu Lake Basin where the tunnel is located is a suspended basin,and there should be no deep lateral recharge across the basin.The basin is a nearly closed ring-shaped erosion datum composed of surrounding basins and basins facing the central erosion.On the plane,the Qilu Lake Basin has the remarkable characteristics of large lakeside zone,small lake surface and small mountain zone,that is,two’small’and one’large’.The watershed between the basin and the surrounding basin or basin is strongly biased towards the side of the Qilu Lake basin.For the topographic slope of the mountain belt on both sides of the watershed,the slope of the inner side of the watershed is obviously smaller than that of the outer side of the watershed.The Qilu Lake basin is strongly captured.The trend of watershed migration to the inner side of the basin is becoming more and more serious year by year,which also indicates the development trend of the basin shrinking year by year.The runoff path（seepage field）of groundwater in the basin mountain belt is also squeezed and decreased year by year.Vertically,the height difference between the lake surface and the lowest point of the watershed is 65 m,and the height difference between the lake surface and the foot of the mountain is 30 m.That is to say,the depth of the basin is only 65 m.If the erosion datum of the foot of the mountain is considered,the depth of the basin is only 30 m.The long-term evolution of the Qilu Lake Basin in a specific geological-geomorphic environment is the root cause of this phenomenon.Combined with the elevation of the tunnel and the spatial location of the basin,it can be predicted that the surrounding rock environment（groundwater environment）crossed by the Dacun tunnel should be an unsaturated zone,and the possibility of tunnel uncovering large-scale water gushing is low.3.The recharge,runoff and discharge characteristics of groundwater in the tunnel area are strictly restricted by the meteorological hydrology,topography and tectonic factors in the area.On the whole,the tectonic framework of the tunnel site area is a large-scale monoclinic with a general south-dipping and a partial west-dipping.The karst rock and non-karst rock interbedded structure is obvious.According to the lithology,groundwater can be divided into pure carbonate karst water and carbonate and non-carbonate interlayer karst water.The recharge of groundwater is mainly atmospheric rainfall,which lacks the strong induction of long-distance and low-elevation erosion datum.The runoff path is simple,and the discharge is dispersed in the form of beaded groundwater outcrop.4.Geochemical analysis shows that the hydrochemical type of groundwater in the area is HCO₃-Mg·Ca type,with low mineralization degree and weak alkalinity.Both cluster analysis and correlation analysis show that the chemical process of water and rock in the area is mainly the dissolution of dolomite in Sinian dolomite,while the lower TDS shows that the residence time of groundwater in the aquifer is short,the path of groundwater runoff system is not long,and there is no long-distance deep lateral recharge.The karst space on both sides of the tunnel line is not piped and the concentration of karst water is low.5.The water inflow of the tunnel is estimated by water balance method and groundwater dynamics method.It is predicted that the normal water inflow of the tunnel is 8791.22 m³/d,and the maximum water inflow is 10171.10 m³/d.This is consistent with the’dry’actual situation of the tunnel as a whole.The actual water inflow of the tunnel should be small and affected by seasonal changes.For the section of the tunnel containing the fault zone,the prevention and control measures are taken in the tunnel construction,which are mainly based on plugging and combined with plugging and drainage,so as to avoid the construction obstruction or the occurrence of life and property safety accidents;for the section without fault zone,it is mainly based on prevention,and the combination of advanced prediction and prevention measures.6.The drainage of tunnel construction may cause the decrease of Q1-Q7 spring water flow in the front foot of Xiaocun,Dacun,Anchangli and Shibi villages near the south side of the tunnel,which has a certain impact on the production and life of local villagers,but in the long run,this impact can be gradually restored.On the other hand,the possibility of karst collapse in tunnel surrounding rock and its overlying area caused by construction drainage is small,and many secondary hydrological and environmental geological problems are controllable.