Research On Groundwater Connecting Experiment And Its Engineering Application In The City Tunnel Of Water-Collective Complex Stratum

A big problem is how to prevent the loss of groundwater and reduce the influence caused by construction to the surrounding water environment, when mountain tunnel in the city is being built in water-collective complex stratum,such as the fault fracture zone.On the basis of the safe excavation, great attention should be paid to the lake or reservoir water, which may inflow into the tunnel by leakage passage caused by fault fracture zone and change the surrounding water environment.Even there is slight leaking water, the leakage passage will be larger and larger under its long-term effects, which will cause lake or reservoir water erosion. Moreover, it will also affect the safety of structures if soaked in the leaking water.The controlled drainage principle is proposed in order to protect the local natural environment and avoid potential safety hazard in later operation. As for the exact measure to control the drainage, the extent, and whether second grouting is necessary for the leakage parts as well as the choice of grouting material, they all depend on whether there is apparent hydraulic connection between leaking water and surface water. Therefore, it is of vital importance to carry out connecting experiment of tunnel of being built in water-collective complex stratum, especially for the mountain tunnel in the city.On the base of the investigation and research for lots of document literature and according to the practice of mountain tunnel in the city of water-collective fault fracture zone, this text discussed and analysed types and applicable conditions of groundwater connecting experiment at home and abroad, and combined Visual MODFLOW, built a concentrated leakage passages numerical model to analyse the groundwater solute transport rule in the fault fracture zone. This text also researched groundwater connecting experiment methods and mechanism of both manual and natural tracers, including hydrogen and oxygen stability isotope, hydrochemical analyse and hierarchical cluster analysis, and adopted corresponding solutions for leaking water combined with site test results. The research conclusions are as follows:（1） The structure factor of fault fracture zone plays an important role in leakage accidents, while the water-collective and hydraulic conductivity of fault tectonic are determined by many factors. When groundwater is moving through large fissures formed by fault fracture zone, the sport forms will be much more centralized, and it is relatively faster, less interfered and the flow along the fracture is bigger. （2） The simulation result of Visual MODFLOW turned out—after the tracer was released, it mainly moved forward in the concentrated leakage passage formed by fault fracture zone, spread slightly outward in the surrounding strata nearby the fault fracture zone and its sports forms were relatively concentrated. In different permeablestratum, the speed of tracer differed from various hydraulic conductivity and it was apparently faster in strongly permeable layer than in aquitard.（3） Data regression analysis concluded that hydrogen and oxygen stability isotope precipitation line equation of Xiamen:δD=7.48δ18O+4.42,correlation coefficient:R=0.9926,which is similar with China’s precipitation line equation, it Can be used to judge the supply source of the seepage water gushing.we found out that hydrogen and oxygen stability isotopes withδD andδ18O value of seepage water gushing down to the atmospheric precipitation line both sides and deviation was not big, basically in line, it showed that isotopic composition characteristics of the seepage water gushing was similar with atmospheric precipitation, Thus indicated that the seepage water gushing caused by atmospheric precipitation and it was recharged by the atmospheric water.（4） Because Cl^- and SO₄^2- has a strong water transfer ability, its stability properties can be used as natural tracer. If the reservoir water and seepage water gushing with the Cl^- or SO₄^2- concentration is the same, explained respectively that there is apparent hydraulic connection between reservoir water and seepage water gushing.Conversely, if the seepage water gushing with the Cl^- or SO₄^2- concentration is lower than reservoir water , there is weak hydraulic connection between them. Test results showed that the seepage water gushing with the Cl^- or SO₄^2- concentration was lower than reservoir water,it indicated that there was weak hydraulic connection between reservoir water and seepage water gushing.（5） Carrying through water chemical composition analysis by using the Pipers’ s diagram, we can see the general geochemical characteristics and the relative content of various ions.A region of water marked on the Pipers’ s diagram, you can analyze groundwater chemical composition of evolution. From the analysis figure, chemical type of seepage water gushing is HCO₃^-—Na⁺Ca²⁺seepage, each of the water chemical compositions of atmospheric water, reservoir water and seepage water gushing correspond to one region in the middle diamond part among the Pipers’ s diagram, it indicated that there was weak hydraulic connection between reservoir water and seepage water gushing,and there was no supply relationship.（6） Based on Spass17 hierarchical cluster analysis, it will turn out to be the same by adopting Euclidean distance or Minkowski distance formula to calculate the distance between different samples, as well as the cluster results concluded from the cluster analysis by the samples. In another word, there is slight difference between the samples of seepage water gushing and atmospheric water during the process of cluster, while there is bigger difference compared with reservoir water. It means that reservoir water has no hydraulic connection with seepage water gushing and its supplying resources is from the precipitation infiltration fissure diving. It also confirms the groundwater connecting experiment result of hydrogen and oxygen stability isotope and hydrochemical analyse.（7） The manual tracer connecting experiment should be also adopted to further confirm the connectivity and find out the leakage path if the natural tracer connecting experiment confirmed that there was supplying relationship between reservoir water and seepage water gushing and the hydraulic connection was strong. The test result showed that there was a weak hydraulic connection between reservoir water and seepage water gushing and the supplying water of seepage water gushing is eventually from precipitation infiltration fissure diving. That is to say, it is unnecessary to carry on the manual tracer groundwater connecting experiments. However, under the consideration of the fact of leaking water in the tunnel, some measures were taken to manage the leakage place during the construction process.