西南岩溶区深埋隧洞涌水条件分析及涌水量预测——以滇中引水工程大坡子隧洞为例

Analysis of water inflow conditions and prediction for water inflow of deep-buried tunnels in the karst area of Southwest China:Taking Dapozi tunnel of central Yunnan Water Diversion Project as an example

隧洞涌水预测是隧洞安全施工的重要保证,而分析涌水条件、识别涌水来源是涌水量预测的前提。本文以滇中引水工程大坡子隧洞为研究对象,通过分析研究区地层岩性、地质构造,结合地下水水化学特征、同位素结果,厘清了研究区地下水的水文地质条件,确定了隧洞涌水补给来源;在此基础上,利用解析法和数值法对隧洞最大单位涌水量和正常单位涌水量进行计算,并将预测涌水量与实测涌水量进行了对比。其中,基于FEFLOW构建的地下水流数值模型,耦合模拟区域宏观流场的同时,采用多时间序列与各类内边界综合赋值的方法刻画动态施工过程。结果表明:解析法可以在工程初勘设计阶段高效计算隧洞涌水量,但不能动态预测隧洞涌水变化且在地下水位较高区段预测精确度不如数值法;数值法能够精细刻画含水系统结构、参数分布以及隧洞施工工况,能较精确预报施工涌水量变化;故采用解析-数值方法可以显著提高涌水量预测效率和精度。本文采用的方法和模型对于大风险隧洞突涌水灾害防控具有重要意义。

The karst area in Southwest China is characterized by complex terrain and karst development,with a large area of exposed carbonate rocks and a wide distribution of karst depressions and valleys on the surface.Therefore,this area is highly subject to water inflow in tunnel construction.The Dapozi tunnel area of the central Yunnan Province is a deep-buried tunnel in the karst area,in which exist many faults and very complex hydrogeological conditions.Therefore,it is necessary to predict the tunnel inflow during construction by analyzing the water inflow conditions and identifying its sources,which can ensure the safety of tunnel construction.Currently,numerical and analytical methods are commonly used to predict water inflow.The analytical methods mainly calculate the water inflow of tunnel through empirical calculation formulas.However,most of these methods are based on some assumptions and specific boundary conditions,which may limit their applicability in complex cases.The numerical method can simulate complex geological conditions by obtaining accurate data about geological characteristic,but its accuracy is restricted by the accuracy of borehole data in numerical calculation.Therefore,the mutual supplementation of numerical and analytical methods can significantly improve the efficiency and accuracy of water inflow prediction.Besides,most of the previous studies only predicted the water inflow of tunnel in the survey and design stage.But they did not describe the actual construction progress in the model,and the accuracy of prediction results conducted separately with numerical or analytical methods were not verified by the actual water inflow data.By analyzing the stratigraphic lithology and geological structure of the strata in the study area,we preliminarily identified the construction sections with the risks of water inflow.Based on hydrogeological surveys,we collected water samples from tunnels,springs,and wells in the study area to measure the geochemical characteristics of groundwater.Furthermore,we analyzed the sources for tunnel inflow and degrees of karst development,using Piper trilinear diagrams and Gibbs diagrams.Based on the procedures above,we used analytic and numerical methods to calculate the maximum and average water inflow of each tunnel unit,and compared the predicted values with the actual ones.We have also built a numerical groundwater flow model based on FEFLOW,which couples the simulation of regional macroscopic groundwater distribution and employs a method that combines multiple time series with the comprehensive assignment of various internal boundaries to depict the dynamic construction process.The results show the study area is developed with three main faults in.Although the permeability of these faults is weak,the gullies formed by these faults will gather atmospheric precipitation and surface water,increasing water inflow risk during tunnel construction.The TDS of groundwater in the area gradually increases from south to north,indicating that groundwater flows from the south to the north of the study area.TheδD andδ^(18)O isotope results prove that the water inflow in tunnel is from the low altitude;the distance of the groundwater runoff path is medium;atmospheric precipitation is the primary supply source.Gibbs diagram shows that the ions in the groundwater are mainly from rock weathering,suggesting that there may be karst fissures in carbonate rocks due to water-rock interaction within the area.In addition,the analytical method can efficiently calculate the water inflow of tunnel in the preliminary survey and design stage of the project.However,this method cannot dynamically predict the change in the water inflow.In essence,it is an analytical formula derived from the theory on the stable movement of the phreatic aquifer to the complete well.Therefore,the groundwater table dramatically affects the result,and the prediction accuracy in the section of the high groundwater table of analytical method is lower than that of the numerical method.The numerical method focuses on the high-accuracy model of engineering scale in the karst system,which is controlled by the macroscopic groundwater distribution.It pays attention to the description of the dynamic process and the conditions of the project area.Specifically,it describes the dynamic construction process by multi-time series.The working conditions of the excavating and lining are described by"GAP"in FEFLOW,which can accurately predict the change of water inflow in construction.Therefore,based on the actual hydrogeological conditions,the analyticalnumerical method can significantly improve the efficiency and accuracy of water inflow prediction.The methods and models used in this paper are significant for preventing and controlling water inflow disasters in high-risk tunnels.