Study On External Water Pressure Of Deep Buried Tunnel Lining In Karst Areas

Under the background of "go west" policy, at present construction of the infrastructures such as hydropower and traffic of our country mainly concentrated in the southwest area, which of high altitude, complex topography and hydrological conditions, often need to build deep buried tunnel, the high external water pressure is one of the major safety hazard of the projects, and the existence of karst geology and other adverse conditions will increase the probability of engineering accidents. Therefore, it has important engineering significance to study the external water pressure of deep buried tunnel lining in karst areas.In this paper, we first introduce the research status of seepage in fractured rock mass, and to the basic seepage analysis theory of continuous medium model to carry on the elaboration. Against the ideal axisymmetric tunnel seepage model, using basic seepage theory derives the corresponding analytical solutions, and analysis the related factors’s effect on the reduction coefficient of tunnel lining external water pressure and the water inflow of tunnel based on analytical solutions. Against the typical tunnel construction measures including "ordinary lining","ordinary grouting and ordinary lining","double layer grouting and ordinary lining","ordinary grouting and draining lining" and "double layer grouting and draining lining", using reduction coefficient and water inflow as evaluation indexes to carry on the analysis and calculation, demonstrates the rationality of "double layer grouting and drain lining" measure.Then, introduces the basic theory of finite element method and the common finite element software ABAQUS, and to explore the feasibility that ABAQUS was used to analysis the tunnel seepage problem, the influence of model dimension on the analysis results. Based on ABAQUS analysis the antihypertensive effect of drainage tube on various areas of lining, points that circular tunnel drainage tube in the lining at intervals thirty degrees layout more reasonable. Through arranging drainage tubes of thirty degree intervals in the lining range to simulate "drain lining", using reduction coefficient and water inflow as evaluation indexes to do numerical analysis of the typical construction measures, demonstrates the rationality of "double layer grouting and drain lining" measure.Next, introduces the connectivity of deep buried karst and ground surface, points out that the catchment capacity and discharge capacity of karst determines inner water pressure of the karst area. And based on ABAQUS studies the impact of the karst pipeline on the tunnel lining external water pressure at "no pressure state" and "pressure state". According to the analysis results, to external water reduction coefficient value of tunnel lining which close to the karst, could make the following recommendations:(1)when catchment capacity is weaker than discharge capacity, the karst is always under "no pressure state", reduction coefficient may be taken as0.8, and if distance between karst and tunnel is short, reduction coefficient could be taken as0.55;(2)when catchment capacity is stronger than discharge capacity,①when the rainfall intensity is low (smaller than the threshold value), the karst would under "no pressure state", the reduction coefficient could be taken the same as (1),②when the rainfall intensity is large (beyond the threshold value), the karst would under "pressure state", and the inner water pressure of the karst would increase with the increase of rainfall intensity, its extreme value is controlled by height of the rainwater entrance of karst, reduction coefficient may be taken as1.2, but when taking the "double layer grouting and drain lining" measure, reduction coefficient could be taken as0.5.Finally, introduces the Adaptive Distributed Model, and through example analysis shows the superiority of the model as follows:Strong adaptability, can fit the function relation of arbitrary between observation components and factor; Can better reflect hysteresis effect on seepage of reservoir water level, rainfall and temperature, its fitting and forecasting effect is better than that of Normal Distribution Model and Rayleigh Distribution Model; Through analyzing the relationship between piezometric level, rainfall, reservoir water level, temperature and time, can timely detect the change of rock seepage character, then infer the changes of the environment of rock fracture.