Meticulous Simulation And Evolution Law Research On The Seepage Flied Of Underground Caverns

Underground caverns are the key generation part of hydropower engineering,the distribution of the seepage flied of rock mass is important to the structural stabilityand antiseepage drainage design of underground caverns.However,the calculation efficiency and accuracy of the seepage analysis cannot meet actual engineering requirements.The main reasons for these problems include the seepage theories of strong permeable media including fault and bedding fault zones are not consummate;The small size and large number of drainage holes;The range of the seepage analysis simulation model is not clear;The topography of the project area is undulating.How to improve the calculation accuracy and efficiency of the seepage field of underground caverns is great significance to the "Dual Carbon" strategy and hydropower longterm planning.In this study,theoretical analysis,numerical simulation,laboratory experiments and engineering verification were used.The non-Darcy flow’s characters of the broken rock mass of fault zones were analysed with different conditions,the computational models of main parameters were established to modify Forchheimer and Darcy’s law.The superiorities of drainage hole explicit and implicit simulation methods were combined based on well hydraulics theory,the simplified drainage hole explicit simulation method was proposed.Taking the underground caverns of pumped storage station as research object,the influence mechanism of model range on seepage field was explored,the value of model range and the location of computational boundary were defined.Integrating the application of information processing,modeling and finite element softwares,sub-model technique and the proposed theories and methods to analyze the evolution of the seepage field of underground caverns,and evaluate the seepage control effect.The main results are as follows:(1)The influence mechanism of broken rock mass gradation and porosity on non-Darcy seepage characteristics was revealed,the application scope of Forchheimer and Darcy’s laws was broadened,the transition between Darcy’s and non-Darcy’s laws was realized.Based on laboratory tests the non-Darcy equivalent hydraulic conductivity was defined,the evolutionary characters of the non-Darcy equivalent hydraulic conductivity,permeability and non-Darcy seepage coefficient was illustrated with the changed hydraulic gradient,broken rock porosity and gradation,the corresponding computational models were established.On the basis of the computational models,the non-Darcy’s seepage theory was perfected and popularized by modifying Forchheimer and Darcy’s laws.(2)The simplifying hole to line and simplifying hole to face drainage hole simulation methods were proposed,which could conduct the contrastive analysis of drainage hole diameter and space in one simulation model and provide technical support for the comparison and selection of large-scale drainage hole arrays layout scheme.The simplified methods were proposed based on the analytical solution of flow rate,and combined the advantages of drainage hole explicit and implicit simulation methods.Compared with implicit simulation method,the simplified methods could provide effective computational boundary.Compared with the explicit simulation method which was represented by the drainage substructure method,the nodes of drainage hole boundary of simplifying hole to line method would be reduced by 3/4,the nodes of drainage hole boundary of simplifying hole to face method would be reduced by 1/2.Besides,when tetrahedral mesh was adopted for the simplifying hole to line method the tedious substructure modeling process could be avoided.(3)The influence zone of underground caverns to seepage flied was defined,the determination method for the simulation model range of underground caverns was innovated,the computational boundary location could be realized quantitatively.The change mechanism of the calculation results of the seepage field with the changed model range was clarified within the influence zone of underground caverns,the conclusion of the seepage flied simulation model range should be larger than or equal to the influence zone of underground caverns was proposed.The influence of groundwater surface slope ratio,maximum water depth above caverns and area of potential seepage boundaries on the values of the influence range of underground caverns was illustrated with theoretical analysis.The mathematic model among underground caverns flow rate,model range and the influence factors was established.(4)The sub-model technique and proposed non-Darcy seepage model,the simplified drainage hole explicit simulation method,model range determination method were applied improve the fidelity and computational efficiency of the geological model.The evolution law of the seepage flied of underground caverns and groundwater level of sub-model boundary revealed during impoundment of reservoir and excavation of caverns.The influence mechanism of the layout forms of drainage hole arrays and excavation process of underground caverns on the seepage control effect of engineering was revealed.The results shown that the drainage system dominated the results of groundwater level and the flow rate of caverns.Increasing the drainage hole diameter and decreasing the layout space could further reduce the flow rate of powerhouses.Decreasing the drainage hole space possesses more significant effect.The decline process of groundwater level and decrease of the flow rate of powerhouse system could be improved with the prior excavation of drainage system.Thus,decreasing drainage hole space could sharply adjust the efficiency of drainage system when increasing drainage hole diameter brought minor adjustment with allowed surrounding rock conditions and engineering planning.Furthermore,it was suggested that the drainage system should be excavated before powerhouse system at same elevation,to reduce the drainage pressure of powerhouses and improve construction safety.