Bearing Characteristics Of Sealing Shell Structure At The Bottom Of Karst Funnel Reservoir

Karst depression is a closed negative landform in karst area,which is formed by karst funnel.The construction of water conservancy project is considered to be one of the effective ways to solve the seasonal water shortage problem in karst mountainous areas,but the problem of bottom leakage is the key technical problem that must be solved first,and it is also the difficulty of reservoir construction in karst mountainous areas.Based on the technology innovation and application demonstration project of chongqing,"technology and engineering demonstration of suitable sealing at the bottom of karst funnel for reservoir formation"(nummber:cstc2018jscx-msybX0328),and taking the cizhuwan reservoir project in pengshui county of chongqing under construction as an example,this paper proposes a karst funnel sealing shell structure according to the topography,landform and geological structure characteristics of the karst funnel in the engineering area.Based on thin shell theory and numerical simulation,the bearing characteristics of shell structures are studied.The main research work and conclusions are as follows:(1)Derivation of geometrical equation of cross section of karst funnel shell structure.The geometric equation of the long and short axis section of the elliptical shell of karst funnel is given,and the method of determining the equation of the section is based on the central angle and span.According to the internal force equation of the thin film of the cylindrical shell with zero Gaussian curvature,the internal force equation of the thin film of the elliptical shell of karst funnel is deduced and simplified under the symmetrical load.(2)Sensitivity analysis of cross section of karst funnel shell structure.By using Midas geotechnical analysis software,the structural sensitivity of the two-dimensional long and short axis section of the elliptical shell of karst funnel is analyzed,including the central angle,thickness and bedrock angle.For the central angle,the larger the central angle is beneficial to the structural stress and deformation,but there will be a limit state up to 110 degrees;the thicker the shell is unfavorable to the overall stress and deformation,and it is too small.The bedrock angles(30 degrees and 60 degrees)are not conducive to structural stability.(3)Three-dimensional sensitivity analysis of karst funnel shell structure under different axial proportions.Four kinds of karst funnel-shell plugging structures with different ratio of long axis to short axis were calculated by three-dimensional finite element method,and the stress range was calculated by combining the internal force equation of the thin film of karst revolving elliptical shell.For the short axis direction,the shell is controlled by compressive stress;for the long axis direction,when the half angle of the center angle exceeds 51°56',thecompressive stress will turn into tensile stress,which is about 10 degrees smaller than the tensile stress area calculated by the mathematical model.The reason should be that the bending stress is not taken into account in the theoretical calculation.Therefore,the middle part of the shell is in compression state,the maximum settlement occurs in the center position,and the maximum tensile stress occurs at the end of the long axis.(4)This paper analyzes the influencing factors of internal force of structure in cizhuwan reservoir.In an engineering example,a three-dimensional model is built according to the actual terrain,and the structure of different schemes is optimized on the basis of the above research.The results show that the tensile stress and compressive stress decrease with the increase of the angle when the center angles of the short axis section are 90 degrees 100 degrees and 110 degrees respectively;the effect of increasing the thickness of the end of the shell on reducing the tensile stress is obvious;the cutting angle of 30 funnel and the cutting angular compressive stress of 45 degrees funnel can meet the design requirements,but the tensile stress produced by the cutting angle of 30 degrees.The results are processed by equivalent stress method,and the calculated results can meet the design requirements.