| Hydro-junction Projects are the main technical way to make full use of hydraulic energy resources and solve the problem of uneven spatial and temporal distribution of water resources,while the seepage control of earth dams have always been a hot research topic in hydraulic structure engineering.It is well known that due to the characteristics of the construction materials of earth dams,seepage is common inside the dams,causing infiltration damage.The combination of seepage prevention and drainage is one of the effective measures to control seepage erosion and seepage deformation in earth dams.It has been shown that the installation of filter can better prevent the loss of fine particles and give full play to the role of the drainage body to release the pore water pressure and reduce the possibility of infiltration damage,thus significantly improving the stability of the dam.Previous studies on the performance of the filter have focused on the case where the seepage direction is perpendicular to the contact surface between the filter and the base soil,and the design criteria for the filter in the current specifications are based on the results of such tests.However,the seepage direction and the contact surface between the soil and the filter not always remain orthogonal to each other in actual projects due to different seepage areas and upstream and downstream water levels,but change dynamically.Therefore,it is necessary to investigate the influence of different infiltration angles on the filtration performance at the soil/filtration contact surface in the seepage field of earth dam.The thesis adopts a combination of experiments,numerical simulations and theoretical analysis to study the effect of different infiltration angles on the filtration performance at the soil/filtration contact surface.The test soil is taken from the Shizi reservoir yard in Xiji County,Guyuan City,Ningxia,the basic physical and mechanical indicators of the soil sample test at first,according to the specification of the filtration guidelines for the soil sample set the corresponding filter,the development of seepage conditions under the soil/filtration contact erosion test program,the use of the subject group developed by the multi-functional infiltration tube surge tester for the relevant soil/filtration contact surface of different infiltration angle of the infiltration test,the soil pressure,pore water pressure,infiltration rate and permeability coefficient and other test data were analyzed.On this basis,numerical simulations based on discrete element theory and PFC-3D software were used to investigate the effect of infiltration angle on the filtration performance from four perspectives:intrusion rate,intrusion depth,porosity and local infiltration velocity,and to discuss the theoretical basis of the test results based on the law of seepage refraction.Finally,the applicability of the current specifications of filtration when considering the influence of the contact surface infiltration angle is discussed in the context of the filtration systems of "Zhangfeng Reservoir" and "Qiaojiagou Reservoir".The main research contents and conclusions are as follows:(1)The results of the filtration tests carried out according to the test protocol developed in this thesis show that,at the same depth,the larger the infiltration angle is,the smaller the pore water pressure is,which indicates that the filtration has stronger drainage capacity;with the increase of hydraulic gradient,the larger the infiltration angle is,the smaller the change of soil pressure is,and the better the infiltration stability of the reverse filtration system is;the smaller the infiltration angle is,the faster the growth of infiltration velocity is,but there is no sudden change of flow velocity curve,which indicates that the stability of the filtration system is,and no infiltration damage occurs;with the increase of hydraulic gradient the infiltration coefficient is decreasing trend,among which the smaller the infiltration angle is,the larger the infiltration coefficient is.(2)The numerical simulation results show that:the larger the infiltration angle,the smaller the intrusion rate of the protected soil particles,with the increase of hydraulic gradient,the intrusion rate curve tends to flatten;the depth of intrusion with the increase of hydraulic gradient,the curve tends to flatten,the smaller the infiltration angle,the greater the depth of intrusion;the porosity within the measured range on the contact surface gradually decreases with the increase of the hydraulic gradient,and the porosity variation within the local range is larger for the model with smaller infiltration angle;with the increase of the hydraulic gradient,the local infiltration rate grows faster when the infiltration angle is smaller,and the local critical hydraulic gradient is lower.(3)Comparing and analyzing the results of backfiltration tests and numerical simulations.The larger the infiltration angle,the smaller the variation of pore water pressure,earth pressure,infiltration rate,permeability coefficient,intrusion rate,depth of intrusion and porosity,and the larger the local critical hydraulic gradient,indicating that the larger the infiltration angle,the better the infiltration stability and the better the filtration performance;with the increase of hydraulic gradient,the change patterns of the above physical parameters for different infiltration angles are basically the same and do not change abruptly,indicating that the filtration criteria of the "Design Code for Crushed Earth and Rock Dams SL274-2020" are applicable to the working conditions of different infiltration angles.(4)Numerical simulations of the filter system of "Shizi Reservoir" designed in this thesis and the filter systems of "Zhangfeng Reservoir" and "Qiaojiagou Reservoir" with different infiltration angles show that,although the local critical hydraulic gradient,intrusion rate and depth of intrusion all reflect that the filtration performance decreased with decreasing infiltration angle,which is consistent with the findings of the previous study.The filtration design guidelines in the current specifications have a certain safety reserve,and the numerical simulation results show that the adverse effect of small angle infiltration on the filtration performance is still within its safety reserve,which reflects the suitability of the specifications for different infiltration angle.The results of the above study can provide a reference basis for the setting of the filtration in actual projects. |
