Research On Optimization Of Filter Layer Material Performance Of Sand-gravel Dam With Clay Core

The key for earth-rock dams to intercept water and raise the water level lies in the safety and stability of the impervious body.To ensure the safety of seepage,a reasonable design of the filter layer is very important.In this paper,taking the Wendegen Water Conservancy Project of the Yinchuo Jiliao Project as an example,the filter layer is taken as the research object,laboratory seepage and seepage deformation tests,joint impermeability tests and saturated-unsaturated seepage numerical simulation were carried to analyze different filter materials.The influence of the combination of particle size composition and different thickness of the filter layer on the seepage law of the dam was studied and the results are listed as the follows.1.Through penetration and seepage deformation tests,it can draws two conclusion.Firstly,the first layer of filter material is relatively fine,and the grain size of the soil material within the upper and lower envelope of the overall gradation has a small effect on the permeability coefficient of the filter material,and has a great impact on the critical hydraulic gradient and the permeation damage slope,and the failure mode changes from flowing soil to piping.Secondly,the sand and gravel particles of the second layer of filtering material are relatively coarse,and the content of fine particles below 5mm in the soil is very small.The permeability of the soil depends on the course for the content of granular material.The permeability coefficient is about 1 cm/s,and the critical slope of coarse-grained soil is relatively small.There is not any obvious damage in the permeability test,which is a transitional failure.2.The joint impermeability test of the core wall clay and the first layer of filter material shows that when the osmotic pressure is small or equal to zero,the different particle size composition of the filter material has little effect on the joint impermeable permeability coefficient under the osmotic pressure.With the increase of osmotic pressure,the coarser particle size of the inverse filter material,the permeability coefficient suddenly increases earlier,and the osmotic pressure continues to increase due to the effect of inverse filtration,the permeability coefficient decreases or stabilizes until the final pressure is too large and the damage occurs.The combined impermeability test of different dry density combinations of filter media shows that the decrease in the dry density of the first layer of filter material will cause the seepage velocity and permeability coefficient of the soil material in the joint impermeability test to increase,but it will have little effect on the initial slope,critical slope and failure slope.3.Through the anti-seepage test of different particle size combination,different dry density combination and different thickness combination,we can see that:(1)The difference in particle size composition of the second layer of filter material has a smaller impact on the percolation slope and seepage velocity of the joint impermeable filter than the first layer of filter material.(2)The decrease of sample dry density of the first layer of filter material will result in the increase of seepage velocity and permeability coefficient of the soil material of the combined impermeability test,but it has little effect on the initial slope drop,critical slope drop and failure slope drop of the combined impermeability test material.(3)increase the thickness of the first layer of filter material,and correspondingly reduce the thickness of the second layer of filter material.The lgi-lgv curve of the first layer of filter material has a smaller change.Seepage velocity increases slightly under the seepage slope,and the lgi-lgv curve of the second layer of filtering material changes significantly.4.Through the numerical method of saturated-unsaturated seepage,the permeability characteristics of different filter thickness combinations and filter particle size compositions are simulated,it can be concluded that :(1)As the increase of the thickness of the first layer of filter materials,the core wall escape ratio shows a downward trend as a whole.Only under the condition of checking the flood level,the core wall escape ratio shows a small increase.As the thickness of the first layer of filter material increases,the dropout ratio of the first layer of filter material shows a downward trend,however the check flood level shows a growth trend.The dropout ratio of the second layer of filter material decreases with the increase of the thickness of the first layer of filter material,and when the thickness of the first layer of filter material is greater than 2.1m under the design flood level,there is an increasing trend.(2)the particle size of the first layer of filtering material increases from fine to coarse wall,and the second layer of filtering material decreases from fine to coarse wall.When the second layer of filter material is fine material,the first layer of filter material changes from finer to thicker.The ratio of effluent between the first layer of filter material and the second layer of filter material decreases.When it is granular material,the filter material of the first layer of filter material changes from finer to thicker.The ratio of the leakage of the first layer of filter material to that of the second layer of filter material increases.When the first layer of filter material is fine material,the second layer of filtering material changes from finer to thicker.The ratio drops of the first layer of filter material and the second layer of filter material decreases.When the filter material is coarse,the second layer of filtering material changes from finer to thicker,and the dropout ratio between the first layer of filter material and the second layer of filter material increases.5.Through laboratory test and dam seepage analysis,it can be seen that the variation of filter material thickness combination and particle size composition have great influence on the escape ratio drop of clay core wall of dam body,and the difference between the two layers is not conducive to the safety of dam seepage.Therefore,the thickness of the first layer of filter material can be increased within a reasonable range,and the thickness of the second layer of filter material can be reduced accordingly,thus reducing the permeability slope of the clay core wall.