Study On Hydraulic Properties Of Wide Grading Gravelly Soil As GCLs’ Protective Layer In Landfill

Seepage control techniques have been improved and in constant innovation with the development of economic and social todays. Geotextile composite impervious material such as geotextile clay layers (GCLs) have gradually been applied because of its excellence property of anti-seepage and easy to be constructed. There are many defects when GCLs are used in landfill as impervious material alone. So the specification recommends that a certain thickness of clay should been used as protective layer for GCLs/GM to make up its shortcomings. However, clay material also has many disadvantages such as resource scarcity, cracking easily with drying-wetting cycles and shear low strength, etc.Existing domestic and foreign research shows that the impermeability of GCLs hydrationed is superior and the impermeability of GCLs has been significantly influence under drying-wetting cycles. As we all know, the final cover liner of landfill in northwestern region with arid and semi-arid climate is always in the special environment of drying-wetting and freezing-thawing cycles, and the permeability of GCLs will increase fast with drying-wetting cycles. In view of this, this research uses the wide grading gravelly soil instead of clay GCLs/GM protective layer, in landfill, to explore and use the characteristics of the wide grading gravelly soil, such as high shear strength, good water retentivity and the hydraulic conductivity can meet the specification requirement after compacted, in order to improve the performance of GCLs.Based on the analysis and conclusion of the domestic and foreign research results, mainly for the shear strength test of wide grading gravelly soils is measured with large shearing equipment, and the meaning of the water retentivity of wide grading gravelly soil that used as GCLs protective layer on the background of the landfill composite impervious system is defined; The evaporation of wide grading gravelly soil samples that taken from local and artificial adjustment is simulated by artificial soil column; The soil water characteristics curve with drying-wetting cycles are measured by pressure membrane instrument; The hydraulic conductivity of unsaturated wide grading gravelly soil is calculated and base on theoretical formula. And research results of wide grading gravelly soil and clay are analyzed by contrast. Research shows that:(1) The shear strength of wide grading gravelly soil with the angle of internal friction is29°and the cohesion is11.94kPa performs well, higher than typical clay obvious.(2) There are three typical characteristics on water retentivity as the protective layer of GCLs in the specific conditions.①Natural precipitation can infiltrate easily.②t has strong capacity to store water that can be used by GCLs easily.③Soil water is not easy to be lost with the strong evaporation on the surface.(3) The rule of compacted wide grading gravelly soil in different initial moisture content and time is discussed by simulated soil column evaporation, and soil evaporation rate goes to slow with the increase of soil depth, and each layer soil water content is reduced with the growth of trial time. And three areas were divided according to the differences of soil water content.①The dehydration is violent from O-10m under the soil surface, and it is extremely difference on the water content of each soil layer.②The dehydration is strong from10-20cm, and the difference on the water content of each soil layer goes to smaller than the above soil.③It is whole to dehydrate water starting at20cm, and there is almost no difference on the water content in different soil layer.(4) The dehydration soil water characteristic curves are tested, by pressure membrane instrument, with0-3times drying-wetting cycles. The wide grading gravelly soil dehydrate rapidly in low suction, and the water content is gradually decrease when the suction is400kPa, by times growth of drying-wetting cycles. Van Genuchte model was better than Fredlund with3parameters to wide grading gravelly soil. And the unsaturated hydraulic conductivity is calculated by van Genuchten-Mualem model. It shows that the hydraulic properties of wide grading gravelly soil are better than clay.The research results can provide reference for the design of composite impervious system on local landfill.