Research On Shear Strength Of Compacted Gravel Soils As GCL’s Protective Layer Under Freeze-thaw Cycles

In arid and semi-arid area of Northwest China, where has the significant temperature difference during spring and winter, belongs to seasonal frozen soil areas. Within the scope of frozen deep, the landfill composite impervious system will be affected by the freezing-thawing environmental and its physical and mechanical traits would be changed so that endanged the stability and security of the landfill. In this thesis, it was summarized that referred to the research results of surface shear strength under the freeze-thaw cycles between GCLs and its protection layer at home and abroad. The key researches focus on shear strength characteristics of compacted gravel soils itself and interface between compacted gravel soils and GCLs liner under freeze-thaw cycles through indoor tests based on the design that compacted gravel soils will take the place of clay for composite impervious system in landfill.In this test, it mainly includes penetration test, freeze-thaw cycle test and shear test. Through the penetration test, it could be obtained the permeability coefficient of composite liner system(the specification requirements:k<1×10-7cm/s) and subsequently prepared compacted gravel soils, clay and different hydrates degree of GCL samples. Used direct shear test to get its shear strength parameters of soil samples and their contact with the GCL composite gasket consisting after different freeze-thaw cycles. Then analysed its trends and obtained the variation of strength parameters(c, cp) under different conditions of freeze-thaw cycles. According to the above research content and the analysis, it could be obtained the following conclusions:(1) Used self-made100×100mm type variable-head permeability apparatus for measuring the permeability coefficient of compacted gravel soils+GCL composite gasket. The test results showed that its permeability coefficient in the initial period was large and decreased with increasing time. Deduced the cause may be:1) bentonite particles had not expanded fully at first;2) GCL cut size was too small, it woud be lead water seepaging from the sidewall, and water was not completely through GCL. Eventually penetration coefficient was around1×10-9cm/s fluctuations, it coude be met the requirements of permeability coefficient in solid waste landfill.(2) Under different frequency of freeze-thaw cycles, the direct shear test results of compacted gravel soils/clay showed that the test specimen shearing strength reduced along with the number of freeze-thaw cycle increased. As the number of freeze-thaw cycles increased its amplitude of shear strength gradually became levelling off. Its cohesive strength decreased when the number of freeze-thaw cycles increased. Angle of internal friction increased when the number of freeze-thaw cycles increased. The strength of compacted gravel soils under non-freeze-thaw was greater than the condition of the freeze-thaw cycle, the shear strength of clay-itself was less than the shear strength of compacted gravel soils.(3) The direct shear test results of compacted gravel soils/clay and the GCL contact face showed that compacted gravel soils/clay and the GCL contact face shearing strength reduced along with the GCL hydration degree increased. Compacted gravel soils/clay and the GCL contact face shearing strength reduced along with the number of freezing-thawing cycle increased, also the clay and the different hydrated degree GCL contact face shearing strength all was smaller than the compacted gravel soils and the GCL contact face shearing strength.(4)It was derived by analyzing the calculation between cohesive force or angle of internal friction and the number of freeze-thaw cycles (n). According to the comparison of the measured values and the fitted value, analysed the trend of cohesive force and internal friction angle varies which was along with the variation tendency of freeze-thaw cycle.