Experimental Study On Unsaturated Shear Strength Of Wide Graded Gravel Soil

Northwest of landfill field are mostly located in the valley foothills,large variation of groundwater level,so in these areas the construction of landfill site closure of the composite impervious liner is bound to be affected by dry-wet and freezing-thawing cycle. In this harsh environment, the use of clay as impermeable material, there are a variety of Engineering defects, so many researchers began to look for alternative materials as the protective layer of the GCL impermeable layer.The existing engineering practice shows that the gravel soil has the characteristics of low compressibility,large bearing capacity,and can be adapted to the environment of dry-wet and freezing-thawing. In this thesis, a series of laboratory tests are carried out for this kind of gravel soil.On the one hand, the previous results are summarized.On the other hand, more deeply explore the wide grading gravelly soil in dry wet cycling conditions change law of soil water characteristic curve and under different suction shear strength characteristics. Finally verify the feasibility of the forecast formula can replace the test value.In this thesis,the wide grading gravel soil of the protective layer of the landfill liner is studied,according to the previous research results,the particle size distribution of 1×10-5cm/s was optimized by 5 groups of permeability coefficient.Particle analysis test, compaction test, variable water head permeability test, soil water characteristic curve test and direct shear test were carried out and the mathematical model of soil water characteristic curve was established for further analysis and treatment of experimental data. Finally, the shear strength test values are compared with the predicted values of the formula and the following conclusions are drawn:(1)On the grade 1,grade 3,grade 5,respectively, after 1,2,3,4times of wet-dry cycles, and then the soil water characteristic curve test results for the:Matric suction is about less than 50kPa,curve of wide grading gravelly soil 1 times of dry-wet cycles was significantly lower than that of two cycles of dry-wet curve,when the matric suction in the 50-500kpa interval,for certain matric suction,water rate decreases with the increase of number of cycles.(2)Through direct shear test for grade 5 after 1 times dry-wet cycles, the results are as following:At all levels of matrix suction, the shear stress increases with the increase of shear displacement and under the suction of a certain matrix, the shear stress also increases with the increase of the vertical pressure; When the matrix suction is less than 400kPa, the apparent internal friction angle and the apparent cohesion nonlinear increases with the increase of the matrix suction;Matric suction is about equal to 400kPa,the apparent cohesion reaches a maximum value, then the apparent internal friction angle increases with the increase of the matrix suction,and the amplitude decreases gradually and slowly tend to a stable value and the apparent cohesion will appear a certain reduction of the amplitude.(3)The shear strength test results can be compared with the predicted values:The error of Oberg formula is small, the error of Vanapalli formula is larger, and the error of Bishop formula is the largest, but the shear strength values predicted by the 3 formulas are not very large; And with the increase of the matrix suction,Oberg formula,Vanapalli formula and Bishop formula,predicted error value is also gradually increasing.(4)When predicting the shear strength,in the 0-500kpa matrix suction range,the prediction value of the Oberg formula and the error value of the test value is the smallest and the formula is simple in use and high in prediction accuracy.