Experimental Study On Shear Strength Characteristics Of Cement-improved Granite Residual Soil Under Dry-wet Cycle Condition

Completely weathered granite residual soil is comprehensively distributed in Hunan Province.With the rapid development of rail transit recently,granite residual soil is often used as filling material for high-speed railway subgrades.But in fact,the residual soil along the road construction generally cannot meet the engineering requirements.On the one hand,the ground rail traffic vibration at a certain intensity will cause the residual soil foundation to liquefy,sink or even sink.On the other hand,in the rainy season in Hunan The seasonal alternation of abundant rainfall and lack of groundwater in the dry season makes the residual soil soften.At the same time,this dry-wet cycle effect leads to a decline in soil strength and resistance to deformation.For ensuring the security and stability of high-speed railways,it is compulsory to further quantify the mechanical properties of granite residual rock and soil in Hunan under the effects of dry-wet cycles.With the support of the Natural Science Foundation of Hunan Province(Research on the Small Strain Stiffness Degradation Characteristics of Cement-Improved Residual Soil under Dry-Wet Cycles No.2019JJ60010).The object of the research is the residual granite soil of Xiangtan.The mechanical properties of the residual granite soil were analyzed in combination with a laboratory experiment.Exploring the strength characteristics of cement-improved soil,and further studing the changing law of the strength properties of the improved soil under the effect of dry and wet cycles.The main research results are as follows:(1)According to the actual environmental conditions on site,indoor geotechnical tests are used to determine the type and degree of weathering of the test soil sample.The research shows that the soil sample has a loose structure,poor physical and mechanical properties,high liquid plastic limit,low cohesion,and poor water stability.,Easy to disintegrate in contact with water.(2)Examine the original dry density and exaggeration of the residual shear strength of the soil.Using a direct internal shear test,the functional relationship between the above conditions and the shear strength index is summarized.The results of decreasing the initial dry density and increasing the water content of the residual granite soil will reduce required internal cohesion and internal friction angle.(3)A series of indoor direct shear tests were performed on cement-modified residual soil with varied ash ratios(3%,5%,7%),and the changes in shear strength,moisture content,ash ratio,and curing age of the modified soil were studied.law.The strength index parameters of cement-improved soil are better than those of unimproved soil.With the extension of the curing age,the shear strength gradually increases.The initial growth is slower in the early stage,and the mid-term range changes greatly,and the growth is weak in the later stage.However,the overall strength can continue to grow for a long time.Generally,28 days of curing can meet the requirements.40% of strength.(4)Considering the influence of the seasonal alternation of climate,through the triaxial test of consolidation and drainage,the strength characteristics of the residual soil modified with cement were studied under the conditions of various wet and dry cycles(N = 0,1,3,5): the addition to some extent weakened the effect of the dry cycles-Wet on the ground resistance,and the weakening of the force gradually accumulates with an increase in the number of cycles.The strength attenuation was most obvious when N=1 and 3 times,and then the attenuation gradually became flat.