| In practical engineering,the change of soil water content caused by rainfall and seepage is the main cause of soil failure.The increase of soil water content reduces the shear strength of soil,resulting in a series of engineering accidents such as subgrade collapse and dam failure.At present,scholars at home and abroad have studied the strength theory of unsaturated soil deeply.However,the research on the strength theory and the law of strength change on sandy soil with different fine grain contents in the unsaturated state is rarely involved in the high suction stage.Therefore,In this paper,unsaturated sand soil with different fine grain content is taken as the research object to study the influence of fine grain content and saturation on strength characteristic,volume variation characteristic and soil-water characteristic curve of unsaturated soil.In this paper,the change law of shear strength for unsaturated sand with different fine grain contents in the full suction range is studied.The main contents of the laboratory test include: We prepare 4 kinds of standard sand with fine grain content of 8%,16%,24%,36%,coarse and fine mixture with fine grain content of 50%,65%,80% and pure fine-grained Qinghai silty clay.Control the same compaction work to make saturated ring knife soil samples with an inner diameter of 61.8mm and a height of 20 mm which are dried indoors to obtain unsaturated soil samples with different moisture content.The filter paper method and WP4 C are used to measure the matric suction and draw the SWCC curve of sand with different fine grain contents.The direct shear test is carried out with the four-linkage direct shear instrument to measure the shear strength under different water states and then study the strength variation characteristics.Based on the laboratory test data,the shear strength parameters of unsaturated soil are calculated,and the existing strength theory of unsaturated soil is used for analysis.Then the strength model of unsaturated soil is tested and verified,and the existing strength model was improved.Through experimental research,the main conclusions are as follows:(1)Through the analysis of the peak strength-saturation curve for sandy soil with different fine grain contents,it can be found that: In the range of total suction,the strength and capillary cohesive force of unsaturated soil will all appear a peak with the change of matric suction,no matter how much the fine grain content is.(2)In the logarithmic coordinate system,the relationship between peak intensity and the logarithm of saturation is almost linear.For sandy soil with different fine grain contents,the peak intensity-saturation relation curve is distributed in a sector area.(3)Saturation and fine grain content have a great influence on the stress-strain curve shape of unsaturated sandy soil.It can be seen from the shear stress-shear displacement relation curve: With the decrease of saturation,the failure pattern of soil sample gradually changes from plastic failure to brittle failure.During the shear process of completely dry unsaturated soil samples,the shear stress will appear a cliff-like steep descent and the shear stress of soil decreases sharply.The residual strength of sand samples with all fine grain contents increases with the decrease of saturation.(4)The saturation corresponding to the plastic and brittle transition points of the shear failure pattern first increases and then decreases with the increase of fine grain content,which is mainly because the increase of the fine grain soil content changes the grain gradation and mineral composition of the soil sample,making the macroscopic mechanical properties of the soil samples to show a big difference.(5)With the decrease of saturation,the dilatancy of soil increases and the dilatancy decreases.For the soil sample with high fine grain content,the lower the saturation is,the smaller the shear contraction volumetric strain of the soil will be,and the smaller the shear displacement is when the soil changes from shear contraction to shear dilatation.The drier the soil is,the greater its elastic modulus is and the smaller the shear strain is,when the peak strength appears from stress-strain curves.(6)When the fine grain content is low,the air entry value and residual values of unsaturated sand increase with the increase of the fine grain content.With the further increase of the fine grain content,the increase of the air entry value and residual values slows down until they are stable at a certain level.The residual saturation of soil samples increases first and then decreases with the increase of fine grain content.When the fine grain content exceeds 65%,the residual saturation basically remains unchanged with the increase of fine grain content. |
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