Study On The Strength Experiment Of Gravel Soil In The Slope Of Daqianshiling Tunnel

As the plan of revitalizing northeast old industrial base has been implemented, the investment of infrastructure construction constantly increases, and infrastructure construction of railways, highways and bridges in the northeast area come to flourish. Accompanied by a great deal of engineering construction, artificial high slope problems became increasingly prominent. A variety of slope instability are often caused due to shear failure, so it is pivotal to grasp the strength characteristic of rock soil Mass accurately when slope stability analysis and control engineering design was carried out.Therefore, research on physical mechanical properties of the rock soil Mass has become an important task of slope stability analysis and prevention of disasters. Shear strength index of rock soil mass is an important mechanical parameters of slope stability evaluation, its value determines the results of stability analysis, which was usually obtained by experiment measures (including laboratory tests and field in-situ tests), then the shear strength parameter index can be obtain based on the statistical regression analysis of experimental data.An artificial slope formed due to excavation at entrance section of Daqianshiling Tunnel in the Tianshifu-Huanren railway which was under construction recently. Slope composition is mainly gravel soil and granular soil. It will be a serious threat to normal operations of the railway and people’s life and property safety once slope failure occurred. Physical and mechanical properties of gravel soil are different from the usual soil and general rock. Gravel soil is a special kind of highly nonlinear inhomogeneous geological body between the soil and rock mass. Because the content of coarse particle in the Daqianshiling Tunnel slope soil is considerably large, and the difference of soil properties is apparent while composition of coarse particle is different, conventional direct shear tests have been unable to meet the full study of its mechanical properties. Taking gravel soil in the slope of Daqianshiling as the research object, this paper conducts research on the basic physical properties, shear properties, shear strength parameters variation and granularity fractal characteristics of gravel soil through the field survey sampling, indoor regular physical mechanics test and indoor large direct shear test, which helps provide reliable geotechnical parameters for the later slope stability analysis and prevention.In this paper, the main research contents and results are as follows:(1) Study on basic physical properties. The nature density, natural moisture, particle composition and so on basic physical property parameters are determined by the field test and laboratory test.(2) Study on shear strength characteristics. Based on indoor large direct shear tests, supplemented by conventional direct shear tests, three groups sample of different stone content (coarse grained content was 30%,50% and 68.53%), three groups sample of different moisture content (moisture content was 9.05%,12.50%, saturated) and three groups sample of different the stone size (three interval size are 60～40mm,40～20mm,20～5mm) are configured combining with the basic physical parameter of gravel soil in the slope of Daqianshiling Tunnel. Large direct shear test and conventional direct shear test under three different moisture contents are conducted, and the corresponding shear strength coulomb fitting formula is established.(3) Study on the relationship between the shear strength parameters and stone content. Combining with the large direct shear test results of three soil samples with different stone content, this paper analyzes the laws of shear strength, strength parameters, stress-strain curve with the variation of stone content changing. The study found that the stone content largely affects the shear strength of gravel soil. With the increasing of stone content, the shear strength increases. Internal friction angle increases as stone content goes up, which increases rapidly with the stone content ranging from 30% to 50%, while it showed a gradual increase in slow steady trend when the stone content was more than 50%. Cohesion rises sharply after declining slightly with the increase of stone content, and when stone hits 50%, cohesion reach the lowest and then rise sharply. There was no obvious peak intensity in any stress-strain curve of different stone content soil samples; it showed three phases that are elastic deformation stage, the yield phase and strain hardening stage. With stone content increasing, the more obvious the yield stage is; under the same normal stress, strain hardening of high stone content samples is significantly higher than that in low stone content. Gravel soil deformation and failure mechanisms affected largely by their stone content.(4) Study on the relationship between shear strength parameters and moisture content. Combining with the large direct shear test results of three soil samples with different moisture content, this paper analyzes the laws of shear strength, strength parameters, stress-strain curve with the variation of moisture content changing. The study found that the shear strength decreases with moisture content increasing, and the internal friction angle decreases with moisture content increasing. Relative to internal friction angle, cohesion is more sensitive with the moisture content changing; it reduces sharply after rising slightly with the increase of moisture content. When the moisture content increased from 9.05% to 12.5%, cohesion increased slightly and then increased sharply with moisture content increasing. There was no obvious peak intensity in any stress-strain curve of different moisture content soil samples. It showed three phases that are elastic deformation stage, the yield phase and strain hardening stage. With moisture content increasing, the less obvious the yield stage is. Under the same normal stress, strain hardening of low moisture content samples is significantly higher than that in high stone content.(5) Study on the comparison between large direct shear tests and conventional direct shear tests. It can be found through the comparison and analysis of large direct shear test and conventional direct shear test under three different moisture content that variation of cohesion and internal friction angle under the two different ways of testing are basically the same, large shear tests concluded that cohesion was significantly higher than that of conventional small indoor shear test while the gotten internal friction are little different. The shear strength parameters obtained by the large direct shear test is closer to the actual mechanical properties of gravel soil.(6) Study on fractal dimension of gravel soil particle size. Introducing the fractal theory, this paper makes dimension calculation to the samples in the stone content of 50% with using the particle quality-size distribution fractal model. The result shows that:Under the same stone content, fractal dimension increases with the stone size increasing; The bigger the fractal dimension is, the worse the particle distribution is; The smaller the fractal dimension is, the better the particle distribution is.(7) Study on the relationship between shear strength parameters and stone size. As research shows, when stone content is the same, shear strength increases with stone size increasing, also its increase range does; the internal friction angle increases with stone size increasing, while cohesion is on an opposite trend, it decreases alone with the increase of stone size. There was no obvious peak intensity in any stress-strain curve of different stone size soil samples. It showed three phases that are elastic deformation stage, the yield phase and strain hardening stage. Stress-strain curve presents strain hardening type in the condition of large and middle particle interval. However, small particle interval gradually shows a weak strain softening under high normal stress. With the increase of the stone size, the yield stage before shear stress reaches peak strength lasts longer.(8) Study on the relationship between shear strength parameters and fractal dimension. As research shows, there is a certain degree of correlation between shear strength parameters and fractal dimension. Cohesion decreased with fractal dimension increasing, while internal friction angle is on an opposite trend, it increases with fractal dimension increasing; cohesion and internal friction angle have approximately parabolic function relationship with particle size fractal dimension value.