Experimental Research On Shear Strength Of Unsaturated Silty Clay Based GDS

At present, the engineering properties of unsaturated soil has become a hot topic in the modern international soil force field, the mechanics properties of unsaturated soil are very different with the classic soil mechanics, and the engineering properties are more complex. In engineering practice, most soil is in non-saturation, especially the soil which is above the groundwater table, its non-saturation is also constantly changing with the seasons and the changing of the precipitation. So, it is very necessary to study the engineering properties of unsaturated soil. The shear strength problem of unsaturated soil is a key to the stability analysis in unsaturated class engineering, especially in unsaturated class slope engineering, the research on its shear strength is a foundation in the buckling deformation analysis of slope.The slope which is in Majiagou is under the influence of water level fluctuations throughout the year, so the slope engineerings in this area, are closely linked with the engineering properties of unsaturated soil. Therefore, the landslide soil in Majiagou is the research object. This paper systermaticly and deeply studied the mechanical properties of the unsaturated silty clay and provided a basis for the deformation analysis of slope engineerings under the influence of water level fluctuations and rainfall in Majiagou. The main research results are as follows:(1) Summarizing the basic theory of unsaturated soil, introducing two types of stress theories of unsaturated soil which are recognized in the world, summarizing the research achievements of predecessors to unsaturated soil strength theory and the constitutive models of unsaturated soil. Thus the begainners can have a preliminary understanding of the unsaturated soil theory.(2) The soil-water characteristic curve is a foundation of the mechanical property research of unsaturated soil. Research on the engeering properties of unsaturated soil must understand the grown form of its soil-water characteristic curve firstly. In geotechnical engeering, due to the related theory of soil water characteristic curve is needed in slope stability evaluation and reservoir level fluctuation on the slope stability, so the soil water characteristic curve theory has got rapid development in the field of geotechnical engineering in recent years. Beyond that, many engeering properties of unsaturated soil, such as the strength, permeability and the volume change of unsaturated soil can be indirectly obtained from the soil and water characteristic curve. The basic characteristic of the soil water characteristic curve and its influencing factors are introduced and the equations of soil water characteristic curve are summarized detailedly in this paper. The soil water characteristic curve tests of Majiagou landside soil and0.2%fibre soil are done using the five Bar pressure plate instrument, the trial curves are contrastive analyzed. From the fitted curves we can know that the air-entry value of0.2%fibre soil is lower than silty clay and the dehydration rate of0.2%fibre soil is gentler. This illustrates the water-holding capacity of0.2%fibre soil is stronger and the water stability is better than silty clay.(3) The unsaturated direct shear tests are done under controlling the matric suction for unsaturated silty clay in Majiagou by GDS unsaturated back pressure shear apparatus (UBPS). GDS UBPS is introduced detailedly; the test procedure and the sample preparation process are indicated. In addition, the test results in different matric suction are summarized and analyzed such as the consolidation deformation result, the dilatancy and the shear strength. And the change mechanism of unsaturated soil shear strength with the changing of matric suction is analyzed detailedly.①In the same matric suction, the consolidation of the specimen is increased with the increasing of the vertical load.②The increase of matric suction will increase the dilatancy, the increase of vertical load can inhibit the flip of soil particles, and thus the dilatancy of sample is reduced.③The relationship curves of the specimen shear stress with the shear displacement show shear harden type. The specimens are remodeling samples and its uniformity is well, so the force in the soil is relatively homogeneous under the action of external force. Therefore, after cut to a certain displacement, the shear brittle failure does not happen in the specimen and the shear strength tends to be stable.④The soil shear strength refers to the limit ability to resist the shear failure, and it is one of the important mechanics indexes of soil. The shear strength of unsaturated silty clay has a good linear relationship with the normal stress. With the increasing of matric suction, the shear strength of unsaturated silty clay increases gradually, especially the increase of cohesion is very obvious.⑤With the increase of the vertical load, the variation rate of shear strength is increasing with the change of matric sacution in unsaturated salty clay. This illustrates that the effect of matric sacution on soil shear strength is increasing with the increase of vertical load.⑥Due to the presence of the matric suction, the additional friction (apparent cohesion) is appeared among the soil skeleton. The increase of moisture content will make the decrease of matric suction in unsaturated soil and cause the reducing of the apparent cohesion, so that the shear strength of unsaturated soil is reduced with the decrease of matric suction (the increase of moisture content). This is the main reason that the shear strength of silty clay is reduced with the decrease of matric suction in Majiagou. In addition, the water into the soil will destroy the structure of the cement material, thicken the adsorbed water film of clay mineralogical composition, weaken the cementation between soil particles, lead to the reduce of true cohesion in silty clay and the soil particles easy to slip.(4) The unsaturated direct shear tests are also done under controlling the matric suction for0.2%fibre soil by GDS unsaturated back pressure shear apparatus (UBPS). The purpose is to research the improvement effects of fibre on unsaturated soil under different matric suction.①After mixing fibre in unsaturated silty clay, the occlusal friction is occurred between soil particle and the cellosilk, making the soil particles more easily flip, so that the dilatation of fibre sample is more obvious.②Fibre material can change the structure of soil and make the holes in soil unevenness. In the shear process, after reaching the maximal frictional resistance of fibre, the occlusal friction between soil particle and the cellosilk will be a repid decline, so that the relationship curves bewteen shear stress and shear displacement in some samples appear shear softening.③Under different matric suctions, the shear strength of unsaturated silty clay is improved obviously after mixing fibre in unsaturated silty clay, and it can be maximum increased to29.52%.④The improved value on the shear strength of unsaturated silty clay is reduced gradually with the increase of matric suction (the decrease of moisture content) in soil.⑤Generally, people just study the fibre improvement effects on the shear strength of soil and ignore the influence of matric suction (saturation) in soil. In unsaturated soil, it is unreasonable to apply the improvement effects of saturated soil to engineering practice, espesally the unsaturated soil engineering. Due to the presence and changing of matric suction in soil, the fibre improvement effect on the shear strength of unsaturated soil is changed with the changing of matric sution. Therefore, in engineering practice, especially in the engineering involved in unsaturated soil, it is very necessary to take into account the influence of matric suction to ensure the quality and improve the benefit of the projects.