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Along with the development of urban construction, due to its convenience, or high cost of the land, or for complying with building regulations and civil defense requirements,underground projects are provoked in China since 90’s at the last century. Followed by the large number of high-rise buildings and municipal works,a large number of pits will be constructed. In medium and small cities,especially in big cities and coastal ones,with the trend of high-rise buildings, underground projects and deep pits are desperate.As engineering geological conditions are very complex and also possible great displacement and settlement of pits,construction will possibly damage surrounding buildings, municipal and underground pipelines.In particular, rainfall,vibration and other external factors affect the stability of foundation pits at a disadvantage. Several support methods are adopted in pits,due to low cost, reliability and convenience of soil nailing, it’s widely used in pits,slopes and tunnels. Under financial support of scientific research project of Hunan Province Education Department(03C579),in this paper, interaction facing, soil nails and expansive soils are studied through experiments and numerical analysis:(1)Based on unsaturated triaxial testing system, with different water content, the strength characteristics of expansive soils of Nanning is studied. Under certain confining pressure, with low water content, stress-strain curve is strain-softening, whereas with high water content, is strain hardening; Under high confining pressure,with low water content stress-strain curve is strain-hardening type, whereas with low confining pressure the stress-strain curve with the increase of axial strain is a slight softening trend. The shear strength of soil has a large sensitivity with moisture content. As the moisture content increases, the expansive soil friction angle decreases linearly. With the increase of water content, firstly its cohesive force increases,it reaches maximum value at the optimum water content, then it decreases with the increase of water content.(2) In order to study the friction characteristics of the interface between soil and structure, through improved direct shear apparatus, two types including Nanning expansive soil and Zhuzhou red clay are adopted. The test results shows that interface strength criterion of both types of clays can be described by Mohr-Coulomb criterion. Strength indexes including cohesion and friction angles increase with increase of dry density and vertical pressure, whreas reduce with increase of water content. The difference of two kind of soils lie in that stress-strain curve of interface is strain-softening between expansive soil and concrete facing, whereas is strain-hardening type between red clay and concrete facing.(3) In order to study the force characteristics of the soil nailing with different water content in the expansive soil slope, a model test is established, horizontal displacement of the facing and earth pressure are also studied in the test. It is shown that horizontal displacement presents a parabola shape, the middle is large, the upper and lower part of concrete facing is small. Due to swelling characteristics of expansive soils, top surface of soil nailing uplifts with increase of water content, the relationship between horizontal displacement of facing and water content is log-linear by regression analysis. Stress in the slope will be redistributed owing to existence of soil nails. Earth pressure of the facing is approximately 0.6 times of the value through Rankine classical theory(4) Based on "the FEM (ADINA), model tests are simulated by three-dimensional numerical, some conclusions are obtained as follows, numerical results of axial force of soil nails and slope displacement are in good accordance with test results, therefore, the numerical model is verified and applicable.
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