Study On Undisturbed Loess Joint Strength Theory And Slope Engineering Applications

Loess is widely distributed in China, especially in the loess plateau in the middle reaches of the Yellow River, where not only distribution range is wide, but also has the large thickness and the whole formation levels is a typical loess area in our country. There were many researches about mechanical properties analysis of loess on the region, only tend to focus on its shear strength, and the study on the tensile strength is less. Although the tensile strength of loess is small, but the development of geological hazard especially the loess landslide, collapse, and ground fissure is related to the tensile properties. Meanwhile, it’s difficult to judge the soil damage state under the condition of a tensile stress. At present, there has not existed a joint strength which could both judge the tensile and shear failure.In this paper, we take the typical loess in Luochuan, Shanxi as the research object to measure the ultimate strength of undisturbed loess under different stress state through laboratory test, fit and deduce the quadratic parabola joint strength theory formula. On this basis, using the plastic limit analysis and the balance method to deduce the slope stability formula with the fracture lines and logarithmic spiral critical slip surface and the slope surface stability formula respectively, and applied to analysis the loess slope stability successfully. The following achievements were obtained:(1) The tensile strength decreased nonlinearly with the increase of moisture content according to the loess tensile strength experiment of direct tensile, radial fracture and axial fracturing, and the tensile value obtained from two types of fracturing experiment was smaller than direct tensile experiment value, the relationship is approximately linear. Therefore, the tensile strength of loess can be determined by the simple and feasible method of fracturing experiment.(2) Fitting and deducing the quadratic parabola joint strength theory formula through the strength results of the tensile, compressive, and shear experiment, which could judge the failure of undisturbed loess on the experience of uniaxial tension, uniaxial compression and triaxial compression under different moisture.(3) The slope stability evaluation method with the failure mode of the crack- linear and logarithmic spiral line was establised based on the plastic limit analysis method; moreover introduce the method of nonlinear value to the slope stability analysis to make the result more practical compared to the traditional method with the constant cohesion and friction angle.(4) On the basis of field investigation, the slope surface stability evaluation formula is derived through the force balance method, where the tensile stress zone is the failure limit. The theoretical analysis shows that slope surface spalling damage tends to occur for the steep slope, with the slope ratio more than1:0.5, caused by dry-wet circulation, and destruction could usually be blocking successive peeling.