Control Mechanism Of Structure On Shear Characteristics Of The Unsaturated Loess Soils

Loess is typically a kind of structural soil, many engineering accidents andgeological disasters are closely associated with its metastable structure. Landslide is oneof the most common types of disasters in loess strata in China, and the landslides’deformation and failure characteristics are controlled by loess’s shear behavior. Theinfluence of structure on shear strength of loess is thus always a hotspot issue inengineering geological region, however, the control degree of loess structure and itsbehind mechanism are still controversial. At present, landslide disasters have become onebiggest challenge facing loess region’s economic development. After reviewing thisquestion, basing on these previous academic researches, this dissertation studies Lanzhouloess’s shear characteristics, summaries and presents four key chapters on experimentalinvestigations and numerical calculations.First of all, through amount of experimental data obtained by means of conventionaldirect shear and triaxial shear test, this paper systematically investigates the variation ofshear strength and its parameters of both natural and remolded loess samples underdifferent experimental conditions. It finds that the peak shear strength and cohesion of thenatural loess significantly reduced once its structure is destroyed and remolded, whileinternal frictional angle of the loess shows little decrease.Secondly, basing on the comparative analysis of shear strength and its parameters(cohesion and internal frictional angle) between natural and remolded loess specimens, theinfluence degree of structure on shear behavior can be directly evaluated via twoquantitative parameters of sensitivity and structural strength. With different test methods,structure doesn’t play the same role in the shearing procedure, however, similarities priorto distinguish, owing to the influence of confining pressure, sensitivity of shear strengthand its parameters of loess obtained from triaxial test are lower than that from direct sheartest.Thirdly, the mechanisms behind structure influence loess’s shear characteristics areanalyzed though three laboratory approaches of scanning electron microscope, mercury injection apparatus and pressure plate equipment. The result shows that the cementationprovided by carbonate, soluble salt and clay-sized grains among skeletal grain contacts,big pore collapse are key factors in the action of structure. Particularly, among all thesefactors, the clay grain’s impact is the largest, carbonate’s is subsequent, while matricsuction is the least important.Finally, a backfill landslide example (HengDa landslide) in Lanzhou city is chosen tocarry out a compared analysis of stability condition between backfill loess slope andnatural loess slope, which fully certify the advantage role of structure in maintaining stablestate of loess slope.The results presented in this dissertation can not only provide practical guidance ofgreat importance on the city construction in loess regions, but also give furtherimplication on formation mechanisms of loess landslides.