The Research Of Characteristics And Failure Mechanism On Typical Landslide Systems Along Shiyan-Tianshui Expressway West Hangzhong Sectiong

This paper takes the expansive soli landslides and gravelly soil landslides distributed along the Shiyan-Tianshui Expressway West Hangzhong Sectiong as the study object, combines the indoor test for physical and mechanical properties and the field test for deformation monitoring, as well asapplies multiple technical approaches like quantitative evolution, fuzzy mathematics, sensitivity analysis, and numerical simulation, to make a systematic analysis and research on the distribution characteristics of landslides along the expressway, the development mechanism of expansive soli landslides under the action of drying-wetting cycle, and gravelly soil landslides under the action of rainfall. Main conclusions as follows are drawn after the analysis and research:1. The results of analysis on the distribution characteristics of landslide hazards along the West Hanzhong Expressway show that the main factors affecting the distribution of landslide hazards include the slope gradient, the location of the landslide on the slope, the geological structure, the formation lithology, etc., and the factors bringing out the landslide hazards involve rainfall and human engineering activities. After analyzing the motion features of typical gravelly soil landslides along the expressway, the author makes summaries and conclusions based on the motion types of typical gravelly soil landslides, and puts forward a new composite motion mode of landslides, that is, the motion of gravelly soil landslides of the area centers on the pull-type motion in middle and early stages and the push-type motion in later stages.2. In combination with the test for expansibility of expansive soil and the test for shear strength under different wetting-drying cycles, the article, referring to the Fredlund Strength Theory, establishes the shear strength equation for expansive soil of north Hanzhong and the attenuation equation of shear strength. The article fits the test results by using the theoretical equation of absorption strength and calculates the effective coefficient of expansibility of expansive soil in north Hanzhong.3. Via conducting the test for wetting-drying cycle of cutting slope of expansive soil and the test for field deformation monitoring, the author, for the first time, uses the entropy theory to explain the development mechanism of expansive soil landslides under the action of wetting-drying cycles. After the cutting slopes of expansive soil are excavated, the energy which is input to the whole expansive soil system is in a non-uniform distribution due to the changes of their original equilibrium state. The difference in energy density leads to the change of the system towards the direction of uniform energy density. With the constant conversion of the internal energy of the system, the structures of expansive soil are destroyed to decrease or offset the relative difference of energy density. The over-consolidation and expansibility of the expansive soil come into effect. With the cycle repeated, the structure of the original soil mass is destroyed, the strength is reduced, the difference in energy density is effectively lowered, the entropy value is increased, and a new state of equilibrium and steadiness comes into being. The process describes the generation of an expansive soil landslide.4. By using such methods as factor sensitivity analysis method, analytic hierarchy process, and improved fuzzy information optimization processing method, the author analyzes the factors affecting the stability of gravelly soil landslides and draws the following conclusions:in terms of the volume weight of rock-soil mass and sliding mass on the sliding surface of gravelly soil landslides, cohesive force, internal friction angle, and dip angle of sliding surface, the factors affecting the stability of gravelly soil landslides are internal friction angle, dip angle of sliding surface, volume weight of sliding mass, and cohesive force in sequence.5. By using the slope wash and erosion theory and soil preferential flow theory, the author analyzes the deformation, disintegration, and destruction process of gravelly soil landslides under the action of rainfall, and briefly explains the mechanisms for deformation and destruction of the landslides. Under the action of continuous rainfall with certain strength for a long time or heavy rainfall within certain duration, the fine-grained soil in the soil mass of shallow surface layers of gravelly soil landslides begins to move under the action of slope wash and erosion, which subsequently brings about some micro-cracks and tension cracks on the slope which facilitate the infiltration of rainwater. As the rainwater infiltrates, obvious tubular passages gradually shapes in the soil mass and the preferential flow of the whole system comes into being, which finally leads to the sharp decrease of the shear strength of rock-soil mass of the sliding surface of gravelly soil landslides, the gradual destruction of stability of the landslides, the sudden decrease of coefficient of stability, and the sliding of slopes, until they are destroyed.6. The author conducts numerical analysis to the stability of typical landslides along the West Hanzhong Expressway and the results show that as far as the expansive soil cutting landslides, as the slopes are gradually affected by rainfall and vaporization (namely, the wetting-drying cycle), the cracks shape constantly, which further gives rise to the decrease of the shear strength of the soil mass. The decrease of the shear strength of the soil mass causes the buckling failure of the slopes.7. The author analyzes the structural features of separate type prestressed anchor sheet-pile walls, chooses the finite difference elastic foundation beam method as the design calculation method, conducts verification through the finite element numerical simulation, proves the applicability of separate type prestressed anchor sheet-pile walls in controlling gravelly oil landslides, obtains some experience in design and utilization, and provides broader thoughts for the design of expressways in mountain areas in the future.