Study On Stability And Mechanism Of Expansive Soil Fill Slope Under Precipitation-evaporation Cycle

This thesis selected expansive soil fill slope in Ankang area as the objects to research slope stability and destruction mechanism under precipitation-evaporation cycle. Based on engineering geological survey, laboratory physical mechanical properties tests, physical model test, quantitative description of the fissure recording during model test, numerical analysis of the coupled heat and moisture, and other means, the dynamic response rule and stability study of expansive soil fill slope under precipitation-evaporation cycle have been carried out comprehensively. According to the mechanism of expansion and contraction, rainfall and evaporation, the comprehensive study on the deformation and failure mechanism of expansive soil slope has been made. The main contents and results as follows:(1) The material composition of the expansive soil, structural characteristics and the basic physical properties is analyzed by engineering geological survey and experiments of the expansive soils in Ankang area, based this, different saturation compaction and compression performance is explored. Then strength properties of different initial state, as well as changes in the strength characteristics under the wet and dry cycle effects. The results show that:the correlation of different saturation compaction and compression performance is negative; the differences of cohesive force between different initial states are very obvious; with the increasement of the cycle number, the speed of shear strength attenuation slower, at last shear strength trend to constant.(2) Soil temperature and humidity, soil pressure, suction and slope deformation of the physical model system which established in Ankang area have been monitored by self-assembled data acquisition system, combined with meteorological observations to analyze the dynamic evolution of the physical quantities and its influencing factors of expansive soil fill slope under the action of precipitation-evaporation cycle and waterlogging. The results show that:the engineering behaviors of expansive soil fill slope is closely related to the change of the water content in the soil; rainfall and waterlogging are the most direct factors which affect water content of expansive soil, and the evaporation of sunshine and cloudy days is indirect effect factors. The coupling of them leads to tend of deformation and failure of slope.(3) The type of the fill slope fissures defined to be weathering fissures with analysis and discussion of fissure classification and causes. Photographed the development process of fissures, then used vector technical quantitatively to descripte the evolution rules of the fissures and obtaine parameters of the geometry and fissures· degrees; then explored the influence of precipitation-evaporation, water content and atmospheric temperature. The results show that:fissures distribution is irregular during the monitoring period, and the system pattern is very chaotic, basically, no cracks paralled to each other; the fissures closed under rainfall, and gradually developped into a mesh during the evaporation of the later interconnected, with the increase of the cycle, the soil was gradually completely broken, to a certain level it basically became stable.(4) Based on the results of previous studies, the expansion and contraction mechanism of expansive soil under precipitation-evaporation cycle could explained as follows:during the rainfall, the water molecules came into the soil and constantly wrapped crystal layer, soil particles and their aggregates, then changed into mechanical effect through complex physical and chemical effects; they interconnected and formed a combination of water film. Due to this combination of the water film thickening, the distance between grains and crystal layers increased, leading to soil expansion; conversely, water film thinning or disappearance during the evaporation, distance between grains decreasing, thereby soil volume reduced.(5) With clearly understanding of physical processes of rainfall and evaporation, firstly, founded physical and mechanical mechanism between water and soil by analysis results of laboratory and model test; then combined chemical action type and main mineral composition of expansive soil, founded water-soil chemical mechanism. Secondly, with deep study of mechanical mechanism of rainfall erosion and evaporation shrinkage cracking, then studied the condition of derivation of soil particles, infered a series mechanical formula of the runoff and erosion on slope macro,as well as soil cracking. Ultimately, combined the comprehensive mechanism of expansion and contraction and the above aspects, to make a comprehensive explanation to effects of precipitation-evaporation cycle to expansive soil fill slope on deformation and failure mechanism. Reach a conclusion that evaporation is an important prerequisite to the deformation and failure, and the rainfall is a direct cause of the deformation and failure; the cycle of them could lead expansive soil fill slope to deformation destruction.(6) Detailed heat and moisture coupling theoretical model, boundary conditions and parameters of the assignment of calculated, then built geometric slope model based on the field test slope model. Its numerical comparison analysis verified the correctness of theoretical model and the boundary conditionst. On this basis,studied heat and moisture coupling properties of fill slope with fissure, the results show that:the presence of fissures on the role of precipitation-evaporation effect was significant, not only improved penetration performance by provide a fast-track for rain water infiltrate to slope internal; but also increased the evaporation role boundary and provided a convenient by fissure surface directly contacted with the atmosphere.(7) For numerical analysis has to consider both rainfall and evaporation characteristics, both sides of fissures are treated as climate boundaries, then moisture can transport both through soil surface and the fissures sidewalls during rainfall and evaporation; moreover, considered clods divided by fissures still comply with unsaturated soil permeability.This method not only takes into account the impact of fissures during precipitation-evaporation, but also correspond unsaturated soil mechanics theory, is applicable to numerical analysis of soil fill slope.(8) Based on the results of above studies and numerical analysis, analysed the state of slope stability under seepage action, with considering the intensity attenuation as well as fissures evolution. The results show that:the potential slip surface of fissured slope under rainfall is closely related to humidity field distribution, the slip surface will become shallower with rain continued, overall,the safety factor is high. While considering the strength attenuation, the safety factor is much smaller than not, and reduced with the increasement of precipitation-evaporation cycle number, ultimately tend to instability. Slope stability analysis result when considered both fissures and strength attenuation is in line with people’s cognitive, could be a true reflection of the slope failure.