A Study Of Water-soil Mechanics Coupling Mechanism And Model For Loess Slope Surface Erosion On Highway

A study of water-soil mechanics coupling mechanism and model for loess slope surface erosion on highwayHighway slope erosion and failure is a phenomenon caused by rainfall and soils will be taken away from the slope sutface, which can destroy subgrade and cutting slope. Slope erosion will cause a plenty of soil and water loss, collapse at slope shoulder, slope erosion hydraulic drop, scouring at toe of a slope and surface erosion furrow. If the surface erosion develops further disasters such as collapses and landslides will take place, which will have adverse effect to normal operation of highway and will deteriorate ecological environment. Loess in study area has a different geological characteristics and water sensitivity from Northwest areas, thus the loess slope erosion becomes a prominent issue in Road-slope Project. By taking the slope in loess highway as the research object, by means of combining with field investigation and physical simulation, the physical composition, structure of loess and its collapsible deformation behavior are studied. The main factors influenced on slope erosion damage are analyzed and the micro mechanism of slope erosion damage from soil mechanics and hydraulics. Then water-soil mechanical and mathematical models are constructed and their corresponding slope stability analysis method is set up. This study can enrich and improve the theory of loess slope erosion damage calculation, and can provide scientific basis for the design and protection of highway slope in western Liaoning. Many main results and achievements of this staudy are obtained as follows:(1) Loess in study area is formed mainly by aeolian effect and water plays a secondary role. The content of silt with particle size of0.05-0.005mm were72.87%. The main mineral component is detrital minerals, and contains a certain clay minerals. Soluble salt in the region is mainly chloride and dicarbonate, contains a small amount of sulfate. In a depth of20m, there are four microscopic structure types from top to bottom:scaffold-macrospore micro-cementation structure, scaffold-macrospore-mosaic micropore semi-cementation structure, flocculated cementation structure, coagulate cementation structure. Collapsibility coefficient is in the range of0.010to0.108in Western Liaoning, mostly of the Grade Ⅰ～Ⅱ non-collapsible, locally with a collapsible, and initial moisture content has a significant impact on collapse deformation.(2) Three-dimensional digital images of loess microstructure surface are established by extracting gray information of SEM images. A new method of average fractal dimension calculation to determine the fractal dimension before and after collapse. The results show that the fractal dimension of loess is2.508before collapse and it is2.590after collapse. The micro-structure surface undulation increases, as well as the complexity of the pore.(3) Slope erosion damage features can be divided into three stages, namely, splash erosion and laminar flow erosion, gully erosion and general demolition. The procedure from rainfall erosion and failure includes a series of actions such as splash erosion, infiltration, surface erosion, rill erosion, shallow trench erosion, gully erosion, scouring erosion, collapsed and sliding. Th slope will failue from the initial splash erosion and sheet erosion under a lower intensity to medium-term gully erosion under a high intensity, and finally leading to the multi-way damage stage of collapse, overall sliding. According to the field investigation and laboratory simulation observation, cross-sectional shape of highway slope gully erosion can be divided into V-shaped, U-shaped, trapezoidal, triangular and airfoil.(4) The major impact factors on slope erosion are studied. The results show that as rainfall intensity increases, the slope erosion damage is getting worse in the circumstances of continuous increasing rainfall. The critical gradient range of highway slope erosion in western Liaoning is between36.5°and44°. Rainfall erosion is the most serious within the scope of critical gradient. For different slopes, succession of the flow erosion mode and gully erosion cross-section shape will be different. Erosivity will enhance with the increases of slope length and the rainfall rechange region, which would increase the runoff potential significantly. Loess in this region possess characteristics such as vertical joints and large-pore structure, strong permeability, high soluble salt content, cement dissolution failure when saturated, so the cohesion reduces significantly.(5) The rainwater infiltration depth gradually increases with raifall time. The rainwater saturation line on the slope is not parallel to the slope line, but presents regularity the upper superficial and the lower dark. Aided by softwares SEEP/W and PFC2D simulation, it can be concluded, from the physical experiment moisture content dynamic monitoring, that for slopes with a small angle, pore water pressure grows fast at the foot of the slope, and the water content increasing rate turns to be greater than the top of the hill. The strength among grains decreases obviously at toe, soil particles startup under multiple external forces and the toe toe begin to damage. For slopes with a larger angle, even though the development regularity of saturation line is insignificant, pore water pressure increases rapidly and the moisture content grows fast on the top. At this time the top firstly occurs damage due to soil particles was washed away by runoff. To the same slopes, saturated time of surface soils will shorten with the increase of rainfall. Meanwhile the overland flow produces rapidly, flow will expand in the slope, rainwater infiltration rate will increase, and slope erosion damage becomes heavier.(6) Highway slope erosion in western Liaoning can be divided into three dynamic processes, namely, raindrop splash erosion and runoff, soil particle separation caused by runoff and the particles transport. Soon after it rains, infiltration rate is larger, surface runfall will not yet produce on slope. Raindrops hit the slope surface, which was mainly dry soil, small particles are splashed. With the accumulation of infiltration, the infiltration rate begins to decrease. Until the moisture content reaches the natural water-holding capacity, it turns to the fill runoff q. Overland flow q will become a sheet flow along the slope surface, and the soil particles present layered erosion. When the flow rate q is close to qc, layer flow will no longer be able to maintain due to characteristic fluctuation of the underlying surface, and the gully erosion begins. As hillslope erosion pattern turned into rill erosion from sheet erosion, the erosion amount will be doubled or even dozens of doubling. Pooling and transport of particles is the last basic dynamic process of soil erosion. Since sediment is always pooled by the slope and transported to the toe, which process is similar to the generation and convergence of small drainage networks, we can get the entire convergence and sediment transport process with the help of river model as well as the water movement and sediment dynamics description.