Rainfall Infiltration Process Of Huangtupo Landslide In The Three Gorges Reservoir

The Three Gorges Reservoir suffers frequent landslides, cliff collapses and other geological disasters due to complicated geological conditions and frequent occurrence of extreme climate events. Landslides induced by rainfall are exacerbated by reservoir storage and resettlement project, which will become one of the the most important issues after the normal operation of the Three Gorges reservoir project. Rainfall-induced landslides is an ancient science. The mechanism of rainfall-induced landslide and evaluation method have always been highly concerned by the engineering sector, but it is still an open question. Therefore, the research of rainfall infiltration process in the Three Gorges Reservoir Area bedding landslides has important theoretical and practical significance.This thesis focus on Huangtupo landslide. On the basis of analysis of Huangtupo landslide landslide survey data and a lot of field work, the research of rainfall infiltration process in the Three Gorges Reservoir Area bedding landslides is carried out. The research contents are as follows:on base of extensive collection of the latest geological data and monitoring data, representative area was selected to do the double ring infiltration test and installed monitoring instrument for the monitoring of shallow moisture content and matric suction. Then the rainfall infiltration rule in Huangtupo area is analyzed, a rainfall infiltration model for Huangtupo is established. At last, Huangtupo landslide stability on condition of rainfall and its influence factors were evaluated by the combination of improved rainfall infiltration model and the infinite slope model. Through the above research, the thesis mainly achieved the following results and conclusions:(1) the basic characteristics and the rainfall influence characteristics of Huangtupo landslide are analyzed. Research suggests that the overall slide will not occur under the influence of a single rainfall, but mainly for shallow ground surface deformation. (2) Double ring infiltration experiments in east of Sidaogou shows that the soil saturated permeability coefficient is4.81e-5m/s, and the test results are fitted, it is found that the wetting front matrix suction is58.4cm..(3) Instruments for the monitoring of shallow moisture content and matric suction are installed in shallow ground. The rainfall infiltration process and its characteristics in Huangtupo area are obtained as follows:①The moisture content changes in the overall trend is decreased with the lapse of time, the partial segment increases, even saturation is affected by rainfall, the maximum volume water content of0.45; The overall trend of the change of matrix suction is increased gradually, the maximum matrix suction is76kpa, rainfall makes it decreases, and even to0, after the rain increased gradually again.②The change trend of moisture content and matrix suction at different depth is different, and the surface soil is more susceptible to climate. The moisture content and matrix suction change is more frequently in shallower soil, but the deeper soil is relatively more dramatic than the shallow soil changes, the variations in the moisture content and matrix suction is bigger in deeper soil.③Moisture content and matric suction has good correspondence, the higher the moisture content, the lower the matric suction. The VG model is applied to fitting the relationship between moisture content and matrix suction, the soil water characteristic curve parameters areθs=0.453.θr=2.12, a=13.35, b=1.58, c=0.37.④When the rainfall intensity is small, the water stored in the upper part of the soil, resulting in the dispersion of the wetting front depth; in the high-intensity rainfall conditions, the soil saturates quickly, wetting front continues to spread downward. The sensor in1.5m does not change significantly after the rain, which suggests that rainfall usually induces shallow landslide when there is not a deep fracture or other soft surfaces.⑤The short duration of heavy rainfall has greater impact on the upper part of the soil matric suction, while low-intensity continuous rainfall is more obvious impact on the lower part of the soil matric suction.⑥The matrix suction and wetting front change laws are different in rainy and dry season. Matrix suction and moisture content in surface soil changes frequently in the rainy season while stable in dry season. The average moisture content was0.304and0.206in the rainy season and dry season. The reaction to the precipitation is different with the seasons change, in drying condition, the soil needs to absorb large amounts of soil moisture, surface are hard to reach saturation,so wetting front transient down slowly, in wet condition, the antecedent precipitation increases the soil moisture content, soil surface saturated in a moment,and wetting front quickly spread down.(4) An improved Green-Ampt model considering the effects of slope angle and small intensity rainfall effect is deduced, and a rainfall infiltration model for Huangtupo is established. The model parameters are as follows:Sf=60cm, θs=0.453,θi=0.266. This model is used to different slope angles and permeability coefficient for the study of infiltration process in Huangtupo. Research shows that:①When the rainfall intensity is low, slope angle has no effect on infiltration; As the increase of rainfall intensity, slope angle influence began to highlight, expressed as the delay of ponding time, more infiltration of the rainfall; With the increase in infiltration time, the slope increases infiltration effect gradually weakened, until it disappears.②The smaller the permeability coefficient, surface soil is easier to reach saturation, the slower speed of wetting front advancing; the greater the infiltration capacity, the wetting front advance faster, expressed as the greater the coefficient of permeability, the wetting front advance faster, in the initial infiltration stage, the infiltration capacity is big, the wetting front goes on quickly, with the infiltration goes on, the slope between wetting front and time gradually reduce, and maintain a constant after stability infiltration.(5) The stability evaluation formula is for shallow landslide induced by rainfall is deduced by Combination of improved GA model and infinite slope model, and rainfall-induced shallow landslides in Huangtupo were evaluated. Research shows:①When the slope angle is less than30, due to the cohesive force, rain will not cause the shallow landslide. With slope Angle increases, the stability drops dramatically, slide surface becomes shallower.②when the permeability coefficient is small, the infiltration rate is slow, also won’t cause the shallow landslide; when the permeability coefficient is larger, rainfall Infiltration is very fast, the soil is hard to reach saturation, but the matrix suction value is low, it is easy to cause the shallow slide.③When the rainfall intensity is small, Little water infiltrates into slope, it does not induce shallow landslides; As the increase of rainfall intensity, rainfall infiltration increase, so slope stability is reduced. When the rain intensity is bigger than infiltration capacity, increasing rainfall intensity is notwill reduce the slope stability.④In rainy season, rainfall is frequent, intensity is bigger, so rainfall is more likely to induce landslide. When Huangtupo area suffers over20years return period of rainfall, rainfall will be greater than460mm, slope whose angle is bigger than40Will be instability; When suffers over50years return period rainfall, rainfall will be greater than552mm, slope whose angle is bigger than35Will be instability.