Study On Water Movement Law In Typical Loess Slope Under Heavy Precipitation In Yan’an Area

Yan’an is located in the hinterland of the Loess Plateau.Due to its special natural environment,Yan’an suffers numerous casualties and property losses due to landslides and other disasters.Strong rainfall is an important factor in inducing loess landslides.Studying the laws of soil moisture migration on typical loess slopes under heavy rainfall from the perspective of hydrogeology can provide an important basis for preventing and controlling loess landslides.Taking the typical loess slope in Baota District of Yan’an City as the research object,field investigations and laboratory experiments were conducted.According to the saturated-unsaturated seepage theory,a two-dimensional numerical model of unsteady saturated-unsaturated seepage in the vertical section of loess slope is established.Analyze the change of saturation distribution under the conditions of rainfall intensity,permeability coefficient,previous rainfall conditions,rainfall duration,slope,slope height,and crack length,and then summarize the law of moisture migration on the loess slope.Finally,the safety factor of loess slope under different conditions was calculated using shallow landslide safety factor formula,its stability was analyzed,and the sensitivity of the parameters was analyzed.The main conclusions are as follows:1.Analyze the impact of a rainfall.At the end of rainfall,the topsoil saturates and the slope foot is most unstable.As for the slope waist,the saturation of shallow soils changes rapidly,and the deep soil changes slowly.It is mainly controlled by the soil moisture content in the previous period.The seepage near the bedrock is blocked,the water is enriched,and the saturation is increased.2.With regard to the slope geometry,the steep and gentle slopes have similar infiltration patterns.Under the three saturations,the maximum infiltration thickness of the slope decreases with increasing slope.The depth of infiltration under different slope height is approximately the same,slope height has little effect on infiltration depth.3.Simulating the effect of parameter changes on the saturation distribution shows that: Rain is stronger than permeability and surface soil is saturated.Above the impact depth of this rainfall,the greater the rainfall intensity is,the higher the saturation is;below this depth,the saturation of different rain strengths is equal,which is mainly controlled by the soil moisture content in the previous period.A drizzle is more likely to increase the slope saturation than a short-term torrential rain.The greater the permeability coefficient,the greater the increase in surface soil saturation.The more moist the soil in the early stage,the higher saturation at the same depth,the drier the soil,the greater the increase in surface soil saturation,the deep soil saturation does not change,and is controlled by the soil moisture content in the previous period.After the rain stopped,the water continued to infiltrate under the effect of gravity.The high saturation range has been reduced,and the low saturation range has been expanded.Wet front keeps moving forward.4.Through the calculation and analysis of cracks of different lengths,it is shown that when the cracks are long and the areas near the cracks are in contact with the high saturation areas on the bedrock surface,a weak zone is easily formed.In addition,the large areas of shallow soil and slope foot are saturated at 0.7 or more,and the increase in weight after absorption of the soil around the cracks makes it very easy to destabilize the slope.The smaller the soil permeability coefficient is,the slower the central saturation decreases;conversely,the larger the crack impact scope,the more likely the slope is unstable.5.Analysis of the impact of various parameters on the stability of the slope.It is known that the rainfall intensity,the permeability coefficient,and the previous soil moisture content all have such law that the larger the value,the lower the safety factor.After the rain stopped,the safety factor continued to decrease.When less than a certain safety factor,it will cause shallow landslides.The wet season has the lowest safety factor.The longer the rainfall,the smaller the safety factor.The sensitivity of each parameter in the shallow landslide calculation formula is as follows: slope,slope height,cohesion,internal friction angle and soil bulk density.