Trial Study on Factors Analysis and Prediction of Landslide Hazard Triggered by Extreme Heavy Rainfall

To meet the service needs of predicting and warning landslides triggered by extreme heavy rainfall, based on mechanism of landslide induced by torrential rainstorm and its predicting theory, by monitoring experiment in eight trial grounds, the impacts of rainfall on underg round water table, pore water pressure, soil stress and landslides stability are studied. The influence of vegetation coverage on landslide is discussed. Combining trial data and statistics method, the precipitation values used to predict and warn the landslides are given.The function of precipitation on landslide is a dynamic process. When raining water is injected into landslide mass, water content and volume weight of rock soil mass can be increased, soil is intenerated, rock soil capacity is enhanced, and soil also becomes lubricant while penetrating into bedrock surface under weathering rock mass or layer cutting with water, which diminishes anti-slide force and causes landslide. Surface landslide in torrential rainfall is caused by short time function of violent precipitation. In the case, landslide mass arrives quickly to saturation situation that leads to ground displacement sharply, as water penetrates promptly and soil saturation degree and pore water pressure are increased, slope substance strength is reduced and it ends with landslide which is the surface landslide mechanism.Critical values of precipitation to forecast landslides are investigated, which provide bases to predict and warn landslide disasters. Eight observation sites in sensitive areas in Jiangxi Province are set to monitoring changes of water table, pore water pressure, stress of gliding zone, and landslide mass movement. Based on observational data and historic records, the purpose is to study models of landslides stability and search criteria forpredicting or warning landslide disasters. By monitoring and experiment, it is proved that landslide hazards can be predicted. Critical rainfall value to induce landslide acquired from field trial is in accordance with statistics results. Changes of water table consistent with precipitation are revealed. Water table has negative correlation with landslide stability, but is dependent on precipitation. The fluctuation of water level lags behind the rainfall wave with half to 1 day, the effect of each precipitation process on water table is different from landslide to landslide, some affecting duration lasts 5 days or less, others 15 days.Near the ground surface, water table changes strongly depending on precipitation.Drop speed of underground water varies with water table and rainfall amount. Changing features of pore water pressure in different depths of landslide body are discussed. Pore water pressure is a useful index to represent saturation degree of landslide mass. Research shows that the increase of rainfall and its intensity vary with the decreasing of landslide stability. Pore water pressure declines gradually from March to August till a lowest value and then increases again. Burying depth is in 6 to 7 meters, descending trend becomes weaker, which is related to evaportraspiration. Variations of landslide mass stress having a consistency with landslide movement are studied. Conditions of slope creeping can be well reflected by stress monitoring, indicating the probability of the mass occurring. It is proved that stress-monitoring data have more sensitivity to indicate the creeping of landslide than that of pore water pressure.Effects of vegetation coverage on landslides are derived from the intensity of precipitation. Whether more or less the degrees of vegetation cover and biology amount, or in mountainous areas, landslide disasters would occur, which are determined by the intensity of precipitation. Forest vegetation plays a role in intercepting precipitation, which can relieve water erosion. When rainfall intensity is 50 mm or more, interception amount decreases sharply. In the same geological environment, while precipitation intensity reaches a threat value that can lead to landslide, events of landslide hazard will be sharpened by vegetation weight adding to soil mass.The threat rainfall value to predict landslides in dense forest area is lager than that in sparse zone.