Study On The Formation Mechanism And Instability Criteria Of Typical Loess Landslides In Tianshui City

Tianshui City is one of the areas which suffer the most densely distributed loess landslides and the most severe disasters in China.In this paper,two typical giant loess landslides,Shuiyanzhai landslide and Malan landslide in Beishan district of Tianshui city are taken as the research objects.The distribution coupling method of SEEP / W module and SLOPE / W module in GeoStudio software is used to quantitatively evaluate the landslide stability in different conditions,which is used to determine the critical conditions of its overall instability.FLAC-3D is used to analyze the landslide mechanism of deformation and destruction and the instability mode under the instability conditions.The research findings are as followings:1.In the natural state,the Shuiyanzhai landslide and the Malan landslide are both in a stable state integrally,but partly unstable on surface,which is consistent with the field investigation results.2.Under the condition of rainfall,the overall stability of the landslide decreases with the decrease of rainfall probability.The influence of rainfall on the landslide consists in the comprehensive effects of rainfall amount,rainfall intensity and rainfall duration,and is more sensitive to the response of rainfall duration.Under the condition of 24 h rainfall which happens once in 100 years,both Shuiyanzhai landslide and Malan landslide are still in a stable state integrally.The Shuiyanzhai landslide is partially unstable at the lower part of the rear edge of the spring opening and the Malan landslide is partially unstable at the foot of the landslide leading edge.The sliding bodies of Shuiyanzhai landslide and Malan landslide are saturated under the rainfall intensity of 158.7 mm / 36 h,and the landslide is in a critical state of overall instability.3.Under the action of earthquake,the stability of Shuiyanzhai landslide and Malan landslide decreases significantly with the increase of earthquake load.Under the influence of 0.2g earthquake acceleration,the Shuiyanzhai landslide partially loses stability in the loess tableland at the top of the back margin,and the Malan landslide partially loses stability in the secondary landslide section at the front margin.Under the action of 0.25 g seismic load,both Shuiyanzhai and Malan landslides will be unstable integrally,reflecting that the impact of earthquakes on the stability of landslides is much higher than that of rainfall.4.Under the condition of extreme rainfall,the instability of Shuiyanzhai landslide and Malan landslide are both in a block activity pattern.The key factors for landslide reactivation of this way are the different degrees of saturation in different parts of the landslide and the different values of pore water pressure after the extreme rainfall.the intrinsic mechanism of rainfall-induced loess landslide is that the soil mass is saturated with soft plastic,the bulk density of the sliding body increases,the strength of the sliding belt decreases and the pore water pressure increases.5.Under the action of extreme earthquakes,the Shuiyanzhai landslide and the Malan landslide all exhibited the characteristics of the overall movement from back to front.Due to the difference of microtopography in different parts of the landslide,the displacement variation is different.Local micro-topography is the key factor that causes the overall downward movement of the landslide in seismic conditions.