Analysis Of The Formation Mechanism Of Loess Landslide Induced By Earthquake In Jiucai Township In Haiyuan County

Haiyuan,a county of Ningxia,is located at the north of south-north seismic belt and the boundary of west Ordos and Qilian Mountain.The district is at the western hilly area of loess plateau,with intensive inner geologic structural activity,complicated landform and physiognomy and frequent earthquake.The 1920 Haiyuan earthquake brought this area many geological disasters,in which Jiucai loess landslide is the most typical one.This paper takes Jiucai loess landslide under earthquake as research point,and a numerical research of landslide was undertaken using FLAC~3DD finite differential program after understanding geological condition,reading relative materials and doing static-dynamic experiment.To understand mechanism of landslide,the paper took depth analysis of force analysis of the landslide was carried out by studying fault movement and seismic waves'direction.Finally,we came to the following conclusions:1.The seismogenic fault of the Haiyuan earthquake was a Haiyuan active fault zone,showing a significant left-lateral displacement.The seismic waves of the Haiyuan earthquake spread along the fault from the northwest to the southeast,and the seismic intensity decayed slowly in the southeast direction and decays rapidly in the northwest direction.2.The static strength of loess in this area is greater than the dynamic strength,and the dynamic stress-strain relationship of loess is approximately hyperbolic.The dynamic modulus and dynamic strength of undisturbed loess are obviously bigger than that of saturated loess,but with the increase of the number of vibrations,the intensity decay rate of undisturbed loess is smaller than that of saturated loess.The dynamic modulus and dynamic shear modulus show a negative correlation with dynamic strain.Damping is positively related to the amplitude of dynamic strain.3 Jiucai landslide is located near the Haiyuan active fault zone and was affected by the Great Haiyuan Earthquake.Landslides 1 and 2 are loess landslides with large cover layers.The slopes'upper part is steep and lower part is smooth with an average slope of 12°to 15°,and the interface of loess landslide is at the boundary of loess and mudstone.4.Jiucai landsliding was a pushing landsliding under the force of earthquake,with its trailing edge presenting tearing destroying characters and the slope was sheared along the contact surface of the loess and rock.The displacement at the trailing edge of the landslide was significantly bigger than that at the middle front.The anti-slide segment of landslide 1was relatively long and the slippery soil accumulated in the middle of the landslide,while anti-slide segment of landslide 2 was short and the slippery soil accumulated at the leading edge of the landslide.5.A method for evaluating the stability of landslides was established based on the consideration of the Haiyuan seismic waves and inertial forces acting on landslides.Through force analysis,landslide 1 slipped firstly under the influence of seismic forces and inertial forces,while landslide 2 was mainly caused by the momentary inertial forces after the left-lateral displacement of the fault.