Study On Formation Mechanism And Early Identification Of Two Free Faces Of Red Bed Landslides In NanJiang

Lots of red bed landslides were triggered by rainfall on September 16 th in Nanjiang, Si chuan, which caused the nanjiang people huge life and property loss. Through to investigation of red bed landslide in nanjiang, founding that there is a special type of landslide which called two free face landslide. The landslide is ladder like distribution in cuesta, the front and side due to valley cutting to form two free faces. At present, there were some studies about red bed landslide. But there is no study about development environment, distribution regularity, influence factors and formation mechanism of two free faces landslides. Besides, there were accompanying relationship and evolution between them.The paper relies on the Chinese Geological Bureau geological survey project that “Southwest important geological disaster investigation and warning division” and “Geological hazard investigation in the Bahe river of east Sichuan”. Based on the two free faces of red bed landslides induced by rainfall on September 16 th, through the classification of landslide in red bed, we can understand the particularity of this kind of landslide. Through the example of typical landslide, analyzing the development environment and formation mechanism of two free faces landslide. Based on laboratory tests, the structural strength of slip soil was studied with different content between consolidated and unconsolidated samples, and then analyzing influence factors of strength. Through the concept model of this kind of landslide, the corresponding failure mode is deduced, and the instability mechanism of the landslide is summarized. The early identification sign of landslide is established based on its evolution process and distribution characteristic. Finally, Arcgis Software is used to carry out the landslide susceptibility zonation by using logitic regression model. The main conclusions and understanding as follows:1?Two free faces landslide developed in slow-inclination cuesta of Nanjiang, the combination of the free faces is divided into three types. The landslide of I type combination of free face is a tongue shaped or rectangular shaped. The landslide of II type combination of free face is a long strip shaped or triangular shaped. The landslide of III type combination of free face is a long fan-shaped or triangular shaped.2?The intensity curve of slip soil is convex with different water content. The strength curve of consolidated sample is less than that of unconsolidated sample to the left. The influence factors of strength of slip soil were water content, density and clay minerals. When the water content is low, the strength of slip soil was positively correlated with water content. When the water content was more than the plastic limit, the strength of slip soil was negatively correlated with water content. Under the same conditions, the strength of slip soil increases with density. The properties of clay minerals are directional arrangement, softening and expanding, that make the strength of slip soil decrease. The water content is the biggest factor influencing on the strength of slip soil.3?Through slope geological model and rainfall infiltration model of two free faces landslide to establish failure mode, including pushing and sliding mode, rotating and fracturing mode, mixed failure mode of phreatic aquifer and confined aquifer4?Combined with typical examples of two free faces landslide, the formation conditions of landslide are summarized, and the formation mechanism of the landslide is analyzed.(1)The intersection between structural plane and rock formation is a prerequisite;(2) Two free faces are favorable factors;(3) The softening and mechanical effects of groundwater are the core;(4)Extreme heavy rainfall is the major inducing factor.5?The evolution of two free faces landslide is divided into three stages. The first stage is the formation of cracks and free surfaces. The second stage is the formation of rock landslide. The third stage is the formation of soil landslide.6?It was used to identify the two free faces landslide by topography, lithological association and slope structure. The most direct and important mark are:(1)The scarp topographic like a ladder, which rock stratum dips inside;(2)the second mark is sand and mud interbeded, in addition the clay intercalation also is.(3)The third mark are two sets of structural plans, which are parallel to the rock stratum's inclination direction and strike direction.7?The space prediction model based on logistic regression is reliable. The contribution rate of impact factor to landslide is from large to small, which is slope structure, slope aspect, slope angle and elevation. Susceptibility zoning and the actual distribution of landslide is consistent.