Research On Progressive Failure Mechanism Of Jurassic Sand-mudstone Interbedded Rock Slope

Landslides occur frequently in the Jurassic strata of the Three Gorges Reservoir area in China.According to statistics,the number of landslides accounts for 39 % of the total number of landslides in local counties.There is a special landslide in this region,and most of these landslides are developed in the sand-mudstone interbedd rock mass with slow dip angle.Due to the difference in strength and structure of sandstone and mudstone,with the effect of time and external factors,mudification and fracture gradually penetrate in the contact zone of sand and mudstone,which leads to gradual destruction of the slope causing landslide disaster.In this paper,starting from the evolutionary process of progressive damage of sand-mudstone slope with slow dip angle,the dominant joints investigation and statistics of Jurassic stratigraphic region in Zigui County were carried out by using UAV photogrammetry,and the rock fracture model of the study area was established;the influence of fracture on the mechanical parameters and damage mode of single lithologic rock mass and laminated composite rock mass was analyzed,and the Kamenziwan landslide in Zigui County was taken as an example to investigate the sandy mudstone interbedded rocky slope.The discrete element simulation of the deformation and damage process is carried out to study the progressive damage mechanism.The main research contents and results of this paper are as follows:(1)According to the distribution of sand-mudstone landslide in the study area,the areas near Kamenziwan landslide,Shanshucao landslide and Majiagou landslide are selected as the investigation areas.The UAV photogrammetry technology and traditional contact measurement method in non-contact measurement are used to conduct the statistical investigation of the dominant joints in the investigation area.A total of six groups of dominant joints are measured,and the ductility and spacing information of the dominant structural plane are interpreted according to the UAV image.The UAV photogrammetry technology and traditional measurement method are compared to verify the accuracy of UAV photogrammetry.(2)Particle flow discrete element software PFC2 D is used to carry out uniaxial compression test on single lithologic rock mass and composite layered rock mass with cracks.According to the laboratory test,the “ trial-and-error method ” is used to calibrate the meso-parameters of rock mass,and the joint bond model is used to prefabricate cracks.Four kinds of fractured rock mass are established,and the uniaxial compression simulation of fractured rock mass is carried out to observe the influence of rock bridge inclination angle and different crack angles on the strength parameter characteristics and macroscopic failure mode of four kinds of rock mass.The peak strength and elastic modulus of the four types of rock mass generally increase with the increase of the fracture dip angle.The peak strength and elastic modulus of the four types of rock mass generally decrease first and then increase with the increase of the rock bridge dip angle,and they are the lowest values at the rock bridge dip angle of 30°or 60°.According to the macroscopic failure mode of rock mass,the failure modes of single lithologic sandstone and single lithologic mudstone are mainly affected by the dip angle of bedding plane,and have little relationship with the dip angle of fracture and rock bridge.The composite rock sandstone and composite rock mudstone are mainly controlled by the dip angle of bedding plane.When the bedding plane is 0°,the failure mode is the lateral bulging failure of mudstone.When the bedding plane is 30°,it is mainly the sliding failure of mudstone along the bedding plane.When the bedding plane is 60°,the failure mode is the bedding plane failure.When the bedding plane is 90°,the failure mode is the oblique tensile shear failure.(3)Through UAV photogrammetry of Kamenziwan landslide,the occurrence,ductility and spacing of the dominant structural plane are interpreted,and the distribution form of the identified structural plane is approximately imported into the discrete element model.Based on PFCD program,the model of Kamenziwan landslide with jointed fracture network is established.Through the observation of particle displacement,velocity and number of cracks in the model and the monitoring of stress and contact force chain in the measurement circle at different positions,the progressive failure process and failure mechanism of the sand-mudstone interbedded side slope are simulated and analyzed,and it is concluded that the mudstone part of the sand-mudstone interbedded slope is seriously deformed.Joint face cracks play a key role in the fracture propagation and connection of sand-mudstone interbedd rock mass,and the failure mode of Kamenziwan landslide is slip-tension failure.The maximum displacement and velocity of particles are 4.2 m and 8.65 m / s,respectively.The maximum number of cracks is 2143.