The Failure Mechanim Of Locking Segment Of Touzhai Rock Avalanche

Rock avalanche is a kind of geological disaster with the strong destructive power,which is characterized as long migration distance,high speed,high monitoring difficulty and extensive influence.Rock avalanche event has always been the research focus of scholars at home and abroad,its inoculation mechanism and failure mode are also the difficulty key point with respect to this kind of disaster.Taking the rock avalanche in Touzhai,Yunnan province as an example,the test results were comprehensively analyzed through field survey,rock shear test,microseismic monitoring technology,AE(acoustic emission)monitoring system and high-speed camera equipment.Through 3D morphological scanning of rock fracture surface and numerical simulation of rock shear,the analysis results of the test are verified,so as to further deepen the understanding of locking mechanism and failure mode of the rock avalanche of Touzhai,and provide a support for disaster monitoring and prevention.The comprehensive analysis results show that:(1)The locking section with the length of about 150 m,high strength and good integrity is located in the upper part of toe of surface of rupture.Columnar joint and structural fracture are developed in the upper slope rock mass of the source area,which are severely eroded by weathering and lost the self-stabilization ability to squeeze down the locking section.The results of particle size distribution and morphology investigation in Touzhai valley show that the size distribution of lock block is mainly 10-100 mm.What is responsible for this is the eroded and fractured upper rock mass of the locking segment and the broken-up boulders during migration.(2)The results of rock shear test show that the maximum acceleration of rock shear process performs a positive relationship with shear velocity and axial pressure.The maximum acceleration in the rock shear process does not necessarily occur when the rock is penetrated and completely destroyed by macroscopic cracks,but may occur during the process in which a certain fracture is formed.When the rock is subjected to shear force,a large number of tensile cracks appear firstly in the region of two loading ends.Small cracks would develop and expand continuously to form macroscopic cracks,and then extend into the rock interior,paralleling or intersecting with the interior original-exsiting cracks.At the end of the shear process,the macroscopic cracks run through the rock for a fracture zone or present as the phenomenon of instantaneous fracture of the rock.This zone is composed of echelon tensile cracks and connects the two loading ends.The effect of rock size is obvious.In the early stage of the shear process,cracks appear in the middle of the rock in the smaller size basalt,but not in the larger size basalt.The reason for this difference is the reflection of Saint Venant Principle in the shear process of samples of different sizes: under the shear action,the two loading ends of the larger size basalt bear more horizontal shear force,and internal shear force is less than that of the cuboid basalt.The comparison results show that a large number of intensive and high-intensity AE energy values occur at the same time when intensive saltations and responses were detected by acceleration sensor.However,the maximum AE energy doesn't necessarily occur at the moment of the maximum acceleration.The AE monitoring results are not sensitive to the change of velocity within a certain range.However,the monitored energy parameters mutate when the shear velocity exceeds a set value.(3)The 3D geomorphic morphology and fine microstructure of the main shear fracture surface of basalt are analyzed by 3D laser scanning equipment and reverse analysis software.The results show that the influence of axial load on the 3D geomorphic morphology of rock fracture surface is more significant than other factors.With the increase of axial load,the overall roughness of rock fracture surface becomes larger and obvious reverse step phenomenon occurs,but the overall undulation pattern becomes slow.The statistical results of 3D geomorphologic maps show that the morphology features are mainly interlaced with large "gullies or basins" and "peaks or slopes".The results of detailed structure analysis show that the fracture surface structure presents the characteristics of tensile failure.Profile detail structure shows that tensile cracks in echelon arrangement,and shear direction angle is between 10°and 40°.When the shear rate increases,the overall roughness tends to decrease.However,when the shear rate is greater than the threshold,the overall roughness increases with the increase of the shear rate.(4)The results of rock shear numerical simulation show that the mechanism of fracture formation and propagation is similar to that of rock shear test.The stress concentration distribution in the shear process of the two size models is reflected in the "fusoid" distribution connecting the two loading ends.The stress concentration on both sides of the shear crack is more significant and the stress concentration area will be compressed to the macroscopic crack along with the shear.During the whole shear process,the stress concentration and regression due to the formation and expansion of tensile cracks occur repeatedly.However,on the whole,the shear stress increases with shear stress.The statistical results show that the cumulative AE quantity of the 20 m×10 m model is 79.8% more than that of the 10 m×10 m model.The cracks caused by grain fragmentation are mainly tensile cracks and a few are shear cracks.When the macroscopic fracture penetrates the rock mass,a large number of shear AE responses are distributed along both sides of the main fracture,while abundant intensive tensile AE responses are still distributed in the rock interior.The failure mode of rockfall locking segment is the instantaneous failure shear failure which was formed by the expansion and evolution of a large number of tensile fractures.The large amount of strain energy accumulated in the rock force chains provides the initial force that induces the global instability of the locking section.The energy exerts a lifting effect on the upper slope,thus effectively reduces the friction and induces the earthquake.