The Predesign,Occurrence And Propagation Mechanisms Of Touzhai Large High-speed And Long-runout Landslide-debris Flow

Large high-speed and long-runout landslide-debris flow,also called rock avalanche,are rapid mass movements when large-scale rock slopes are transformed into debris flows.It usually causes heavy losses of lives and properties for its sudden occurrence and long travel distance in low gradient channels.However,the predesign,occurrence and propagation mechanisms of the rock avalanche have not been unified.The Emeishan basalt of the upper Permian(P2β)are widely distributed in Southwest China,and there are several catastrophic rock avalanche have taken place in this rock series successively since 1960s.Hence,the study of Emeishan basalt(P2β)rock avalanche is of great practical importance,which could help people to deepen understanding of this kind of hazardous disaster and provide sufficient scientific basis for the disaster risk assessment,prevention and controlIn this thesis,the catastrophic rock avalanche which occurred in Touzhai valley in Zhaotong,Yunnan in 1991 is studied as the research object,and its predesign,occurrence and propagation mechanisms are explored using field investigations,laboratory and field experiments,analytical analysis,numerical calculation and simulation.The main research results are as follows1.Influenced by the primary and secondary structures,the source rock mass is pre-fragmentation with the appearance of "fractured but undisaggregated".The weathering process promotes the formation of the "soil-rock" structure,in which the core stone is surrounded by the basalt saprolite carapace,thus weakens the rock mass quality and becomes one of the critical factors in the predesign of the Touzhai rock avalanche.The locked segment formed by the fresh in the toe of the shear surface is another critical factor in the predesign and occurrence of the Touzhai rock avalanche2.The large scale rock mechanical tests for simulating the dynamic instability of Emeishan basalt locked segment could provide lots of informations.The cracks first appear in the position near the load-end,where the excitation acceleration signals are also generated The strain values in the monitoring points near the load-end are higher than that in the middle monitoring points.The excitation acceleration among the failure process of the lock segment samples are pulse like and the peak acceleration value is positively correlated to the shear rate and the normal load.The wavelet packed analysis turns out that,more than 86.9 percent of the energy collects at the 0～1.953 Hz frequency range(8-1),and the energy in the 0～1.953 Hz frequency range increases with the normal load and the shear rate3.The released sound energy and infrared radiation temperature in the failure process of the large scale samples are positively correlate with the shear rate and normal load.During the loading period,the acoustic emission characteristic parameters present the ladder form rise,while the infrared radiation tempreture of the monitoring points in the empty rock surface also increase with the form of undulatory rise.The rupture surface of the shear zone presents the morphology of serrated-stepped shape,especially in the samples with a larger locked area,which is coincidence with the(counter-inclined)microtopography of the locked segment in the source area4.The concept failure model of the source rock mass in the Touzhai rock avalanche is that the creeping slide mass deforms firstly,thus the loads are transferred to the representative element volume(REV1)in the locked segment,which leads to deformation,crack initiation,formation of smaller scale "rock bridges"of the REV1.After the strength limit of the REV1 is exceeded,the loads are transferred to the next REV and cause a series of progressive failure like dominoes.The intense elastic wave around the shear zone could generate the support and ejection effects,and provide the initial velocity and acceleration for the slide mass5.It is showed that the locked slope system is more prone to fail when the angle difference between the weak slip zone and the locked segments increases or the rigidity of the lock segment decreases.The energy released in the failure process is positively correlated to the brittleness and equivalent height(buried depth)of the locked segment,and negatively correlated to the thickness of the shear zone6.The deposit of Touzhai rock avalanche contains a wide spectrum of grain sizes and the particles are poorly sorted.The grain size distribution shows that there is no apparent grinding effect along the travel path.The "Gobi" landscape is the result of rainfall leaching,not the presentation of "inversing grading".The deposit debris is not dominated by the dynamic fragmentation among the debris travel process,but the pre-fragmentation of the source rock mass7.The propagation mechanism of the rock avalanche is related to the trapped air among the poorly sorted debris.The particle,particle aggregate s and even the mega blocks might have been surrounded by the pore air,which could weaken the interaction between the particles This might have helped the debris float in the compressed air and maintain the long run-out propagation of the rock avalanche.The numerical calculation of the air-dynamic pressure shows that,the relative velocity,the morphology and thickness of the air film could all influence the distribution of the excess pore pressure between the particles.The simulation of particle flow based on the discrete element method(PFC3D)under different bond strengths proves that the decrease of the bond strength could promote the propagation of the debris.