Tensional Rupture And Low-friction Initiation Mechanism Of Bedding Landslides Triggered By Mega-earthquakes

The formation mechanisms of landslides triggered by mega-earthquakes such as the Wenchuan earthquake are very complicated,which are quite different from the characteristics of landslides under gravity force only.The most prominent feature is that many large-scale landslides are firstly shattered at the sliding surface(generally present tensional rupture),and then slide with a high speed and even eject.This study,relied on the National Natural Science Foundation: An initiation mechanism of transient ultra-low friction with respect to rip-slide landslides induced by mega-earthquakes(41472245),investigated the dynamic mechanism of tensional rupture and low-friction initiation of bedding landslides triggered by mega-earthquakes.In view of this,the basic features and failure mechanism of mega-earthquake-triggered bedding landslides,the dynamic tensile and tensile-shear properties of rock discontinuities and strength criterion,the dynamic friction properties of sliding surface and sliding friction criterion,and the dynamic model of landslide shatter-initiation were investigated systematically from theory to application,using the methods of engineering geology analysis,rock mechanics and shaking table tests and the theories of earthquake dynamics,rock mechanics,frictional mechanics,and kinematics.The main work and conclusions of this study are as follows:?The features of typical bedding landslides triggered by the Wenchuan earthquake were analyzed and summarized based on engineering geology information.The mechanism of the differentiation of the failure modes in slopes under earthquakes,i.e.tensile failure to tensile-shear failure and to compression-shear failure from the top down,was discussed.Specifically,the dynamics mechanism of the tensile,tensile-shear and compression-shear rupture mechanisms and dynamic low-friction phenomenon of bedding sliding surface were analyzed.Then,the tensional rupture of the bedding sliding surface and the effects of dynamic low normal compressive stress and the degradation of dynamic friction coefficient with increasing sliding velocity on low-friction,high-speed initiation of landslides under mega-earthquakes were confirmed.?The dynamic,direct tensile tests on rock discontinuities(limestone or sandstone bedding plane),including monotonic tesion under different strain rates,amplification cyclic tension,and amplification cyclic tension and compression,were carried out using the INSTRON 1342 electrohydraulic servo-controlled dynamic and static material testing machine in combination with a designed direct tensile connection device.The dynamic tensile fracture morphology,deformation,strength and damage evolution characteristics of the discontinuities were investigated,and the effects of loading rate and frequency on that were indicated.The tensile strength is mainly influenced by the maximum strain rate at fracture occurs,meaning that the strain rate of the dynamic load control the strength while the cycle of the dynamic load slightly weakens the strength.?A rock tensile-shear device was invented,which can be used in a compression-shear testing machine.The direct shear tests on rock discontinuities(limestone bedding plane or intact sandstone)under different normal tensile stress,different shear rates,and amplification cyclic shear were carried out.The dynamic tensile-shear fracture morphology,deformation,strength and damage evolution characteristics of the discontinuities were investigated,and the effect of shear rate on that were indicated.Combining with the compression-shear test results under different shear rates,the strength criterion in the full region of tested normal stress was discussed(the power function is of better fitting effect),and a unified strength criterion of tensile,tensile-shear,and compression-shear for rock discontinuities considering both shear and tensile rates was established.?Based on the shaking table test of sliding block(sandstone block,placed horizontally),the movement and sliding friction dynamic evolution of sliding block under sinusoidal horizontal vibration,vertical vibration,and horizontal-vertical phase difference coupled vibration were investigated,and the effects of amplitude,frequency and phase difference on movement of sliding block and friction behavior were indicated.And the dynamic conditions of friction reducing of sliding surface were confirmed.A velocity dependent sliding friction law considering initial increase and then degradation of friction coefficient with the increase of sliding velocity was established.?The concept of whole process dynamic analysis of earthquake-induced landslides considering both shattering stage and sliding stage was proposed.By improving the residual thrust method to calculate the acceleration time-history of the slope body and combining with Newmark method to calculate the sliding velocity and displacement,a dynamic time-history analysis theory model considering the whole process of bedding landslide shatter(considering tensile and shear strength criterions)and low-friction initiation(considering sliding friction law)was established.Then,the calculation is programmed according to the model,and a calculating-example analysis considering sinusoidal horizontal vibration,vertical vibration,and horizontal-vertical time/phase difference coupled vibration were conducted.Some results in accordance with the table test results and actual landslide features were obtained.?The shattering and initiation process of the Wenchuan earthquake-triggered Daguangbao landslide was calculated and analyzed using the proposed dynamic model(the horizontal and vertical seismic waves recorded by the Qingping station,Mianzhu,were used).The stage,from the through-going rupture forms in sliding surface with friction coefficient began to degenerate to the friction coefficient degenerates to stable state,was defined as the initiation stage of the landslide,which takes about 6 s and the corresponding sliding displacement is about 3.7 m.The sliding velocity when the friction coefficient degenerates to stable state was defined as the initiation velocity of the landslide,which is about 3 m/s.After the initiation stage,the sliding velocity increases obviously,i.e.high-speed landslide.And the average velocity is about 36.4 m/s,which is in the range of 35~50 m/s estimated by characteristic parameters of landslide movement.In addition,the calculated rupture modes of sliding surface are more correspond with actual conditions,i.e.tensile rupture in crown and tensile-shear rupture in sliding surface,and the duration of the initiation stage significantly decreases if the elevation amplification effect of seismic acceleration is considered.