Research On The Evolution Dynamics Of The Consequent Bedding Rockslides

China is one of the countries with the most widespread landslides and it is severely affected by landslide hazard. Tens of thousands of landslides occurred every year in our country and the landslides almost spread over every province. As China’s western development campaign proceeds further, lots of landslides are triggered during the engineering construction and operation, leading to significant casualties and property losses and these landslides restrict the progress of the project. In the water conservancy and hydropower, highway, railway, mining and other series of important engineering practice, it is found that consequent bedding rockslides are widely developed and have significant impact. Among the25giant, large landslides which located in the main stream in three gorges reservoir region, consequent bedding rockslides account for16, approximately64%of the total.It is hard to grasp the development trend of the landslides and make accurate judgments on their stability in the engineering practice, due to insufficient understanding of dynamic mechanism and evolution regularity of the landslide. Carrying out the study on evolution dynamics mechanism of consequent bedding rockslide and revealing the evolution regularity and disaster mechanism will do help to promote landslide prediction theory. Therefore, research on evolution dynamics of consequent bedding rockslide is of great theoretical and engineering application value.Evolution dynamics of landslide focus on exploring the interaction regularitybetween force and motion of the slope with a certain constitutive propertyin the whole evolution process including deformation, failure, movement and accumulation. The purposes of studying the evolution dynamics changein different landslide evolution phases. The long-time evolution process before instability is regarded as deformation-failure stage. The purpose of studying the evolution dynamics in this stage is to provide theoretical basis for landslide instability prediction. The stage after instability is termed as movement-accumulation stage, which is to determine landslide kinetic velocity and deposit scope by studyingitskinematiclaw. This isan important theoretical basis for landslide spatial prediction.At present, research on evolution dynamics of landslide in its deformation-failure stage is mainly based on the displacement-time curve, usually lacking fully consideration of mechanical response onrock soil mass.To the bedding rockslide, the deformation is mainly controlled by the soft interlayers. The properties of rupture zone are the main factors that determine the evolution stage of bedding rockslide. Therefore, it is necessary to start the research of dynamic evolution of bedding rockslide from the evolution of rupture zone. But just some local information from a certain part of the landslide is unable to reflect the whole evolution state of landslide, making it rather inevitable to combine the research with the concept of "field".High-speed long-distance landslide is the main object of the research in the evolution dynamics of landslide in movement-accumulation phase. Velocity anddistance are the two importantkinetic parameters in the study. Existing research on starting velocity of consequent bedding rockslide only consideredelastic strain energy accumulated in locking section of rupture zone, but ignoredthe ability of accumulating elastic strain energy in otherpositionsoutside the locking section. Currently, although there is much research on the dynamicmechanism of high-speed and long-distance landslide, but they are lack of universality due to that they are carried outunder a specific geological environment. It is the final goal of researchers to establish a reliable predicting model for travel distance through the research into dynamics in landslide movement-accumulation phase.Considering the importance of research on the consequent bedding rockslide, the study also take theproblems existing in the current dynamics research of landslide evolution into account.A typical consequent bedding rockslide, Jiweishan rockslide,is taken as a case here to do research on the dynamics deformation-failure to movement-accumulation of consequent bedding rockslide throughout the whole evolution process. Start with the mechanical property evolution regularity of rupturezone in consequent bedding rockslide,its dynamic modelof evolution is established.Then the temporal-spatial evolution regularities of stress field, displacement field and strain energy field is analyzed by numerical simulation method, thus to further reveal the failure mechanism of consequent bedding rockslide. On the basis of constitutive study ofrupture zone, the research on starting velocity of consequent bedding rockslide is also carried out.With the discrete particle flow (PFC2D) software, simulation for the whole movement-accumulation process of Jiweishanrockslide is conducted, revealing the dynamic mechanism of remote motion and accumulation characteristic of landslide. Finally the systematic and classified study is carried out on these large high-speed landslidesin our country while analyzing the factors which affect movement-accumulation characteristic.Based on the equivalent friction coefficient method and analysis of factors affecting accumulation, a set of traveldistance prediction system is put forward, providing basis for landslide spatial prediction.According to the above research approach, research on the evolution dynamics of the consequent bedding rockslide is carried out, obtained the following results: body,stress field, displacement field and deformation field of Jiweishan rockslide is obtained as follows:1) Inthe initial evolution stage, the rear part of rupture zone playsthe main anti-sliding part. Stress in the rear part of rupture zone reduces to a stable levelalong with thelandslide evolution, and the forepart of the rupture zone provide main anti-sliding force;2) Deformation zone in slide body and rupture zone presents forward expandingcharacteristics;3) On the aspect of time, elastic strain energy in rupture zone and slide body show an increasing trend, gravitational potential energy conversion rate undergoing the period of early increasing and later decreasing; Spatially, elastic strain energy in the rear part oflandslide would release after its accumulation, when it comes to failure, elastic strain energy mainly concentratedin the forepart of landslide.In the whole evolution process, elastic strain energygradually transfers from back to front. Multiple-field evolution regularity implies that advancing failure pattern inconsequent bedding landslide is the mechanicalmechanism of failure which caused by the rear part ofslide body driving forepart.(4) Definition of high-speed long-distance landslide is discussed as well as the relationship among high-speed landslide, high-speed long-distance landslide and high-speed long-distancedebris flow landslide. Due to different formation mechanisms, the high-speed long-distance landslide generally can be divided into four stages, namely starting stage, departurestage, remote motionstage and accumulation stage. Then, the startingelastic-impulse acceleration mechanism of the consequent bedding rockslide is systematically elaborated and two acceleration effects about the "rupture zone releasing energy" and "lockingsection releasing energy" are emphatically analyzedin acceleration mechanism. Starting-elastic-impulsevelocity calculation formula of the consequent bedding rockslide is derived under two different accelerations. In the meantime, elastic-impulse velocity formula under the combination of these two different accelerations is composed. By calculation, the initial integrateelastic-impulse velocity of Jiweishan rockslide is about1.2611m/s. Further analysis implied that the reasonable maximum elastic-impulse velocity range of general rockslideis about2.6-4.4m/s.(5) The results of the simulation with the discrete particle flow software PFC2D show that the process of the movement and deposit of Jiweishan rockslide experienced three stages include departed acceleration, high-speed remote motion and deceleration accumulation. At startup, the landslide gradually disintegrated from the leading edge and trailing edge to the middle part. After it slides out of the shearing opening, it goes into the ditch bottom and the overall average speed of the landslide reaches its maximum value. Since then, although the speed has a decreasing tendency, it still makes the high-speed movement Ultimately, the materials of the landslide gradually accumulate and make remote movement with the reduction of the speed of the landslide. In the process of the Jiweishan rockslide, the overall average maximum velocity of the landslide is35m/s. In some local parts, the extremes maximum velocity can reach60-70m/s, nearly twice of the overall average maximum velocity value. The disciplines of the velocity change of the material in different layers on the same section are basically the same, synchronously acceleration and deceleration. But the magnitudes of the velocity are different as they gradually decreasing from top to bottom. The velocity of landslide materials at different parts on the same layer that the rear substances start slowly then get a higher speed for a short time and last short.The front edge has a fast start and the movement lasts longer. Before the overall average velocity of landslide reaches a peak, the landslide gravitational potential energy mainly transformed into kinetic energy. After reaching the maximum overall average speed, the friction and collision dissipated energy dissipation significantly increases and the collision energy dissipation dominates. Numerical simulations also reveal that the landslide debris accumulation has obvious positive sequence features, anti-particle deposition characteristics and sorting stacking feature. The motion features is basically consistent with the landslide site survey. Force chain transmitting from the surface to the bottom of the landslide gradually and concentrating on are important mechanisms for the anti-particle accumulation. Secondary acceleration effect is an important mechanism for the sorting stacking and remote movement of the landslide.(6) According to the landslide’s formation conditions and characteristics of deposit, the high-speed landslide are divided into four categories. They are loess high-speed landslide, general soil high-speed landslide, non-seismic high-speed rockslide and earthquake-inducedhigh-speed rockslide. Then dynamics mechanism of various types of landslides is preliminarily explored. The four factors affecting the accumulation of landslide movement are summarized as topography, landslide properties, volume and external power when landside occurred. This research goes further in the relationship between four factors and the characteristic parameters of landslide movement accumulation. Data analysis shows that the key factor in whether the landslide transmits high-speed movement into long-distance travel is whether they have sufficient length of linear-shaped terrain. The equivalent coefficient of friction and the logarithm of the landslide volume are negatively correlated. The differences in the correlation coefficient of the different types of high-speed landslide also reflect the characteristics of the landslide movement have certain regional characteristics. It also reflects the dynamic mechanism of the rocky landslide is more complex than the soil one. The equivalent friction coefficient of loess landslides are generally lower than the general soil landslide while the equivalent friction coefficient of the rockslide which triggered by earthquake is higher than the general rockslide. Based on the equivalent friction coefficient method and analysis of the influencing factors of accumulation, here a set of complete distance prediction system is proposed. Jiweishan rockslide is taken as an example to predict its travel distance in different volume. By comparing the results with the actual values, the proposed travel distance prediction system is feasible and can provide a scientific basis for the determination of the scope of the high-speed landslide disaster.Here are the innovation points of this research.(1) Weakening coefficient and weakening coefficient state curve are defined to depict the temporal and spatial variability of mechanical parameters of rupture zone during the evolution. Based on these, evolution dynamics equation of consequent bedding rockslide is constructed and it is applied in the landslide evolution simulation and the dynamic stability assessment.(2) Based on damage theory of rock constitutive energy dissipation, a coupling elastic-plastic constitutive about the rupture zone is established. Then, the releasable elastic strain energy of bedding rockslide is studied. Further, a solving equation of the starting velocity is deduced.(3) Based on the analysis of four factors topography, landslide properties,landslide volume and external power, combining with the friction equivalent coefficient method, a complete landslide travel distance prediction system is put forward as the basis for high-speed landslide spatial prediction.