Research On Motion Characteristics And Impact Force Of Rockfall

Rockfall is a common phenomenon in mountainous terrain, usually the scale of which is small, however, because of its high frequency, significant burstiness and randomness, rockfall can easily induce to hazards. As China's western development campaign proceeds further, the exploitation of mountain area's resource and the construction of the infrastructure are developing rapidly unprecedented. Accompanied with ongoing construction of the railway from Yichang to Wanzhou, west part of High way from Shanghai to Chengdu, and west-east gas transmitting project, etc, there have been a substantial number of cases of rockfall triggered by human engineering activities causing death. Therefore, systematically research on rockfall is of great theoretical and tremendous engineering application value.In order to effectively reduce the rockfall hazard, firstly, the geological environment of rockfall occurrence should be investigated and which make its possibility to determine the sensitive area of rockfall, then motion characteristics should be understood well and through that the affecting area of motion of rockfall could be assessed. Finally, impact force should be calculated. From this procedure, designing basis could be provided for protection projects.During theoretical calculation of rockfall motion, rockfall is assumed as a centroid point or a circular rigid body. The Rockfall movement is divided into separate phases including free fall, bouncing, rolling or sliding. Then each process was studied respectively. Among which the bouncing and rolling are controlled by restitution coefficient and rolling friction coefficient, respectively. The study of rockfall motion characteristics focus on determining the key parameters, namely restitution coefficient and rolling friction coefficient. These parameters characterize the energy dissipation during rockfall movements, in which restitution coefficient affects rockfall movement more strongly. Based on impact laboratory trials or in situ tests performed by launching single blocks on selected slopes, a number of authors suggested value of restitution coefficient, however, these suggested value vary considerably and are sometimes contradictory.Impact force determines strength grade of protection projects. Mostly, the impact completed in 10-3-10-2 seconds, based on theorem of impulse, the impact force is large because the impact lasts shortly. As its difficult to explain the impact of rockfall in theoretical method, current research on impact force mainly depend on established empirical formula through measure kinematic parameters of falling rock, such as acceleration, impact duration, and responding of impacted material, etc. However, established formula usually lack of universality due to most impact experiments carried out under particular conditions.This study concerns on mitigations of rockfall hazards, determines sensitive area of rockfall by engineering geological analysis, records rockfall motion by FASTCAM SA1.1 high speed camera, analyzes kinematic parameters, combines with numerical simulation to study the effects of size, impact velocity, and incident angle of rockfall on restitution coefficient. Based on the experimental results, an artificial neural network is trainedto forecast the maximum runout distance, also the degree of slope relative parameters and rockfall relative parameters effect on runout distance are studied. Based on energy theory and ideal failure model of Pulangdeer logarithmic spiral foundation bearing capacity, simplified model of rockfall impact foundation is established, according to that a method of calculating the maximum impact force is proposed. Finally, an unstable rock and a natural gas pipeline which below the unstable rock, which locate at Yesanguan, Enshi, west of Hubei, are taken as research objects. Analyze the engineering geological conditions of the area, forecast possible runout distance of potential rockfall, calculate the range of impact force caused by rockfall, and adopt dynamic finite element software, namely ANSYS/LS-DNYA, to simulate the response of natural gas pipeline under impact. Therefore, the main tasks completed in this paper are as follows:(1) Through comparing the definitions of rockfall of existing literatures, combined with the objects of this study, relationship between rockfall and collapse, rockfall and unstable rock, are determined. The form of rockfall is controlled by geological environmental conditions, maily topography, strata lithology, geological structure, weathering and unloading; and the major factors trigger rockfall including human engineering activities, root growth and animal activities, rainfall and underground water, freeze-thaw processes and earthquake. According to the physiological characteristics and corresponding failure mode of potential unstable rock masses, they are subdivided into six types, namely, rolling or sliding of isolated type, pull-apart drop of hanging type, sliding of sticking slope type, expansion pull-apart of soft base type, buckling failure of catactastic structure type, and topple over of plate-column structure type.(2) High speed camera, FASTCAM SA1.1, is introduced to research the motion characteristics of rockfall. The high speed camera was used at 5000 frames per second to record the motion of rockfall. From the recording then motion trajectory, motion mode, velocities and acceleration are measured. Moreover, velocities at impact for each rockfall are measured accurately. The results show that slope angle, slope surface conditions, mass and shape of rock blocks affect motion obviously. As the rockfall movement model easily changed in the collision, the initial model of rockfall movement has no clear effect on subsequent movement. Lateral displacement-time curve of rockfall show that there are 96.2% rockfall coming with lateral displacement less that 10%,61.5% rockfall coming with lateral displacement less than 5%.(3) Based on in situ tests, effect of relevant factors on restitution coefficient is studied. Not only strength of slope surface material affects restitution coefficient, factors such as incident angle, impact velocity affect restitution coefficient obviously. Normal component of restitution coefficient decrease with incident angle apparently. Inversely, tangential component of restitution coefficient increase with incident angle. When the ratio of the resultant velocities and ratio of kinetic energies before and after impact are used to define the restitution coefficient, a very clear decreasing trend in the restitution coefficient with incident angle is observed. Based on in situ experiments, dynamic finite element software, ANSYS/LS-DYNA, is adopted to establish numerical model to study the normal restitution coefficient. Numerical simulation shows that the higher impact velocity result in the smaller normal component of restitution coefficient; the bigger rock block result in smaller normal component of restitution coefficient, however, for the identical foundation material, block size does not affect normal restitution coefficient after it increased to a given value. In addition, the bigger plasticity parameters of foundation material result in bigger normal restitution coefficient, when the plasticity index increase to some extent, the normal component of restitution coefficient reaches a steady state.(4) Artificial neural network was applied to predict runout distance of rockfall. The trained artificial neural network that achieves instant prediction of runout distance takes slope angle, slope height, slope surface conditions, mass and shape of rock block into account. Sensitivity analysis show that:①runout distance increase with slope angel and achieves a maximum at roughly 45°, before decreasing again at 50°.②runout distance of rockfall increase with the slope height, however, once slope height increased to a certain value, runout distance stop increasing. Moreover, this certain value is affected by slope angle.③less slope cover, or slope cover material strength, the more conducive of falling rock movement.④spherical rockfall come with longer runout distance, then followed by square block; Runout distance of plate-like block and rod-like block does not show apparent difference due to their runout distance affected by motion model strongly. The analysis of connection weights indicate that decreasing order of relevant parameters affecting runout distance is as follows:slope angle, slope height, slope surface condition, mass of rock block and shape of rock block.(5) Based on energy theory and ideal failure model of Pulangdeer logarithmic spiral foundation bearing capacity, simplified model of rockfall impact foundation is established. The equation for calculating impact force which takes soil bulk density and internal friction angle into account is proposed. Studies show that maximum impact force increase with soil bulk density and internal friction angle.(6) The proposed guideline for rockfall hazard protection is applied to Shimaling unstable rock, which locate at Yesanguan, Enshi, west of Hubei Provence. Firstly, according to the engineering geological conditions of area which unstable rocks locate at, determine the morphological characteristics and relevant failure model of unstable rock. Assess affected area by trained artificial neural network, the results show that no matter the shape of rock block, once mass of rock is more than 60 kg, the rock block could reach to the location of natural gas pipeline. The range of impact force caused by biggest potential rockfall is calculated.(7) Numerical simulation is carried out in order to research response of natural gas pipeline under impact of rockfall, ANSYS/LS-DYNA, a dynamic finite element software is adopted. The main conclusions are as follows:①the deeper of pipeline result in smaller maximum impact force, and attenuation of impact force in the buffer soil layer is more obvious.②when the pipeline depth is shallow, impact velocity of rockfall affects penetration depth apparently, as depth of pipeline increase, the effect of impact velocity on penetration depth gradually decrease. This indicates properly increasing pipeline depth is helpful for protection of pipeline.③if horizontal distance between collision point and top of pipeline is longer, the impact which pipeline suffer will be lighter. Under the impact of rockfall, the most dangerous location is on the top of pipeline.The research of this paper could provide scientific basis for determining location of proposed infrastructure or construction and design parameters of rockfall protection projects in rockfall area. According to above content, the main research results are shown as follows:(1) High speed camera is introduced to the research of motion characteristics of rockfall, based on the application of high speed camera, restitution coefficient of rockfall is in-depth studied, the effect of incident angle on restitution coefficient is demonstrated. The range of restitution coefficient under different incident angle is determined, based on which the range of restitution coefficient under different slope angle could be determined.(2) According to five easily obtained parameters, namely slope angle, slope height, slope surface condition, mass of rock block and shape of rock block, an artificial neural network is established to predict maximum runout distance of rockfall, and influence area of rockfall could be predicted instantly. The effect degree of parameters relate to slope and rock block on runout distance is studied.(3) Based on energy theory and ideal failure model of Pulangdeer logarithmic spiral foundation bearing capacity, a method to calculate the impact force is proposed. The method takes soil bulk density and internal friction angle into account, it offers reliable source for engineering application.