Rockfall Movement Characteristics And Engineering Risk Assessment In Parlung Zangbo Canyon Area

Rockfall is a common bad geological phenomenon and occurs frequently in the great elevation difference valley area in southwest China.The characteristics of accidental rockfall disasters bring great challenges to the design of prevention and control engineering.Smallscale rockfall disasters will affect the safe operation of railways and highways.Large-scale rockfall disasters will not only seriously threaten the safety of buildings,but also cause heavy casualties.Parlung Zangbo is located in Bomi county,Linzhi city,Tibet autonomous region.It has the features of alpine and canyon landform with wide and narrow river valleys,deep rivers and river systems developed along structural lines.Rockfall and other adverse geological development in the area.The rockfall problem in Parlung Zangbo is chosen as research object in this paper.Based on field investigation and theoretical research,the factors affecting the characteristics of rockfall movement are studied by means of model experiment.Combining with the numerical method,this paper analyzes the movement path of rockfall on a tunnel canyon slope of Piaoda river in the research area,and predicts the area of rockfall.Finally,some suggestions on prevention of rockfall are put forward.The main research contents are as follows:(1)The movement of rockfall on the slope is divided into four forms: flying,collision,rolling and sliding.Based on the kinematics theory,the trajectory and parameters of the four movement forms of rockfall are discussed.(2)Based on the control variable method,several factors affecting the characteristics of rockfall movement are studied by model experiment.Research shows:(1)The furthest distance of rockfall is directly proportional to the falling stone mass,quantity,falling height and the slope height.And the furthest distance reaches the maximum value when the slope is about 50°.(2)The offset ratio value of rockfall is directly proportional to the slope height and the amount of rockfall,but it is inversely proportional to the mass of rockfall.And the offset ratio value has nothing to do with the slope and the falling height of rockfall.(3)The bounce height and collision restitution coefficient are related to the overburden of the slope,but they have nothing to do with the rockfall mass and falling height.In addition,the bounce height and collision restitution coefficient reach the maximum value when the slope is about 50°.(3)The model experiment shows that the movement form of rockfall on the slope is closely related to the shape of rockfall.The movement form of sheet rockfall on the slope is mainly sliding.The movement forms of strip rockfall on the slope are sliding,rolling and collision.The movement forms of cuboid and cube rockfall on the slope are rolling and collision.The movement form of cylindrical rockfall on the slope is mainly rolling.The more regular and nearly spherical the shape of rockfall is,the farther the horizontal movement distance is,the smaller the offset ratio value is,the higher the bounce height is,and the greater the collision restitution coefficient is.(4)Based on the model experiment results,according to the contact condition between the rockfall and the slope,the collision forms of rockfall are divided into surface collision,edge collision and point collision.It is suggested that the value of point collision restitution coefficient should be between 0.3 and 0.67,the value of edge collision restitution coefficient should be between 0.13 and 0.3,and the value of surface collision restitution coefficient should be between 0.02 and 0.13.(5)Statistical analysis was performed on the data of the lateral offset distance and the horizontal farthest movement distance obtained from the model experiment.The statistical results show that 99.4% of the rockfall offset ratios are less than 0.3,all of the rockfall offset ratios are less than 0.4.There are 99.83% of the rockfall offset angles are within 60°,and all of the rockfall offset angles are within 65°.(6)The numerical method was used to simulate and analyze the movement trajectory of the rockfall on the slope of Piaoda river in the study area.Combined with the offset ratio of the model experiment and the field structural plane survey data,the rockfall movement area after considering the structural plane was predicted.Based on the numerical calculation results,the design suggestions of using flexible protective nets or sheds to prevent and control the bad rockfall geology are proposed.