Motion Parameters And Striking Surge Of Rockfall From High-steep Rocky Slopes And Its Engineering Effects

Large hydropower projects are often built in deep valleys and the scientists usually focus their researches on stability of overall slope in reservoir sites. Although there are big and less stable rockmass on the surface layer of steep slope in reservoir sites, whose unstability would pose rockfall risk on the reservoir safty both in construction and during its operation, researches about this issue reported few till now. This dissertation focused on study of motion of these big and less stable rockmass when rockfall takes place, bouncing surge of waves triggered by rockfall rockmass striking into the water in reservoir, and the its impact on reservoir safty.In the dam area of Laxiwa hydropower station, the mountain is high and the canyon narrow. A large number of different size dangerous rockmass spread on the high-steep slopes, which is one of the main potential geological hazards menacing the safety of dam in construction and operation. based on field investigation, data analysis of discontinuities and using three-dimensional laser scanning technique, the dissertation described the development and distribution of dangerous rockmasses in study area. With software RocFall employed, the rockfall motion was simulated and gained the motion parameters, including the trajectory of rockfall motion , impact energy, and impact velocity. Based on these quantitative parameters, the height of bouncing surge and its propagation length of the water waves are suggested, which is triggered by rockfall striking into the water in the reservoir from the high-steep rockmass slopes. These significant results would provide the base information for the early warning for potential hazards in this hydropower station and also for similar researches for method reference.The research has made the following main achievements:(1)Through the research of the dangerous rockmass on the right bank high-steep slope of Laxiwa hydropower station, the distribution for those rocks was gotten. The studies have linked the dangerous rockmass to many factors, such as terrain condition, structure of rockmass, rock weathering, stress condition, unloading, earthquakes, and water pressure of void space.(2)Failure of the dangerous rockmass often occurs during the rainy season. When the rainwater infiltrates into the adverse structure face which control the development of dangerous rockmass, the friction of those structural face was reduced on the one hand, and the increase of water pressure of void space provided a thrust for the dangerous rockmass to the free surface of the thrust on the other hand, leading to dangerous rockmass out of control.(3)Using the geological survey, ILRIS 3D laser scanning technique and block theory, the author collected the information about the dangerours rockmass in the study area, and analyzed their location, shapes, sizes, and instability characteristics.(4) According to the position of dangerous rockmass, nine profiles perpendicular to the rivers flow were drawn, which were used as actual calculation profiles. As a tool, the computer program RocFall assisted us with probabilistic simulation of 26 rockfalls. The dissertation analyzed and explained the location of the rock stop and trajectory of rockfall motion, and also presented the result of the kinetic energy, the velocity and the bounce-height when the rock fall into the reservoir.(5)The dissertation proposed a concept that high-speed rock falling into the reservoir will cause a striking wave, called the surge due to rockfall. The formation mechanism of such surge due to rockfall was theoretically analyzed as following. When the high-speed rock falls into the water with a high speed, it has an impact to the range of water bodies around the rockfall by the rockfall volume and rockfall velocity. They will cause the surrounding water in accelerated motion. Since the obstruct from the quiescent water surrounding it, these water will move to the surface of the reservoir and form a water column above the water surface. During the water column fall to reservoir, it promote the sport around the water forming waves, and spread to the surrounding gradually.(6)According to the test analysis, the volume of initial surge due to rockfall consists of two parts, part of a rockfall size, and the other part is the impact volume which is engendered by the retarded motion of the falling body in the water due to the friction of water. Learning from the calculation method for landslide generated waves, and based on water dynamics, the dissertation presented the equations and the algorithm to calculate the height , the propagation and the incidence of the surge due to rockfall. The formula has strong adaptability and could be applied to similar engineering projects.(7)By using the computing formula, the initial wave height and spread are calculated. This surges were caused by 26 rockfalls from the right bank slope of Laxiwa hydropower station falling into the reservoir assumed .(8)The dissertation studied the main factors and influencing factors of controlling rockfall. The necessary condition of strike wave is that rockfall has certain volume and falling velocity.(9)According to the calculation results, wave heights of rock falling produces from W5, W6, W8,W9, W13 spread to the front of the dam are greater than 11.72-63.3 meters.