Study On Motion Feature Of Rockfalls

The disaster of rockfall has became one of research subject which is second only to slope stability, particularly in engineering of high and steep rock slope. The movement of rock in the slope is very random because the movement is influenced by many complex factors. At present, the main research tools are field test, model experiment and numerical simulation. In particular, the method of numerical simulation has already more and more aroused people's attention because it has the advantages of low-cost, simple, rapid and high transplantation comparing to other methods. But there is defect in present commonly used calculation formula of rockfall movement characteristics that there is no parameter can characterized the mass of the rock. So the computed result can not reflect the influence of the different rock mass to the movement characteristics and is quite different from the actual movement of falling rock.This paper takes Huangjinping Hydropower Station "6.18" Collapse as an example, based on lots of field survey result and combined with the discussion of the influencing factors of rock fall, and then summarizes three main motion trajectory of the rock in "6.18" collapse, which is classified as Track1, Track2 and Track3 from downstream to upstream, and further more judge the region of influence, which is divided into regions of different influence degree. After the qualitative analysis, the experiential formula and the kinematic formula based on probability are used to calculate the character of the falling rock movement. The result approximately reappears the movement of the falling rock in "6.18" collapse, then we also compare and analyse the calculating result of two means. And according to the different influence degree during the collapse because of the different rock mass, the method of using the deformed slop gradient as the calculating slope gradient when using the kinematic formula. And on the basis of measured data of the pits that were left behind after the large mass falling rock impacted on the slope surface, the linear relationship between the original slope gradient and the calculating slope gradient to the large mass falling rock is established, and thus to the medium and small falling rock development. In the end, the relationship between the calculating slope gradient and the falling rock mass is initially established based on the influence region of "6.18" collapse. On the foundation of the research, the modification of the kinematic formula based on probability is summed, and the more universal calculation method is put forward. In the end, the suggestion of passive protection for the dangerous rock mass in high position near the "6.18" collapse source according to the calculation results. The main conclusions of this paper are summarized as follows.1 According to the destruction situation and the movement trace, combined with the evolution history analysis of the collapse, the influence region is judged to be approximately fan-shaped. And it is can be divided into four smaller regions in the influence region, which is divided into three influence ranks based on the destructiveness. And Track2 is the main trajectory, the falling rock at the end of which the falling rock has the highest energy.2 The intersection angle between the falling rock velocity and the slop surface during the collapse has great influence on the movement characteristics of the falling rock. And there is obvious relationship between the mass of the falling rock and the deformed slop gradient, which should be reflected in the calculation through the means of modifying the calculating slope gradient. Thus the defect that is unable to consider the mass of the falling rock is improved in the kinematic formula.3 The calculating results of the the kinematic formula based on probability are much better in accordance with the actual situation than which of the experiential formula, when using the modified calculating slope gradient in the kinematic formula based on probability.4 The motion feature of the falling rock has a significant difference when the mass of the rock has a large difference. The large mass, medium mass and small mass falling rock which is distinguished in the "6.18" collapse has different motion feature respectively. Especially the large mass falling rock has its remarkable features of the great speed, high bounce and far hop distance. As the result, the large mass rock has the quite high movement energy and is the main protection object. The bouncing height appears close relationship with the terrain while it's related with the velocity. So it is impertinence to generally discuss the motion feature without considering the mass of the falling rock. 5 According to the synthetic analysis of the motion feature of the "6.18" collapse, the passive safety net is suggested to set at the height of approximately 1780～1800m. The protection energy level of the net should be above 1800KJ, and the height of the net may be 5m.