Study On Stability, Movement Characteristics And Countermeasures Of Potential Unstable Rock Mass In High-steep Slope

Potential unstable rock mass disaster often happens in high-steep slope and steep cliff zone, which is a kind of common geohazard in mountainous area. Every year, it always induces people's casualty and much economic loss in China. This thesis took the potential unstable rock masses on Jinping I and II hydropower engineering as a typical example. Based on a number of site investigation and rolling rock experiments, the author relatively systematically studied the classification of high-steep slope potential unstable rock masses, its deformation and failure model, evaluation method of its stability, movement characteristics of rolling rock and Countermeasures of potential unstable rock masses. Followings are the main achievements gotten.(1) Investigation on the high slope of Jinping I and II hydropower stations told that there were 333 blocks of potential unstable rock masses. The maximum volume among them was 122549m~3, and the minimum was 0.1m~3. Most volume ranged from 10 to 1000m . The development of potential unstable rock masses had a strong relation to topography, lithology of stratum, geologic structure, rock mass structure, weathering, and unloading. Detailed mentions were as follows:①Most of potential unstable rock masses developed on the steep slope whose slope angle is beyond 50 degree and on the high elevation.②The number of potential unstable rock masses of unit area in marble zone was far larger than in slate zone; But good-sized and oversize bulk of potential unstable rock masses had a larger proportion in slate zone than in marble zone.③The number of potential unstable rock masses of unit area of consequent slope was much larger than of obsequent slope and tangential slope; And their number of unit area in mosaic structure～cataclastic structure and cataclastic structure～slack structure zone was much more than in block structure～sub-block structure and sub-block structure～mosaic structure zone.④If other engineering geology conditions were similar, the number of potential unstable rock masses of unit area would increase with increasing bottom depth of strong slope unloading.(2) According to potential unstable rock masses structure and morphology, they were basically subdivided into 7 types, i.e., hanging type, sticking slope type, toppling type, wedge type, soft base type, masonry block type, and isolated type. Every type potential unstable rock mass had its deformation and failure model. The 7 types corresponded to 7 types of deformation and failure models in turn, that is, hanging type corresponding to vertical dislocation model, sticking slope type corresponding to planar sliding model, toppling type corresponding to toppling model, wedge type corresponding to block sliding model, soft base type corresponding to compressure-toppling (shearing) model, masonry block type corresponding to bulking failure model, and isolated type corresponding to eccentric rolling (sliding) model.(3) Based on limit equilibrium element method, quantitative evaluation method of potential unstable rock masses stability was established, and the stability calculation formulas for different type of potential unstable rock masses were deduced. Using them, typical potential unstable rock masses stability of high slope in Jinping I and II hydropower station were calculated.(4) Discontinuous deformation analysis was done for the stability analysis of hanging type and masonry block type of potential unstable rock masses, and conceptual model of hanging type of potential unstable rock masses was made. In addition, the influence of the ratio between height and width on hanging type of potential unstable rock mass was analyzed. As a result, the judgment formula was put forward for its stability. Through the stability analysis of masonry block type of potential unstable rock masses, and the analysis of strength parameters relation of prominent structural plane steeply dipping out of slope, we got that inner friction angle had stronger influence than cohesion.(5) Six basic factors were selected as evaluation criteria of potential unstable rock masses, including dipping angle of main controlling discontinuity in potential unstable rock mass, base condition, slope gradient of geomorphology, state of recessed cavity, rock mass structure and state of relief and slack. These geologic parameters of potential unstable rock masses were all typical and easy to get in situation. With relation matrix, the importance and weight of these criteria were semi-quantitatively analyzed. Based on the importance and weight of rapid evaluation criteria, unstable masses instability index (UMII) method of rapid evaluation for potential unstable rock masses was put forward, and rapid quantitative evaluation method of potential unstable rock masses stability was established. Moreover, they were applied to the rapid quantity evaluation of potential unstable rock masses stability of high slope on the left bank, at the lock location of Jinping II hydropower station.(6) A number of rolling rock experiments were done to study the common characteristics of rolling rock movement on slope surface and changing rule of kinematic parameters. Experimental results showed:①gentle slope, overburden and platform on slope made evidential resistance to the rolling rock movement on slope; For rolling rock moving on slope, its kinetic energy when it rolled down at the foot of slope was 1/15～5/12 of that when it freely fell off at the same location, average 5/27. The movement type of rolling rock mainly was bounce. But at the site of thicker overburden, it could be turned into rolling, and there was probably local sliding; The collision between rolling rock and slope resulted in a large number of fragments. When part of them dropped to the foot of slope, individual kinetic energy would increase much due to much loss of individual mass.②The characteristics of slope overburden and vegetation made key contribution to speed restitution coefficient before and after collision, and they could be described by a linear correlation. Detailed mentions were as follows: If the overburden was thicker than 1m and vegetation developed, restitution coefficient before and after rolling rock collision could select 0.40～0.55 If the thickness of overburden ranged from 0.5 to 1m, and vegetation developed, the value could select 0.50～0.60; If there was locally overburden and there was locally rock cropout, and vegetation did not develop, restitution coefficient could select 0.60～0.70; If base rock cropout was widespread on slope, and there was no or seldom vegetation, the value of restitution coefficient could select 0.70～0.80.③There were many relevant factors to influence movement acceleration, but slope angle, shape of rolling rock, and the characteristics of overburden and vegetation, were key influence factors to the acceleration of rolling rock. The mutual relationship of rolling rock acceleration, slope gradient, rolling rock shape, slope overburden characteristics and vegetation characteristics could be expressed by a linear regression formula: . Its application of engineeringshowed its stronger applicability. So it had practice sense.(7) Through the analysis of rolling rock movement characteristics on a platform, the calculation method was gotten for the collision speed between rolling rock and platform, and horizontal movement distance of rolling rock moving on the platform. At the same time, their influence factors were also analyzed. Meanwhile, to study the platform tarriance effect on rolling rock, a large number of site experiments were accomplished. Their results showed:①The mass of rolling rock, initial position (slope height), and the roughness of the platform had a certain influence on the stop position. In other words, the stop position of rolling rock and its mass accorded nearly with power function. The more rolling rock mass was, the longer displacement on the platform was, and the higher slope height was, the longer displacement on the platform was for same mass rolling rock; There was a threshold mass value, m_临. If m> m_临, rolling rock with the same mass and initial position would move longer on the platform whose surface insisted of block and debris than on the platform whose surface insisted of debris with block; If m< m_临, it was quite the contrary condition; If m= m_临the displacement was equal.②For rolling rock moving on platform, its stop position was influenced by rolling rock mass, shape, velocity when just arriving at platform, and the roughness of platform surface. Their relation could be expressedby the formula, .(8) To study trees barrier effect on rolling rock and analyze the collision probability between rolling rock and trees, a large number of rolling rock experiments were accomplished in site. Results were as follows:①Collision probability between rolling rock and trees was related to trees space and rolling rock diameter. The expression of trees rows was gotten to assure that collision at least happened k times between rolling rock and tree.②Collision between rolling rock and trees made the rolling rock speed decrease on a large scale. If only the collision between rolling rock and trees happened once, the speed would lose 45%, and the kinetic energy would lose around 70%.(9) Base on the study on rolling rock movement characteristics, platform tarriance effect on rolling rock and trees barrier effect on rolling rock, the feasibility and design steps of countermeasure for potential unstable rock masses were analyzed, when using the methods of platform, trees, or platform and trees together. The new passive countermeasure were put forward, namely, platform, trees, or platform and trees together.