| Rock avalanche is one of the main geomorphic hazards in the Three Gorges of the Yangtze River and constitutes a great threat to the safety of peoples' lives and property and the navigation safety of the Yangtze River channel.Slopes of soft-hard alternate strata formed by the interbedding of carbonatite with shale or mudstone are widely distributed in the incised valley of Three Gorges Region under the control of geological structure and geomorphic conditions.The soften rocks easily flow and subside towards free face due to the weak weathering resistance under the function of rainfall and groundwater.It is easy to produce instability of upper limestone rocks caused by the strengthen of the softened decreasing bases intensively.The author takes the Wangxia unstable rocks on the WU Gorge bank slope as example,has studied the formation and failure mechanism as well as the lasting effect and hazard assessment by geomorphology,geology,fracture mechanics,damage mechanics,energy principle,discrete element method and SPA.This study is a cross edge discipline between geography,applied mechanics,geotechnical engineering and hazard prevention.The innovation achievements have obtained as follows:(1)Based on the investigation and measurement of landslide and rock avalanche were made in Chongqing section of Wu Gorge,the geomorphic hazards of the study area were divided into three types: rock slumping,colluvium-debris landslide and rock avalanche.Examples of the local geomorphic hazards were cited to discuss their main characteristics,formation mechanism and failure mode.The development process and vertical distribution characteristics of the hazards were illuminated from the perspective of dynamic evolution processes of layered slopes in the Three Gorges karst area.The relationships between typical geohazards such as rock avalanche,landslide,debris flow and karst collapse were analyzed and the mechanism of geohazard chains were also discussed.(2)Classification of carbonate rock structure and mechanical characteristics in the Three Gorges area were systematically summarized,the modification by supergene deformation structures such as stratification,stratum,fold and corrosion fault in the study area were discussed.The slopes in the Three Gorges area were divided into horizontal bedding rock slope,consequent bedding rock slope and anti-dip bedding rock slope.Under the control of stratum,weak intercalated layer and weak structural plane,the failure mode and mechanics mechanism of the slope evolution process could be summarized as plastic flow-tension,creep-tension,sliding-tension,sliding-bending,bending-tension and sliding compression-tension.(3)The X-Ray diffraction,SEM and polarizing microscope analysis on the soft foundation mineral of the Wangxia unstable rock in WU gorge have indicated that quartz accounted for more than 60% and clay minerals such as montmorillonite and illite-montmorillonite series accounted for about 11.2%.The shear strength of layered silicates such as chlorite,mica and montmorillonite are low.Terefore it is easy to slide along the potential failure surface in the direction of the major structural surface and leads to the whole rock body sliding down;According to the triaxial compression test result of carbonaceous shale under different confining pressure by use of RMT-150 C mechanical testing system,the strength curve and Mohr circle curve were obtained by Linear regression method.The linear relationship among the yield strength,peak strength,residual strength and confining pressure were also established.(4)The geological and mechanic model of W1 unstable toppling rock and W2 unstable sliding rock were established.Based on fracture mechanics and damage mechanics,the calculation of energy transfer,energy dissipation and energy release of Wangxia W1 unstable rock has been deduced.The constitutive curves for the locked patch in dominant fissure of Wangxia unstable rock mass W2 with two kinds of medium materials has been proposed.Based on energy principle,the cusp mutation model for the failure of unstable rock was built and formulas for the transient elastic and impulsive acceleration and elastic and impulsive velocity of perilous rock abrupt failure have been calculated.The numerical simulation results by PFC2 D have indicated that great speed difference between the top and the base leads to the retroverted sliding failure of W2 rock mass and it coincides with the fact.(5)The movement equation of collapsed rock mass was established and the influential factor such as slope form,slope gradient,slope height,slope covers,rock lithology,rock weight and rock shape were also analyzed.The risk assessment model of unstable rock was built by set pair analysis theory,and the results have shown that Wangxia collapsed rock mass will finally stop on the bank slope,about 1180 m from the avalanche spot and 687 m to the Yangtze river channel,so it would not impact the channel directly. |
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