| Perilous rock is the unstable rock block that cut by rock fissure on the cliff or steep slope. Perilous rock's form, unstable and movement belong to the dynamical evolution process of the slope physiognomy. Perilous rock avalanched have group-occurring and regularity characters on the cliff. This paper explains the regularity of Perilous rock group-occurring avalanche by geomorphology and mechanics. The main contents are as following.(1) On the basis of investigations in real-time field to several typical cliffs and geomorphologic analysis principle, the authors verified chained regularity of perilous rock in the area of the Three Gorges Reservoir. The regularity includes both micro-chain and macro-chain. Micro-chain is designated for perilous rock to form and avalanche gradually from foot of cliff to top, while macro-chain identify for cliff to retreat from slope surface. The regularity shows two models, one is simple model which includes one macro-chain, another is complex model which includes many macro-chains at the same time. Formation processes of rock cell and rock unloading above the rock cell, locating in the base of cliff, belong to initial dynamics for perilous rock to develop in the area.(2) This paper reveals the formation mechanism of perilous rock's control fissure through the relationship of tectonic stress field and fissure. Base on the fracture mechanics principle, this paper received the calculate method of stress intensity factor of perilous rock's control fissure. Damage division of end area of control fissure was defined by authors, established in damage methods of timed-Poisson's ratio and timed-Young's modulus were established in damage mechanics theory. Through direct shear tests for eleven various transfixion rates of mortar samples, the method to calculate C andφof control fissure is established through considering various linking ratio and safety grade of control engineering, which is defined as linking ratio method.(3)The time of perilous rock formation contain the time of rock cell formation and the transfixion time of control fissure. Through survey and observation in the Three Gorges Reservoir Region, it can be found that, to inner surface of rock cell in cliff or steep slope bottom, more than 60 percents of cracks are compression-dilatancy or compression-shear. These can express the notable fracturing of mudstone weathering, and its weathering velocity is 2～7 times faster than that under nature condition. Then a deduction model that rock cells' back surface recess parallel is put forward by geomorphologic analysis. The rule that vertical displacement and root compression increase approximately linearly with height of rock cell with FEM numerical simulation is revealed. The formation time of critical length of perilous rock's control fissure include fissure creep propagation's time under Gravity Load and fatigue fracture tirne under fissure water pressure.(4) In order to study the mechanics process of the law of perilous rock's avalanche, divided them into interaction mechanical mechanism of micro-chain and mechanical deduction of cliff retreat under coupling action of multi-macro-chain. The interaction of perilous rock estimated by the angle of rocks turned. First, figure out the angle of rocks turned without regard to the reciprocity of perilous rock. Then, acquired the angle of rocks turned which have considered the reciprocity of perilous rock by law of conservation of energy. At last, obtained the calculate formula of the rock's equivalent bending moment by angle. Also obtained time calculate formula for rock collapse by the principle of fracture mechanics and damage mechanics. Taking flexivity of various perilous rocks as the intermediate variable, the authors establish the method to calculate interaction loads for adjacent perilous rocks by equilibrium limit principle. Further, drawing lessons from the general Kelvin creep fracture model, derive and build the method to count fracture tenacity factor considering creep effect of perilous rock. Based on discriminate factor for perilous rock rapture, mechanical status every perilous rock is made known obviously. Case analysis verifies the mechanical mechanism for steep rock slope retreat evolution is in consistent with that in real time field.
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