Study On The Vibration Induced By Collapsing Of Horizontal Tensile Fracture-sliding Unstable Rock

Unstable rock collapsing is a common type of geological disasters in China.According to statistics from the Ministry of Land and Resources,collapsing accounts for about 26 percent of all geological disasters.The near-horizontal rock with relatively steep dip fissures is common in the mountainous area of central and western China and in the Three Gorges reservoir area,and its instability often causes great loss of life and finance minister to local people.At present the research achievements of unstable rock block mainly focused on a single potential fissure and the single failure mode of the unstable rock block,the research on the unstable rock with double potential fissure is not thorough,It's not considered about the vibrate excitation between adjacent unstable rocks and the demage of the rocks after collapsing affected by the chain vibration effect.focusing on the geomorphic evolution,the study built the geological model of the tensile fracture-sliding unstable rock.According to the interdisciplinarity of fracture mechanics and limit equilibrium theory,energy theory,the seismic wave dynamics theory and the numerical simulation,the instability failure mechanism of the tensile fracture-sliding unstable rock was researched.This study is devoted to the mechanical mechanism of formation and instability of the the near-horizontal rock with relatively steep dip fissures.Taking the interlocking excitation effect on the residual rock block and other issues after collapsing into account,This study made the following progress:(1)The steep cliffs in the Three Gorges Reservoir area are usually composed of nearly horizontal hard rocks such as sandstone or limestone and soft rocks such as mudstone or marl.The lithology has the spatial characteristics of interbedded distribution,and the lateral unloading causes the unloading cracks to be almost parallel to the steep slope of the mountains,the unloading cracks is intermittent called the second potential fissure.Under the influence of the continuous weathering and unloading of the rock cavities,a " tensile fracture-sliding unstable rock " with near horizontal bedding(the first potential fissure)and steeper unloading cracks(the second potential fissure)is formed on the steep cliffs.The stability of this unstable rock is controlled by two main potential fissures.Under the action of load,the first main potential fissure of unstable rock is firstly extended by tension,and then slides and deforms along the second potential fissure when it reaches the critical state.Finally,the unstable rock block falls,showing the mechanical characteristics of tension fracture-slide.(2)According to the geological and mechanical model of the tensile fracturesliding unstable rock,based on the theory of fracture mechanics and limit equilibrium method and Mohr-coulomb strength criterion,respectively build the tensile instability discriminant equation of the first potential fissure and the sliding instability discriminant equation of the second potential fissure.According to the strain energy theory and seismic wave dynamics theory,the calculation formula of the transmitted and the reflected vibration wave in the two potential fissures which were caused by the unloading rebound and the elastic strain energy releasing were established.Considering about the vibration excitation,the tensile instability discriminant equation of the first potential fissure and the sliding instability discriminant equation of the second potential fissure were both corrected.(3)Taking tensile fracture-sliding unstable rock collapsing accident above the national highway G318 in gaoyangzhai area hubei province as an example,and take the influence of artificial blasting source in the construction tunnel into account,the instability discriminant equation of the two potential fissures were calculated.Through calculation result,it is found that the first potential fissure of the first and second unstable rocks in the lower part is not fully apart from each other so they are stable,and the three unstable rocks are damaged as a whole.After the sudden simultaneous collapse of the three unstable rock blocks,the horizontal first potential of the fourth unstable rock block on the upper position is still does not reach the ultimate tensile strength,and will not be cracked under the condition of considering the vibration excitation.The calculated results are in good agreement with the actual rockfall situation.(4)FLAC-3D numerical simulation software is used to simulate the collapse process of Gaoyangzhai unstable rock under the action of blasting-induced vibration.In the initial stress field without vibration,the phenomenon of the tensile stress concentration could be seen which was caused by the rebound of rock block near the steep slope.In the collapse process of unstable rock,the failure mechanism of the gradually cracking of the first potential fissure and the gradually increasing of the pressure on the second potential fissure could be seen clearly,and then the first potential fissure is completely aparted,the second potential fissure slips,and the three unstable rocks overall instability and caving finally.after the rockfall of the three unstable rocks,the fourth unstable rock in the immediate vicinity remained safe.The above research results have a significant guidance on the prevention and control technology of the tensile fracture-sliding unstable rock in the area with steep and high slope.