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Research on Dynamic Response and Failure Mode of Tunnel Surrounding Rock under Blasting Loading

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Tutor: Cui
School: Tianjin University
Course: Hydraulic Structure Engineering
Keywords: tunnel,blasting excavation,surrounding rock stability,dosagecontrol,stress rotat
CLC: TV554
Type: Master's thesis
Year:  2012
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Abstract:
Blasting is the important way in underground engineering excavationconstruction, but the shock and vibration effect caused by explosive energy whenbreaking rock can also lead to the damage of the surrounding rock, it has negativeeffects on completeness of surrounding rock and the safety of engineering.Based onthe numerical simulation, this paper studies the dynamic response of surrounding rockcaused by tunnel blasting excavation, evaluates the influence of blasting on thestability of surrounding rock, and carry on blasting quantity control analysis based onthis; the changes of stress path of surrounding rock induced by circulatory excavationis also researched, along with the evaluation on the influence of the change of stressmagnitude and direction on surrounding rock. The main research work is as follows:(1) Take Column blasting charge as example, the damage process and blastingvibration attenuation process of rock in the action of explosion is studied. The damagezone of rock mass caused by blast develops from bottom to external along the wall ofthe blast-hole, with the extension in the radial. Larger blasting pressure concentrate onthe rock mass within the region around the hole, the region is as big as5times ofcharging radius. Under the condition of different charge, the pressure peak andvibration velocity peak all appear in100μs after explosion. When the charge weightper blast hole is3times of the original, the damage radius increase about50%, andthe pressure peak and vibration velocity peak increase about70.5%.(2) The dynamic response of tunnel surrounding rock under bursting loads andits influence on the stability of surrounding rock are studied. The influence of blastingshock on surrounding rock in popping peripheral1.0m range is more obvious. Thepeak of principal stress generally appear in800μs time. Larger pressure peak mainlyappears in the position close to the blasting hole. The attenuation amplitude ofblasting wave in the internal surrounding rock in front of working face is greater thanwhich behind the working face. The plastic zone of surrounding rock is develop alongthe radial of blasting hole, and it extension in the internal rock mass in front of theworking face and behind the working face along a cone shape. When the chargeweight per blast hole is3times of the original, the principal stress peak and vibrationvelocity peak can increase by300%, but there is not too much expansion in the range of plastic zone. It shows that greater charge weight will aggravate rock breaking butthe range of broken zone is influenced much less.(3) According to the tunnel excavation, the influence of stress path changes oncrack propagation and stability of surrounding rock are researched. To surroundingrock, excavation is a repeated loading and unloading process, it can lead to the changein magnitude of stress and the rotation of stress, and the surrounding rock stabilityproblem caused by stress rotation should not be ignored. Based on the study ofengineering, the maximum rotation angle of principal stress of rock mass nearsidewall is close to90degrees and the angle stays in peak until the end,while theprincipal stress of rock mass near hanging wall return to the initial direction after themaximum rotation angle appears. The rotation of principal stress in surrounding rockcan lead to significant changes on the depth and direction of the extension of the crack.The stress rotation in the surrounding rock close to the top of the tunnel is moreviolent, and the rock mass near the top very likely appear macroscopic damage.During the blasting excavation of tunnel, explosion shock load will lead to thedestruction of surrounding rock in short time, then the surrounding rock is mainlyinfluenced by blasting vibration; the increase of dosage will aggravate the degree ofcrushing, but the range of crushing zone is influenced much less; stress rotation caninfluence the direction and the depth of crack propagation, the lead to the broken ofrock; all above should be pay attention to in actual engineering.
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