Study On Dynamic Response Of Connection Tunnel By Different Blasting Excavation Methods In Shaft

The ventilation shaft is part of the supporting projects for long highway tunnel, shaft is contacted with main tunnel through connection tunnel. When the excavation near the connection tunnel, blasting vibration would cause the endoplasmic point generating large vibration velocity in connection tunnel, and may lead the connection tunnel cracking, falling block, even collapse. Under the influence of blasting excavation, the stress would be redistributed, even affect the stability of primary support and surrounding rock in connection tunnel. Therefore, in order to make the shaft blasting scheme satisfying excavation response, we should focus on the dynamic responses of connection tunnel caused by blasting excavation of shaft.This paper introduces the mechanism of rock blasting damage and the formation process of surrounding rock loose circle and calculation formula of thickness. The extension principle of the surrounding rock loose circle under the action of blasting dynamic load is also studied, then puts forward a design of excavation scheme based on the mechanism of millisecond blasting to reduce the dynamic impact on the connection tunnel. Combined with the engineering characteristics of ventilation shaft of Guantian tunnel of Zhangyong Expressway in Fujian Province, the pros and cons between well method and anti-well method are compared. This paper select anti-well guide well as the free surface of blasting, determining the blasting parameters, and designing five different blasting schemes based on different millisecond time. Using Midas/GTS software to build a three-dimensional model, calculating the blasting load curve and the formula of peak load, then superimposed by Matlab software to get the curves of blasting load and simulate the blasting excavation.Through the numerical simulation, we analyze the influence rule of vibration velocity and stress caused by blasting excavation of shaft, the following conclusions can be obtained:(1) When selecting the different programs for blasting excavation of shaft in the same construction segment, the vibration velocity and stress are increased with the millisecond time reducing. When the millisecond time is 75ms, the impact is minimal with connection tunnel by the velocity, stress and so on.(2) The maximum combined vibration velocity and variation in initial support of connection tunnel are close to the value of Y direction, also the values of maximum combined vibration velocity are larger than the other two directions. When the millisecond time is 25ms, the combined vibration velocity has exceeded the national security allowed limit 10cm/s. And the maximum combined vibration velocity distributes in the longitudinal axis of the connection tunnel vault from the combined velocity cloud picture.(3) In the process of shaft excavation, the surrounding rock’s maximum and minimum principal stress near the cross-contact surface between shaft and connection tunnel did not exceed the allowable tensile strength of rock mass, it will not cause the surrounding rock to be destroyed. When the millisecond time is 25ms, the minimum principal stress is maximum, and with the decreasing of the minimum principal stresses, the rock loose circle gradually reduced.