Exploration Of Laws Of Blasting Vibration And Air Blast Propagation For Urban Shallow Tunnels

The effect of shallow tunnel blasting on the surrounding buildings and constructors is an inevitable problem caused when a city builds subways. Due to the fact that different geological conditions need different construction plans, it is necessary to explore the laws of blasting vibration and underground air blast propagation for urban shallow tunnels. This paper is based on the construction of Dalian Metro, to analyze the principles of blasting vibration in different geological conditions with different measures.According to the actual circumstances, this paper gives the allowable value of velocity of vibration suitable for the tunnel blasting of Dalian Metro. Tests of blasting vibration have been made under different geological conditions, such as weathered marl,weathered limestone and middle weathered dolomite limestone based on the allowable values. The test results show that in the area near tunnel blasting source, the difference between the resultant velocity (Vmax)and the resultant velocity of the maximum speed of each three directions(VHmax) is about 15%, which means it is feasible to use resultant velocity as the reference value. Based on a large number of analysis of test result, it is clear that the dominant frequency on the horizontal direction of shallow tunnel blasting is lower than that on the vertical direction; however, buildings have selective amplification effect on low frequency, so attention also need to be paid to the vibration on horizontal direction rather than vertical alone. It is clear that the blasting energy from low frequency is suitable for the blasting design and test of dilapidated buildings according to the graph of power spectrum density. It also has proved the hollow effect on shallow tunnels, and has provided Sodev curve of attenuation for the three typical geological conditions mentioned above, which has the guiding significance for the future construction.The result from test of air blast wave in tunnels and exploration of underground air blast propagation show that, compared with the blasting vibration velocity on the gound,the peak of overpressure of blast wave in detonator segments of detonators is obvious, and can be used for checking the quality of blasting design of the cutting area. According to the analysis of air blasting overpressure,the rate of decay of overpressure of blast wave in underground tunnels is slow and is accompanied with high decibel noise. Different ways of cutting have obvious influences on the figures of overpressure of blast wave. The impulse analysis of the test result shows that although the absolute value of negative pressure of blast wave is smaller than the value of overpressure, in a long working-period, the difference between the two figures is small, and the latter may even exceed the former one. According to the test results, the attenuation curve of the impact wave is fitted,it has also provided the energy emission coefficient of overpressure of blast wave under different working conditions and has further concluded the allowable safety distance of air blasting under different working conditions.