Finite Element Study of Blast-Induced Vibration from Construction of Tunnels with Particular Emphasis on its Effect to nearby Slopes

Abstract

In Hong Kong, the vibration effect of blasting from construction of rock tunnels is often limited to a defined peak particle velocity (PPV) value for each sensitive receiver affected by the blasting. Slope is considered as a sensitive receiver in Hong Kong whereby the PPV limits will continue to be the default-controlling factor over explosive charge weight and thus rates of tunnel advancement. The blast-induced vibration limits generally used in Hong Kong or issued by facility owners are prescriptive only. The adopted empirical attenuation equation was based on surface blast vibration monitoring data collected territory wide by the Mines and Quarries Division of the Hong Kong Government. It is unduly overly conservative in many cases as observed in the industry. The Department of Civil Engineering of The University of Hong Kong has collected and analyzed the blasting data from the construction of the Eagle’s Nest Tunnel, which is 2.1 km long connecting traffic between Sha Tin and the Kowloon area. The tunnel has an arch-shaped profile with a finished internal diameter of 15 m in span and 10 m in height. Assessment has been made on the vibration effects to nearby slope stability with the use of an advance finite element program that considers the dynamic properties of the soil material and the propagation of dynamic waves. Attenuation constants derived from the blasts in certain sections of the tunnel are reviewed, using square root scaling, along with the statistical goodness of fit tests, to determine the reliability of empirical predictor equations. From the basis of the field data and numerical simulation, it is found that vibration data from blasting must be scrutinized to take into account the geology, and the free surfaces between the source and the receivers. The attenuation law changes rapidly when data is recorded close to the source and therefore monitoring of vibration close to the blasting source is needed to better define the attenuation constants. With proper calibration of field data, a very good match can be found between the measured attenuation constant and that generated by the finite element program. Using the dynamic stresses generated by blasting at each stage of tunnel advances; finite element analysis on slope stability has been carried out using the stability factor approach. The reduction of factor of safety for slope due to tunnel blasting is small when compared to the analysis assumed in simple closed form solution that is based on greatly simplified boundary condition and elastic theory. There is a great potential to relax the vibration limits currently adopted for slope in Hong Kong, thus improving the efficiency of blasting and shortening the construction time.