Large structure response to high frequency excitation from rock blasting

Abstract

This paper describes measurement and interpretation of response of two, multiple story, older, urban structures to small charge weight, ultra-high frequency rock blast excitation from contiguous excavation. Time correlated responses were measured at the ends, top and bottom of the structures as well as in the foundation rock below the bottom. Observations based on the ten, instrumented positions during eight blast events provided over seventy time histories for analysis. The case study and measurements allowed the following conclusions: close-in blasting with direct rock to building wave transmission imposes short wave length excitation which fails to excite the large, massive structures synchronously. The structures respond predominantly in wave transmission mode where there is a noticeable difference in time, frequency, phase and amplitude of motions measured at the bottom and top corners of the structure. Excitation motions along the base also differ in time, frequency, phase and amplitude. The short wave length of the excitation motions leads to attenuation of the peak particle velocity along the base. Ultra-high excitation frequencies, which are much higher than the expected structural natural frequencies, lead to deamplification for all events