Modal testing as a means of seismic assessment of mining structures

A study was conducted for seismic assessment of underground mining structures by experimental modal testing. This study involved two field experiments in an existing mine. One is modal testing, the other is blasting-induced seismic excitation testing. Three structural surfaces in two different tunnels were tested. In modal testing, a ''broad band roving hammer impact test'' was used. It was found in this test that the structural attenuation of the vibration created by hammer impact was very high. Five to six mode shapes were identified from each of the testing surfaces. The modal frequencies ranged from 25 to 577.5 Hz. The wavelengths were comparable to the dimensions of the structures tested. Modal parameters were also estimated for the underground structures tested. In the seismic excitation tests, the dynamic energy of the structural seismic response was concentrated around modal frequencies found from modal testing. Discussion is provided in this paper as to the possibility of using modal testing for seismic response assessment of underground structures during strong earthquake.