Dynamic response of mine pit walls

Any wall control program at a mine site should naturally include an understanding of the dynamic response of a pit wall to blast loads. If blasts are fired close to the base of the pit wall and have sufficient energy over an appropriate frequency range then they could induce dynamic motion of a significant portion of the wall. In this regard it is important to determine the natural (resonant) behaviour of a pit wall. Unfortunately, this resonant behaviour cannot be determined using a full-scale blast simply because the natural response is contaminated by the spectral influence of the blast delay sequence. Thus the pit wall of interest is loaded (at its base) by a single blast event simulated as a short duration, broadband displacement pulse. In the present investigation, the wall response to such a load is determined using the dynamic finite element method (DFEM). The DFEM results from two- and three-dimensional models show that even for the elastic case it is not possible to excite the wall in any strongly resonant modes. This weak (lossy) resonance is due to significant radiation damping, whereby vibrational energy dissipates into the surrounding medium and does not remain trapped near the wall region. Detailed responses were obtained for walls of elastic and viscoelastic material as well as walls with horizontal layers and a damaged crest zone; in all cases the most significant motions were invariably found in the wall crest regions. However, due to the weak resonances, it is not possible to unambiguously identify wall resonant behaviour in any practical sense, and this is shown to be consistent with field measurements taken for the firing of single (seed) blastholes. In light of such results it is shown that, irrespective of the delay accuracy in any full-scale blast, there is little potential to control the wall response by varying the blast delay sequence.