| Description |
An ever-present challenge at most active mining operations is controlling blastinduced damage beyond design limits. Implementing more effective wall control during blasting activities requires (1) understanding the damage mechanisms involved and (2) reasonably predicting the extent of blast-induced damage. While a common consensus on blast damage mechanisms in rock exists within the scientific community, there is much work to be done in the area of predicting overbreak. A new method was developed for observing near-field fracturing with a borescope. A field test was conducted in which a confined explosive charge was detonated in a body of competent rhyolite rock. Three instrumented monitoring holes filled with quick-setting cement were positioned in close proximity to the blasthole. Vibration transducers were secured downhole and on the surface to measure near-field vibrations. Clear acrylic tubing was positioned downhole and a borescope was lowered through it to view fractures in the grout. Thin, two-conductor, twisted wires were placed downhole and analyzed using a time-domain reflectometer (TDR) to assess rock displacement. Fracturing in the grout was easily observed with the borescope up to 3.78 m (12.4 ft) from the blasthole, with moderate fracturing visible up to 2.10 m (6.9 ft). Measured peak particle velocities (PPV) at these distances were 310 mm/s (12.2 in./s) and 1,490 mm/s (58.5 in./s), respectively, although no fracturing was observed near the depth of the vibration transducers located 3.78 m (12.4 ft) from the blasthole. TDR readings were difficult to interpret but indicated rock displacement in two of the monitoring holes. Three methods were used to predict the radial extent of tensile damage around the blasthole: a modified Holmberg-Persson (HP) model, a shockwave transfer (SWT) model, and a dynamic finite element simulation using ANSYS AutodynTM. The extent of damage predicted by the HP and SWT models is similar to field measurements when using static material properties of the rock, but is underestimated using dynamic material properties. The Autodyn™ model significantly overpredicted the region of damage but realistically simulated the zones of crushing and radial cracking. Calibration of material parameters for the AutodynTM model would be needed to yield more accurate results. |
