| Description |
Ground vibrations caused by bench blasting are a challenge that most active surface mining operations face. Ground vibrations resulting from blasting have been studied since the early 1930s. Most of these studies have focused on damage criteria relating to surface structures. This research focuses on blast-induced damage that can occur in an open pit mine slope. Three inspection holes at depths of 6.9 m (22.5 ft), 5.3 m (17 ft), and 3.8 m (12.5 ft) were drilled in a line perpendicular to the perimeter row of a 131-hole production blast. Each of these holes was fitted with a clear, hollow, acrylic tube. A quick setting hydro-stone™ cement was used to fill the holes around the acrylic tube. Geophones were placed at the collar of each hole and connected to a data collector. The data collector was set up to measure near-field ground vibration levels. After the blast was fired, a borescope inspection camera was inserted down the clear acrylic tube of each of these holes to record the degree of damage. Small-to-moderate fractures were recorded 10.7 m (35 ft) from the perimeter of the blast. Medium-to-large fractures were recorded 7.6 m (25 ft) from the perimeter of the blast. Large fractures were recorded 4.6 m (15 ft) from the perimeter of the blast. The surface particle velocities recorded 10.7 m, 7.6 m, and 4.6 m from the perimeter of the blast were ≥ 254 mm/s (10 in./s), ≥ 254 mm/s (10 in./s), and ≥ 439.2 mm/s (17.3 in./s), respectively. A method for predicting near-field ground vibrations was used to predict the magnitude of ground vibration at the nearest inspection hole. Results show that calculating the near-field response for the nearest blast hole may not be a good way of determining the maximum ground motion expected at a point. The critical particle velocity of the limestone in the Guilmette Formation was calculated to be 952.9 mm/s (37.5 in./s). Even though the seismic records were clipped, they still show ground vibration greater than 25% of the critical velocity, a percentage required for cracks in the formation to be extended. |
