Experimental Investigation of Decoupled Charge Effect on Rock Fragmentation by Blasting

Fragmentation of granite cylinders having a centric charge with different decoupling ratios was investigated in blast experiments. Conclusions are made from studying the post-blast specimens and strain measurement. Eight blasting tests were carried out on granite cylinders having a diameter of 240 mm and a length of 300 mm. All the specimens had a charge (PETN) of approximately 3 g, and the charge diameter was around 6 mm. A centric hole with a diameter of 10 mm, 14 mm or 20 mm was drilled for each cylinder, and the decoupling ratio was in the range of 1.8–3.6. Specimens had a decoupled charge with either water or air. Strain gauges were mounted on the lateral surface to measure circumferential strains. After each blast, fragments were collected and screened with a set of sieves. The specimen with a low decoupling ratio and the water-decoupled charge was mainly fragmented into small fragments, while the specimen with a high decoupling ratio and the air-decoupled charge mainly broken into large fragments. The normalized median size tended to have a linear relationship with the decoupling ratio, and the slopes for air-filled blastholes and water-filled blastholes were 0.179 and 0.094, respectively. With increasing decoupling ratio, the measured maximum strain decreased from 4910 με to 2420 με , and the tension part of dominant strain waves increased from 490 to 780 μs. The findings of this experimental investigation indicate that the decoupling ratio and the filling medium have a great impact on rock fragmentation as the charge weight and other conditions remain the same.