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.