Experimental study of surface constraint effect on rock fragmentation by blasting

In rock blasting, there are different constraint conditions such as a free surface, a partial free surface and a free surface with a barrier nearby. These conditions were modelled by three groups of blasting tests in this study: (1) A cylindrical granite specimen was blasted with complete free surfaces to model the free surface condition. (2) A cylindrical granite specimen was placed in a steel tube larger than the specimen to model the free surface with a barrier. In this case, the gap between the specimen and the tube was empty. (3) A cylindrical granite specimen was placed in a steel tube larger than the specimen and the gap between the specimen and the tube was filled with pebbles to model the partial free surface. A high-speed camera was used in some model blasts. After blasting the fragments including fines were collected, weighed, and sieved. The results showed that: (1) The specimens with a free surface and a barrier yielded the best (finest) fragmentation. (2) The specimens with a partial free surface produced the worst (coarsest) fragmentation. (3) The specimens with a complete free surface gave a fragmentation result in the between.