The production of fines in a blast is a topic that has not been properly quantified. This study examines the fines production as a consequence of a change in several variables of the explosive/rock interaction. The variables studied fall into two groups. Group 1 is the change of blast design parameters, such as: explosive VOD (choice of explosive), Powder Factor (quantity of explosive used) and borehole coupling (how the explosive charge is placed in the hole). Group 2 is the change of the rock mass characteristics that are blasted (compressive strength, tensile strength, density, and grain size). Both groups were examined in several statistically designed experiments. The volume of fines produced was modeled against the parameters in Group 1 and 2. To do this, 86 lumps of rock and concrete grout were blasted and sized down to 38μm. Four different geologies and two different concrete grouts were part of this experimental design. The 46 concrete grout blasts were used to quantify the influence of various different blast design variables. The 40 natural rock blasts were used to examine the influence of a change in rock mass characteristics.
To compare the size distributions of each blast, a method of calculating the surface area of fragments to a very fine size fraction was developed. It is appropriate to use this method to a ultra fine size of 38μm. The surface area of each size fraction for each sample was calculated. In doing so, various geologies of different characteristics were ‘normalized’ with regard to fragmentation. As such the true influence of a change in a parameter like rock density can be quantified.
The energy released by the explosive was related to the surface area of the fines generated. Two clearly distinct components of this energy applied/fines generation relationship were observed. It was postulated that different mechanisms were influential in these two components. The influence of several rock mass parameters was examined as well.
Dynamic crack propagation and micro crack mechanics can explain the mechanisms behind an increase of fines generation and its surface area as a function of applied energy. Micro crack branching could explain the mechanisms of how fines are created along the surface of larger cracks.
Blast induced pre-conditioning was also examined in the context of micro crack generation as a function of applied explosive energy. This was examined due to pre-conditioning being a source of energy absorption and having similar mechanisms to that of fines generation (dynamic crack propagation). It was found that the rate of energy application (explosive VOD) influenced the magnitude of pre-conditioning and not the quantity of energy applied (Powder Factor).
It was observed that each rock type had a unique surface area signature across the size fractions. This signature maintained similar shape and form with different explosive energies applied in the blast. As the applied explosive energy increased, the number of fragments increased but at the same shapes and sizes. Several sub-populations were observed in the size distribution of the surface area of fragmentation. One population was the material below 1mm and another was the material below 100μm.
Using the Surface Area Model, it was shown that the surface area in each size fraction could be related mathematically to the surface area of other size fractions. It is implied that blast fragmentation is fractal in nature. As such, the surface area of fines can be related to the surface area of larger fragments. It was postulated that very small fragments (fines) are generated along the surface of larger fragments.
Also the 3 dimensional spatial position of finer fragments at their genesis was experimentally examined. This study has examined the production of fines in a blast in a holistic sense through a rigorous experimental program.