This thesis describes model-scale bench blasts on mortar blocks with distorted drill hole patterns, done in 2013-2014 at the Montanuniversitaet Leoben. The main aim of the project is to see if the fragmentation from and the crack development behind blasts with and without drillhole deviations differ significantly.
The dimensions of the test blocks were 660×280×210 mm (L×W×H). They were mounted inside a yoke that allows the blast waves to escape. Three rows in each block were shot row by row with a nominal pattern of B×S=70×95 mm. The same nominal delay 73 μs (i.e. 1ms per m burden) between holes was used throughout. Blocks with holes of stochastic collar position errors (variations in burden or in burden and spacing but constant row volume) and systematically shifted collar positions (blasts with staggered pattern) were shot. The results were compared to reference blocks without collar deviations.
Apart from the sieving analysis, the remaining bench faces were measured with a stereo-photogrammetric method and horizontal roughness profiles for each row were constructed. Sawing slabs from block remains after the 3rd row and use of dye penetrant allowed the study of internal blast damage.
The analysis showed that none of the changes in bore hole collaring had a significant effect on the sieving curve (median). The sieving results confirm earlier findings though that the fragmentation gets finer with the number of rows shot, implying that blast damage from earlier rows has an influence on the blasting results. The tests demonstrated that the nequiv (equivalent n-value) has increased 10% by shooting staggered pattern and decreased by 7% by pattern with burden and spacing errors. For the rest of the patterns the nequiv was roughly the same.
The surface roughness investigation showed that a smooth surface was achieved by the staggered pattern. The other deviations patterns did not show a clear trend.
The internal blast damage, which was measured in the remaining part of blocks, showed that out of eleven, four crack families seemed to be most affected by blasting. The comparison of the reference and distorted patterns showed that less cracks were apparent for the staggered pattern, i.e. less damage was introduced.