Improvement of Fragmentation by Blasting - Investigation of the influence of delay-times on the crack development in the surrounding rock, the characteristics of the blasted bench face and the fragmentation of the further rows

Peter Schimek

Research output: ThesisDoctoral Thesis

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This thesis summarises the findings of 27 model-scale bench blasts in magnetic mortar, which were done from 2011 to 2014 at the blasting site of the Montanuniversitaet Leoben. The aim of the study was the investigation of the influence of blasthole delay-time in small scale bench blasts on the face characteristics and the cracking damage created in the surrounding rock, which affects the fragmentation in subsequent rows. The test specimens or blocks, which were made of magnetite-mortar, were roughly 1:100 in scale to normal bench blasts. The confinement of the testing blocks at the sides and at the back ensured the similarity to normal bench blasts with regard to the wave transmission into the surrounding rock. The specimen contained three or four rows with 5 blastholes each and were shot row by row, with virgin (undamaged) material in row 1 and increasingly damaged or preconditioned material in the following rows. The tested in-row delay-combinations varied between 0, 28, 73 and 140 µs which corresponds to 0, 0.4, 1.0 and 2.0 ms/m of burden. The measured material properties showed that the magnetite-mortar used modelled the inhomogeneous behaviour of rock but some unexpected differences of the different production cycles were still detected. The characteristics of the bench after each blast was evaluated along three horizontal lines out of a 3D-model of the bench face. The tested delay-time sequences of the virgin (1st row) and single pre-conditioned blasts (2nd row) produced more overbreak for longer delay-times while the 3rd row shots resulted in a flatter surface independent of the chosen delay. The visible cracks at the top of the testing blocks were documented and divided into several crack families according to their angle, length and origin. The remaining block was broken out of the blasting site after blasting and cut into several horizontal and vertical slices and dye-penetrant spray was used to visualize the cracks created. Some of the detected crack families showed an influence of the delay-time. The calculated mean crack density (MCD) showed that longer delays resulted in a higher degree of damage. The fragmentation results were well reproduced by the basic three parameter Swebrec function. The blasts showed coarser fragmentation for the shortest delays while the longest delays generated the finest material. The observed improvement of fragmentation in the 2nd and 3rd row blasts was linked to the pre-conditioning of the burden for the following row while blasting the actual row.
Translated title of the contributionUntersuchung des Einflusses der Verzögerung von einzelnen Bohrlöchern auf die Rissausbreitung ins anstehende Gebirge, die Beschaffenheit der entstehenden Bruchwand und die Zerkleinerung der nachfolgenden Reihen
Original languageEnglish
  • Ouchterlony, Finn, Assessor A (internal)
  • Schunnesson, Håkan, Assessor B (external), External person
Publication statusPublished - 2015

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