Analysis of drillhole deviation during drawbell construction in block caving

Abstract

In Block Caving drawbells allow for flow of broken rock. The drawbell construction starts with a drawbell design, which is constrained by the extraction layout shape and development drives sizes. The resulting shape of the drawbell is highly influenced by drilling performance, particularly drillhole deviation. This paper presents the results of a comprehensive analysis of drillhole deviation data undertaken from more than 2,000 holes of 76 mm diameter, having an average length of 12 m and drilled in more than 500 ring positions. In terms of drawbell design, the analysis indicates that the holes less affected by drillhole deviation are those with plunge angle ranging between 65° to 90°. Those holes are located near the centre of the drawbell. The holes less inclined show more deviation. They correspond to those forming the edge of the drawbell shape. Therefore, these areas can be exposed to possible blast damage due to confined charges. The analysis also indicated that 59% of the measured points have a drillhole deviation percentage (DD%) greater than 2%, while 2% of the points present a DD% over 10%. An industry standard for drawbell drill deviation along drilled depth was developed as a tool to check the effectiveness of a blast design, in terms of drill patterns that are likely to appear due to drillhole deviation.