Back analysis of roof classification and roof support systems at Kestrel North

Kestrel Mine is a Rio Tinto owned underground longwall operation that mines the German Creek coal seam in Queensland‟s Bowen Basin. Kestrel Mine can be separated into two parts North and South. Kestrel North has been in operation since 1990 (called Gordonstone at the time) while Kestrel South is a comparatively new mine having begun in-seam development in the first quarter of 2011. Over the recent history of Kestrel North several methodologies have been employed to characterise the roof and floor conditions, with a view to optimising the roof support design system and process. The objective of this study was to review various roof classification systems against actual conditions encountered during extraction of coal from the 300 series longwall panels at Kestrel North. The aim was to determine what systems would work well in the deeper Kestrel South environment. The study also reviewed different UCS sonic relationships in use and derived a new correlation for the entire Kestrel area. The back-analysis for the primary support was conducted by comparing the actual conditions and installed bolting patterns versus the rock mass conditions predicted using a variety of different roof classification systems. For secondary roof behaviour, extensometer data was used to review roof performance. The systems reviewed were the Roof Strength Index, sonic derived UCS and Roof Mass Rating. The study confirmed that UCS is a good first predictor for the primary roof conditions, whereas the Roof Strength Index showed the best correlation with the secondary roof conditions. It is inferred that formation of a beam in the primary support horizon is more closely related to rock strength, compared to the secondary support horizon where the influence of the stress regime appears more critical.