| Summary: | Assessment of drillability of rocks is vital in the selection, operation, and performance evaluation of cutting tools used in various excavation machinery deployed in mining and tunneling. The commonly used rock drillability prediction methods, namely, drilling rate index (DRI) and Cerchar hardness index (CHI) have limitations in predicting the penetration rate due to differential wear of the cutting tool in rocks with varied hardness and abrasivity. Since cutting tools get blunt differently in different rocks, the stress beneath the tip of the bit decreases until it reaches a threshold value beyond which the penetration rate becomes constant. In this research, a new composite penetration rate index (CPRI) is suggested based on the investigations on four metamorphic rocks viz. quartzite, gneiss, schist and phyllite with varied hardness-abrasivity values. The penetration-time behavior was classified into active, moderate, passive, and dormant phases based on the reduction in penetration rate at different stages of drilling. A comparison of predicted penetration rate values using DRI and CPRI with actual penetration rate values clearly establishes the supremacy of CPRI. Micro-structure and hardness-based index was also developed and correlated with CPRI. The new indices can help predict cutting tool penetration and its consumption more accurately.
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