This research evaluates the variation in the Drilling Rate Index (DRI) and its relationship with thermal damage on thermally treated rocks. Samples from ‘Prada’ limestone, a lower cretaceous formation in the southern Pyrenees (Lleida, Spain), were subjected to temperatures of 105, 300, 400 and 500 °C and then cooled at a slow rate. Ultrasound P-wave velocity tests were performed before and after heating the samples to evaluate the thermal damage experienced by the rock. Sievers’ J miniature drill and brittleness tests were conducted on intact and thermally treated samples, and then resulting SJ and S20 values were combined to determine DRI. The obtained results show that thermal treatment allowed an increase of 34% in the DRI of ‘Prada’ limestone at 500 °C. DRI exhibited the same variation trend than S20, so we can conclude that thermal variation in DRI is more influenced by S20 than by SJ in ‘Prada’ limestone. We also report a strong relationship between DRI and P-wave velocity, confirming a tight dependence between the drilling performance and the thermally induced damage on the limestone. The observed substantial improvement in the drillability of the rock when heated, measured in terms of DRI value increase, could help in the advance on the development of thermally assisted mechanical excavation methods.
Modern mechanical excavation strongly depends on the efficiency of the means involved to optimize investment costs. Rock properties influencing drillability have been discussed by different authors. Uniaxial Compression Strength, Brazilian Tensile Strength, Shore Scleroscope Hardness, Point Load Strength are among the most important parameters [1–6]. Additionally, physical and mechanical properties of rocks show dramatic variations with temperature that condition the drillability of the thermally treated rocks. The use of fire to ease rock excavability is an ancient art that was extensively employed during the Roman Empire [7], but few research exists on the effects of high temperatures in the drilling performance of rocks. Rossi et al. [8] explored the effects of using a flame jet to achieve high local heating rates, and determined a severe decay of 30% in the rock strength for temperatures up to 600 °C. Rossi et al. [9] studied a combined thermo-mechanical drilling (CTMD) and stated that thermal treatment of rocks causes extensive thermally induced cracks in the rocks, which significantly enhance the penetration performance of the cutting tool. Jamali et al. [10] used high powered laser technology to reduce the rock strength, drilling resistance and fracture toughness. Existing research is still scarce in the drilling performance of thermally treated rocks, even when that would help improve the efficiency of mechanical excavation means. In this study, thermal variation in the drillability of rocks will be studied through Drilling Rate Index (DRI).
