## 4 Discussion  
Decay in mechanical (UCS, elastic modulus, and Poisson's ratio) and physical (P- and S-wave velocities) properties were accompanied with a general increase in the Siever's J  
Figure 13: Correlations between variation of S\({}_{20}\) and P- (**a**) and S-wave velocity (**b**) for air- and water-cooled samples. value and in the brittleness S\({}_{20}\) value that, together with the DRI index, point to a sustained increase in Prada limestone drillability. DRI, SJ, and S\({}_{20}\) values for the Prada intact limestone (before thermal treatment) have been compared with three types of intact limestones studied by [PERSON] and [PERSON] (2018). Although those limestones presented high, or very high, drillability categories, a lower DRI in Prada limestone (DRI = 44 corresponding to a'medium drillability' class) is attributed to a greater UCS. Indeed, a linear correlation between UCS and DRI was proposed by [PERSON] and [PERSON] (2018) for intact limestones. This type of correlation has proven an excellent prediction of DRI from UCS for intact Prada limestone (UCS = 168.09 MPa; DRI = 43.71). In the case of SJ, we obtained a value of 11.35 \(\pm\) 4.41 1/10 mm for the intact Prada limestone, and this corresponds to medium surface hardness, or resistance to indentation, according to the classes proposed by [PERSON] et al. (2018). In the case of S\({}_{20}\) we obtained a value of 42.77 \(\pm\) 10.94% for the intact rock and this is consistent with a medium rock brittleness, or the ability to be crushed by repeated impacts (also according to [PERSON] et al.) (2018). Thermal treatment resulted in a total increase of 40% in the DRI of Prada limestone at 600 \({}^{\circ}\)C, with no significant differences between cooling methods for the studied range of temperatures. Research is scarce on the variation of DRI with temperature, and so results cannot be compared with other lithologies. Research is scarce on the variation of DRI with temperature and so results cannot be compared with other existing experiences. Methodologies and variables considered in the existing research on thermally assisted drilling differ greatly from those presented in our research. [PERSON] et al. (2018) reported a drop of 30% in the UCS of sandstone and granite for temperatures at 600 \({}^{\circ}\)C when using a flame jet to achieve high local heating rates, and UCS is strongly related to the drillability of the rock (2018; 2019; 2020). Although such a decrease in drillability is in the same range as that observed in our research, the lithologies and heating rates (20 \({}^{\circ}\)C/s) are very different. [PERSON] et al. (2018) applied high powered laser technology and performed scratch tests to indirectly measure reductions in rock strength, drilling strength, and fracture toughness at rates of 60% in granite and 30% in sandstone, but such results cannot be directly compared to the DRI reduction observed in our research, because the achieved temperatures and applied heating rates cannot be deduced. A decrease in the DRI with temperature for water-cooled samples coincided with a marginal porosity and a volume decrease at 200 \({}^{\circ}\)C (2019). This effect is related to the closure of pores and fissures by thermal dilation of calcite (2018; 2020) and is a factor that hinders penetration (2019). Thus, closure of pores and fissures is behind a decrease in thermal drilling performance at low temperatures for water-cooled samples. SJ and S\({}_{20}\) represent different effects in a rotary-percussive drilling process, since the impact action of the bit is expressed by S\({}_{20}\), whereas thrust and rotation match with the SJ value (2020), and so we study their respective variations with temperature separately. Temperature influence is remarkable because for the highest temperature of 600 \({}^{\circ}\)C, SJ tripled and S\({}_{20}\) doubled the initial values of the intact Prada limestone.