## 1 Introduction
[17] reported an increase in open porosity, as well as water absorption and reduction in mechanical properties when heating limestones up to 400 \({}^{\circ}\)C, as a consequence of the anisotropic thermal deformation of calcite crystals. [PERSON] et al. [18] reported an increased variation in the physical and mechanical features of carbonate rocks when cooled by water immersion. [PERSON] and [PERSON] [19] observed a clear reduction in uniaxial compression strength (UCS) from 500 \({}^{\circ}\)C on calcareous and dolomitic rocks from Apulia in Italy, with temperatures above 600 \({}^{\circ}\)C usually marking a dramatic decline in UCS [20; 21] [PERSON] et al. [22] explored colourimetry to determine thermal damage in buildings and described a trend of limestone to redden (later confirmed in Prada limestone and related to oxidation of iron compounds) [23]. Natural limestone becomes lighter in appearance with increasing temperature [23; 24]. [PERSON] et al. [25] determined that from 200 to 500 \({}^{\circ}\)C porosity and pore size rapidly increase, and from 500 to 600 \({}^{\circ}\)C UCS, elastic modulus, Poisson's ratio, and hardness decreased. Later, [PERSON] and [PERSON] [26] described a strong relationship between mineral content and thermal damage in limestones (China). [PERSON] et al. [23] described a significant contribution of the thermal oxidation of pyrites in the explosive behaviour and thermo-chemical damage of Prada limestone from 400 \({}^{\circ}\)C. [PERSON] et al. [27] identified micro-structural changes produced by high temperatures and cooling methods in Prada limestone, and related them with severe variations in the physical and mechanical features of this type of rock. Such dramatic changes are mainly explained by the anisotropic expansion of calcite [15; 28], and by other physicochemical processes such as the mismatch in thermal expansion coefficients between mineral particles [29] and the quartz phase transition [30; 31]. The greater thermal damage in water-cooled samples is due to tensile stresses that nucleate cracks [32]. Current drilling methods are based on mechanical abrasion, and this produces substantial drill bit wearing and low rates of penetration in hard rocks, resulting in high drilling costs [33; 34]. Therefore, researchers aim to improve drilling performances using emerging drilling technologies, and one alternative approach is to thermally assist conventional rotary drilling by heating the rock. [PERSON] et al. [35] explored the feasibility of thermally assisted drilling using a flame jet to achieve high local heating rates, and determined a drop of 30% in UCS for temperatures up to 600 \({}^{\circ}\)C. [PERSON] et al. [36] used high powered laser technology to decrease rock strength, drilling strength, and fracture toughness at rates of 60% in granite and 30% in sandstone. [PERSON] et al. [37] studied a combined thermo-mechanical drilling (CTMD) using a flame jet and stated that the thermal treatment of rocks causes extensive thermally induced cracks in granite and sandstone, which significantly enhances the penetration performance of cutting tools. Later, [PERSON] et al. [38; 39; 40] implemented this technology in the field and demonstrated an increase in the removal performance in hard rocks by up to a factor of three when compared to conventional drilling methods, and concluded that integration of thermal assistance to conventional rotary drilling constitutes an interesting approach to facilitate the drilling process. DRI [41] is among the most used testing methods to determine the drillability characteristics of rock. NTNU/SINTEF registered as trademark the DRI test [42], which is assessed on the basis of two laboratory tests, the brittleness value (S\({}_{20}\)) test [43] and Sievers' J-value (SJ) miniature drill test [44]. In this study, SJ and S\({}_{20}\) tests were performed on thermally treated samples from two boreholes drilled during the design stage of the Tres Ponts Tunnel in Prada limestone to determine drilling rate index (DRI) variation with temperature.