## 2 Materials and Methods  
Prada limestone is a Lower Cretaceous formation located in the Serra de Prada, a range of mountains in the southern Pyrenees (Lleida province, Spain). Rock samples were taken from two horizontal boreholes drilled during the design stage of the Tres Ponts Tunnel, which is planned to be entirely excavated from Prada limestone in the municipalities of Organya and Figols, close to a narrowing of the Segre river as it passes next to the Serra de Prada, in an area known as the Congost de Tres Ponts. The tunnel will be oriented north-south on the C-13 road, measures 1273 m in length, and its maximum depth from the ground surface will be of 285 m. Figure 1 shows the area of study including the Serra de Prada, Congost de Tres Ponts, the Tres Ponts Tunnel, and the position and spatial coordinates of the two horizontal boreholes. Samples were very homogeneous and only presented changes in the grey tone and very thin veins of calcite (Figure 2). The effects of high temperatures in textural, physical, and mechanical features from the Prada formation were described in previous research [27]. A dark grey fraction from Prada limestone exhibited an increased thermal damage and explosive behaviour when heated to above 400 \({}^{\circ}\)C, and this is related to an increase in the pore pressure caused by SO\({}_{2}\) released during the thermal oxidation of pyrites [23]. Due to such different effects produced by the thermal treatment, the dark grey fraction was separated from the rest of samples and was not considered in this research. Optical microphotographs (Figure 3) enabled Prada limestone to be identified as a grainstone or biosparite with abundant bioclasts, where cement is abundant and mainly consists of crystalline mosaics of calcite spar that fill the interparticle porosity. Micritic matrix is minor and irregularly distributed, and discontinuities are abundant and consist of fissures, calcite veins, and a small number of stylolites. Some angular and sub-angular grains of monocrystalline quartz, and sub-rounded grains of iron sulphides dispersed in the rock matrix appeared in a minor proportion. Figure 1: Location of the Tres Ponts Tunnel in the Congost de Tresponts area. Borehole coordinates are expressed in meters in the UTM 31N/ETRS 89 reference system. Image modified from Institut Cartografic i Geologic de Catalunya (www.iccc.cat accessed on 25 February 2021). Figure 2: Changes in the grey tone and very thin veins of calcite observed in the intact samples. A dark grey texture was separated from the rest of samples and does not form part of this research. A temperature of 105 \({}^{\circ}\)C was applied to a total of 110 samples to remove moisture and these are considered references for the determination of intact rock properties. The average initial values for physical and mechanical properties of the intact rock are summarised in Table 1. Subsequently, five groups of 20 samples were separated and heated in an electric furnace at a slow rate (a gradient of 5 \({}^{\circ}\)C/s was applied) to target temperatures of 200, 300, 400, 500, and 600 \({}^{\circ}\)C. Target temperatures were then maintained for one hour. Heated specimens at each target temperature were then separated into two groups of five samples and cooled by one of two methods: (i) at a slow rate to room temperature of 21 \({}^{\circ}\)C; or (II) by quenching through water immersion, according to the procedure described by [PERSON] et al. [18]. Temperatures inside the furnace were monitored with a PicLog 6 data logger. Figure 4 illustrates the number of laboratory tests performed for each temperature and the number of samples used in this research methodology. Optical and scanning electron microscopy (SEM) in backscattered electron mode was used to study the petrographic features of representative samples from Prada limestone. Sample surfaces were polished with alumina and diamond powder; the finest abrasive