Micro and macro-structural effects of high temperatures in Prada limestone: Key factors for future fire-intervention protocols in Tres Ponts Tunnel (Spain)

Abstract

Temperature and cooling methods strongly affect the stability of tunnels drilled in rock masses and so condition interventions for fire emergencies. Samples from two horizontal boreholes drilled in 'Prada' limestone during the design stage of the Tres Ponts Tunnel in the Catalan south Pyrenean zone (Spain) are heated from 105 to 600º C, and subsequently cooled with air or water to simulate fire extinguishing interventions. Changes in chemical composition and microstructure, physical properties (open porosity, volume, dry total weight, P and S-wave velocity), and mechanical properties (uniaxial compressive strength, elastic modulus, and Poisson's ratio) are analysed. Rock weakening is observed even at low temperatures (T<300º C). The influence of the cooling method appeared at intermediate temperatures of 300-400º C and open porosity, P and S-wave velocities, and elastic modulus show greater variation for water-cooled samples. A temperature of 500º C is of paramount importance for 'Prada' limestone as it leads to a dramatic increase in porosity and a notable decrease in P and S-wave velocities under both cooling methods. Trans-granular micro-crack progression is observed at 500º C using scanning electron microscope (SEM), and water-cooled samples show a greater loss in uniaxial compressive strength (UCS), and this is due to micro-crack connections and growing fissures. Thermal damage at 600º C is also greater when samples are quickly cooled. An ANOVA and a simple regression analysis are performed to discard the influence of the natural location of the borehole samples in the obtained experimental results. Finally, correlations to predict UCS and elastic modulus from volume, open porosity, and P-wave velocity after thermal treatment are proposed using simple exponential and potential functions to help make preliminary decisions after a tunnel fire. These predictive results on the effects of fire on Prada limestone will be considered for the definition of future fire intervention protocols in the Tres Ponts Tunnel.

Thermal Effects on the Drilling Performance of a Limestone: Relationships with Physical and Mechanical Properties

“…We attribute this effect to the increase of porosity and the propagation and coalescence of micro fissures due to thermal treatment. That is noticeable in the case of S 20 , where the trend is consistent with microstructural changes due to thermal treatment [27]. Incipient transgranular fissures and porosity developed when heated to 400 • C match a gentle increase in S 20 .…”

“…Variations in the micro-and macro-structural features of Prada limestone with temperature were already documented in detail in previous research [27], where SEM and MIP analyses refer to thermal treatment as a cause for dramatic micro-structural changes in Prada limestone in terms of porosity and micro-crack growth and coalescence. New SEM performed in this study confirms the presence of trans-granular fissures and porosity when heated to 400 • C (Figure 7a), and well-formed and connected fissures developed at 600 • C (Figure 7b), both in water-cooled samples.…”

“…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 • C, and this is related to an increase in the pore pressure caused by SO 2 released during the thermal oxidation of pyrites [23].…”

“…In other words, since DRI and UCS tests are destructive, for each temperature they are performed using different fragments (whose values are averaged and then correlated). Thus, regression curves are calculated using the mean values of each property determined at each temperature in line with other authors [10,27,50] since the pairs of correlated values of each sample that define the scatter cannot be considered for adjusting such functions. Different correlation functions exist between SJ, S 20 , and other physicomechanical parameters of the intact rock, but they do not refer to thermally treated samples.…”

“…Later, Zhang and Lv [26] described a strong relationship between mineral content and thermal damage in limestones (China). Martínez-Ibáñez 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 • C. Martínez-Ibáñez 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].…”