Experience gained from the most relevant fire catastrophes on road tunnels, and research on the effect of high temperatures on rock, show that heating causes the weakening of rock and this implies great risk for people and infrastructures. Finding indirect methods for evaluating the decrease in strength induced by temperature in intact rock could facilitate damage analysis and reparation phases after a tunnel fire. The research developed in this work involves laboratory tests carried out on samples of limestone from the south zone of the Catalan Pyrenees (Spain). The samples have been heated to different temperatures under laboratory conditions. Changes in their mass, volume, dry density, and P-wave attenuation have been evaluated to find relationships with the variation in uniaxial compressive strength. Simple regression functions have been selected, as they are easy to use, and provide quick indicative values that could be critical after a fire in a tunnel. The influence in the precision of the data obtained in laboratory tests for the various correlations has been also studied. Variation of dry density and P-wave attenuation predict strength variation values lower than those experimentally observed. Predictions using dry density variation appear to be sensitive to the precision of the data obtained in the laboratory tests. The variation of open porosity offers the best correlation among those considered in this research. The evaluation of open porosity involves a simple laboratory procedure, and simple potential regression enables quickly computing indicative values to evaluate the degree of damage to this specific type of rock when heated to high temperatures.
Experience gained from the most relevant fire catastrophes in road tunnels, such as the Montblanc in France and Tauern in Austria (Leitner, 2001), and in full-scale tests (Keski-Rahkonen et al., 1986), enable us to determine that high temperature causes a weakening of rock structures in tunnels and the fall of rock blocks, which poses a great risk for people and infrastructures. The effect of high temperature on rocks has been studied by several authors. Focused specifically on tunnel safety, Smith and Pells (2008) and Smith and Pells (2009) centred their research on sandstone tunnels in Australia. Nordlund etal. (2014) focussed on igneous rocks in Sweden. Furthermore, full scale tests were performed by Keski-Rahkonen et al., (1986). In the case of limestones, a severe variation in rock strength was detected above 400-500°C by González-Gómez et al. (2015). A temperature of 600°C is set as the upper limit in most cases, as it is related to a severe decrease in uniaxial compressive strength values (Sengun, 2014).