Rock Engineering and Rock Mechanics: Structures in and on Rock Masses – Alejano, Perucho, Olalla & Jiménez (Eds) © 2014Taylor & Francis Group, London, 978-1-138-00149-7 Identification of the dependency to water content of the mechanical properties of Channel tunnel chalk S. Maiolino ERA 29 – Risques Géologiques et Géotechniques, Département Laboratoire de Lyon/Risques Rocheux Mouvements de Sol, CEREMA/DirectionTerritoriale Centre Est, Bron, France ABSTRACT: In this paper, we present the rock mechanics tests performed at Laboratory of Lyons of the CEREMA (known at the time as CETE de Lyon), following the fire of September 2008 in che Channel tunnel. Whereas chalk marl was unaffected by the fire even in the most damaged zone, several laboratory test were performed, during the shutdown of the north tunnel. At the request of Eurotunnel society, we determined that the rock behavior was characterized by an extreme sensitivity to water content: • Young Modulus and Uniaxial Compressive are function of water content , with the same power law. • Unless conserved in a fully saturated atmosphere, chalk marl of the Channel tunnel loses water at an important hourly rate and is totally dry in a few days. This experience was useful for the SAFE project: in the first major modification to the Tunnel infrastructure since it was built, 4 SAFE fire-fighting stations were built. They became operational at the end of 2011. Those stations permit now to contain a major fire. The results of rock mechanics tests performed on samples were conform to the observation made a few months before, in a zone characterized by a higher water content. 1 INTRODUCTION On the 11th of September 2008 a fire broke out on a HGV (Heavy GoodsVehicle) carried by a shuttle com- ing from the UK side through the North tunnel. The fire lasted 20 hours. Civil works damage investigation were performed in a very short period of time on the lining (Delga, Lévy, Ducroq, & Huchon 2010). Most of the investigation concern the concrete lining, as the rock mass was unaffected by fire or its aftermath, as no lining collapse occurred even in the most damage zone. 2 CALENDAR OF EVENTS OF THE EXPERIMENTAL CAMPAIGN Main constraints were linked to time. Rock mechanics test were performed, for reasons of time and avail- ability, both at École des Mines de Paris and CETE de Lyon. 10 cores were sent at Lyon. Samples were to be tested from both extremities. As the cores were extracted from the same place of the tunnel, all the tests were supposed to be rather homogeneous. Tests were to be performed in a really short time. For reasons of availability, two series of ten tests were scheduled. An incident occurred on site, so the samples des- tined to Lyon were cored and sent a few days later. Figure 1. Places where grouted mortar (even grey) or chalk (light grey) can be seen (Delga, Lévy, Ducroq, & Huchon 2010), after removal of damaged concrete by hydro-demolition. By the end of November, the first serie was completed. Comparisons with results from the École des Mines show important variations for the Uniaxial Compres- sive Strength(UCS) andYoung modulus (E). The ratio was roughly 4:1, so that it couldn’t be explained by normal variations, due to the difference of location of the samples. 229