Rock Mass Strength Characterisation from Field and Laboratory: A Comparative Study on Carbonate Rocks from the Larzac Plateau

The southern edge of the Larzac carbonate plateau in Occitanie (France) is subject to various landslide processes from rock falls, toppling, and roto-translational slides to rock spreading. To constrain the strength of the carbonate rock mass involved, field and laboratory approaches have been employed. The first approach involves field investigations using the Rock Mass Rating method and the Geological Strength Index, as well as estimations of uniaxial compressive strength conducted with the Schmidt hammer. The second approach consists of various laboratory tests under uniaxial compressive and triaxial stress. A comparison of mechanical parameters obtained from the field and laboratory approaches for characterising the rock mass (cohesion, friction angle and Young’s modulus) illustrates that laboratory measurements provide much higher (70%) cohesion values than field measurements. The friction angles derived from the field investigations are also weaker (20%). Such differences are considered the result of a scale effect, which is less evident for the Young’s modulus. The field approach appears to be more representative of the in situ rock mass strength as it characterises the rock mass by considering both the intact rock and discontinuities.