Rock mass properties for tunnelling

Estimating the strength and deformation characteristics of a rock mass for tunnel design is generally based on empiricism as it is simply not practical to test the rock mass at a scale large enough. However, the database from which empirical methods are derived and against which analytical methods are tested, is rather limited in terms of large rock masses. This research contributes to estimating rock mass properties for tunnel design in the following ways.• Collates strength and deformation data from case histories of large scale rock masses:o tunnels in Hawkesbury Sandstoneo underground coal pillars from Australia, South Africa, the USA and India o underground mine pillars in igneous, metamorphic and sedimentary rocks• Uses the collated databases to appraise existing:o intact rock failure criteriao means to estimate defect stiffnesso rock mass classification systemso rock mass strength criteriao means to estimate rock mass modulus • Recommends robust methods that the tunnel designer can use to estimate rock mass strength and modulus:o New relationships between the Geological Strength Index and the Hoek-Brown criterion parameters are giveno An extended Damage Initiation Spalling Limit approach is shown to predict the strength of Hawkesbury Sandstone and of coal pillars very wellThe development of a technique for back-analysing the in situ stress ratio and the modulus of anisotropic rock masses from ordinary displacement data recorded during underground excavation