ROCK MECHANICS 445 Figure The Hoek Brown criterion for fractured rock masses, with typical values of the strength constants m and s for various qualities of different rock types Figure Distribution of hydraulic head in a two dimensional fracture network invoked in the determination of in situ stress, characterization of intact rock, and analysis of stresses induced around underground excavations The DIANE nature of real rock masses means that the effects of fractures, variability with respect to location and orientation, and the fact that stress changes will usually lead to irreversible deformations in the material must always be considered A further important consequence of the DIANE nature of rock masses is the scale effect In general, because of the presence of fractures, it is found that large volumes of rock and small specimens of intact rock have different properties This means that in situ rock mass strength and stiffness will be much lower than that determined in the laboratory Similarly, the large-scale permeability of a fractured rock mass is very likely to be much greater than that of a specimen of intact rock The scale effect is thought to apply to other properties, such as in situ stress, but in these cases the consequence of it is less predictable For example, under