The mechanical behaviour of discontinuities in rock, such as joints, is kno
wn to be size-dependent. It is also suspected that the behaviour of larger
size features, such as faults, is also size-dependent. This size dependence
has serious implications for performing numerical response simulations of
geological media. In this paper, we develop a new mathematical theory for s
caling of one particular discontinuity property, namely the interface norma
l stiffness. To accomplish this, we idealize an interface to have fractal g
eometry, and we develop analytical relations which show that the interface
normal stiffness, which is commonly thought to be a size-independent proper
ty, is in fact a size-dependent property and has fractal characteristics th
at may be exploited to develop a fundamental theory for scaling. Copyright
(C) 2001 John Wiley & Sons, Ltd.