Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 396

Rolofs (1981b) and Beyer (1982). In more recent literature, block size distributions gen-
erated by computer simulations based on measured discontinuity spacing values and
by stochastically generated discontinuities are used (Mathis 1988, Maerz 1990, Peaker
1990, Maerz & Germain 1996, Kalenchuk et al. 2006, Rafiee & Vinches 2008).
Block size distributions are also needed for rock mass classification, since block volume
serves as an input parameter of classification systems (Section 12.4). However, the use
of this cannot be recommended, as substantiated in Sections 12.5 to 12.7.
Discontinuity roughness
In order to describe recorded roughness profiles, the nomenclature suggested by the
ISRM (1978e) may be used (see Section 2.7.2). Quantitative mathematical descrip-
tions of roughness profiles using JRC values, statistical parameters and fractals are
suggested in a large number of papers (Wu & Ali 1978, Krahn & Morgenstern 1979,
Tse & Cruden 1979, Dight & Chiu 1981, Brown & Scholz 1985, Lee et al. 1990, Maerz
et al. 1990, Power & Tullis 1991, Yu & Vayssade 1991, Sakellariou et al. 1991, Huang
et al. 1992, Odling 1994, Den Outer et al. 1995, Kulatilake et al. 1997, Shirono &
Kulatilake 1997, Kulatilake & Um 1998, Kulatilake & Um 1999, Xie et al. 1999, Yang
& Di 2001, Grasseli et al. 2002).
Some have also attempted to describe the shear strength of discontinuities on the basis
of recorded roughness profiles (Barton & Choubey 1977, Dight & Chiu 1981, Peek
1981, McWilliams et al. 1993, Kulatilake et al. 1995). However, from the author's point
of view the practical value of such approaches are limited given the uncertainties that
are associated with the determination of JRC values (Hsiung et al. 1993). At best,
roughness profiles allow general statements on the expected shear behavior of disconti-
nuities: undulating surfaces can indicate dilatant behavior during shearing, and rough
surfaces have larger friction angles than smooth or slickensided surfaces.
Discontinuity aperture
Because of their importance with regard to permeability (Chapter 6) various methods
for measuring discontinuity apertures in the field and in the laboratory have been de-
veloped (Section 13.9.1). It should be kept in mind, however, that apertures visible on a
rock exposure are often disturbed due to weathering or due to rock excavation (ISRM
1978e). In addition, the amount of measured apertures or aperture profiles cannot be
statistically representative for a discontinuity set or even for a single discontinuity.
In the author's opinion statistical evaluation of measured apertures or aperture
profiles does not lead to a confident assessment of discontinuity apertures and thus
of permeability. The interpretation of in-situ permeability tests (Section 15.8) is
therefore believed to be a more reliable indicator for rock mass permeability than
aperture measurements.