Site Investigation - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 381

Discontinuity roughness
Roughness profiles of discontinuities can be recorded in two dimensions (linear pro-
filing) and in three dimensions (planar profiling). They may be relevant for the shear
strength in the direction of potential sliding. In many cases the relevant direction is the
dip direction. However, in cases where sliding is controlled for example by two intersect-
ing discontinuity surfaces the direction of potential sliding may be parallel to the line
of intersection of both discontinuities which does not coincide with the dip direction of
either of these discontinuities.
Linear roughness profiles can be recorded using a folding rule, which should be in con-
tact to the highest point or points of the discontinuity. The measurement of roughness
amplitudes (z' direction) should be undertaken at small intervals (x' direction) and the
profile containing x' and z' readings should be plotted to the same scale (ISRM 1978e).
In Fig. 13.24 the procedure for measuring a linear roughness profile of a discontinuity
D1 is illustrated (Wittke 1990). In Section A-A parallel to the estimated direction of po-
tential sliding, the dip angle β 1 of discontinuity D1 appears as the so-called “apparent

dip” β a1 . The relationship between β and the apparent dip of a discontinuity β a is given

in general form by (Wittke 1990)

tan β a = tan β ⋅ sin Δα (13.3)

where is the angle between the discontinuity's strike angle and the vertical section

parallel to the relevant direction of potential sliding. In Fig. 13.24 this angle (

Δα

Δα 1,3 ) is
the difference of strike angles of the discontinuities D1 and D3, the latter being parallel
to the relevant direction of potential sliding (Section A-A in Fig. 13.24).
Rengers (1970) described a method for recording linear roughness profiles up to
2 m in length, using a so-called “profilograph”. A faster field method for record-
ing linear roughness profiles using a contour gauge and pattern marker is de-
scribed in Stimpson (1982).

Planar roughness profiles of different sizes can be recorded with the aid of a Clar com-
pass and circular disks according to a method described in Fecker & Rengers (1971)
and ISRM (1978e).
Roughness profiles can also be established by digitizing images of rock surfaces
obtained from photographs or videos (Maerz et al. 1987, Franklin et al. 1988,
Maerz et al. 1990).
Another method for obtaining linear and planar roughness profiles is the sampling of
natural discontinuity surfaces or casts of them in the field and to digitize them in the
laboratory using a microscope (Durham & Bonner 1993).