Engineering Rock Mechanics

1 24 Discontinuities

Discontinuity set I parallel to plane BCD Number

"2 "3 " " '. .' " " of SetS N : 1 Discontinuity 2 4

"4 " '. .. ABC --.

. , .. .. I-
, , .. ,.
,,, 0' /
Loci of
discontinuity frequency A,
I, =h2=h,=h4= 1.0 2-D case : Polar plots of A,
(Vertical)

N-x

/ /-.

'I {O) \

\

'--/

Random geometry

As 4 2h

270' 90'

3-D case : Equal area projections contoured for As As -+ 0.5

Figure 7.10 Variation in discontinuity frequency for a sampling line passing

through multiple sets of symmetrically orientated discontinuities-two- and three-

dimensional cases illustrated via polar and lower-hemispherical projection plots.

180"

of sets results in a 'peneplain'. The variation for one, two and three

discontinuity sets (which are perpendicular and have equal frequencies) is

represented by the three-dimensional polar plots shown as isometric

sketches in Fig. 7.11. The format of this diagram is a 3 x 3 matrix with the

individual set loci along the leading diagonal and the binary combinations

of sets shown in the off-diagonal positions. The principle of this type

of presentation is explained in Chapter 14. The shaded areas in each

locus are equivalent to the 'horizontal' two-dimensional loci shown in

Figs 7.9 and 7.10.

Because of this variation in discontinuity frequency with direction, the

RQD will also vary with direction, bearing in mind the relations given in

Section 7.2.2. So, a statement such as 'the rock mass has an RQD of 80%'

is inadequate: an RQD value can only apply to measurements made in a

specific direction. In fact, the RQD is a vector-like quantity with both

magnitude and orientation, directly calculable fom the discontinuity

frequency. The variation with direction is particularly relevant to site

investigation results containing the ubiquitous parameter RQD with the

possibility that the RQD measured in a vertical borehole has been applied

to, say, a horizontal tunnel.

7.2.4 Discontinuity orientation, discontinuity sets and

block sizes

If we assume that a discontinuity is a planar feature, then its orientation

can be uniquely defined by two parameters: dip direction and dip angle.