Page 116 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

86 Engineering Rock Mass Classification

TABLE 8.1 Rock Quality Designation

Condition RQD

A Very poor 0–25

B Poor 25–50

C Fair 50–75

D Good 75–90

E Excellent 90–100

Where RQD is reported or measured as 10
(including 0), a nominal value of 10 is used to
evaluate Q in Eq. (8.1a). RQD intervals of 5, such
as 100, 95, 90, etc., are sufficiently accurate.

Source: Barton et al., 1974.

Joint Set Number (Jn)

The parameter Jn, representing the number of joint sets, is often affected by foliations,
schistocity, slaty cleavages or beddings, and so forth. If strongly developed, these parallel
discontinuities should be counted as a complete joint set. If there are few joints visible or
only occasional breaks in rock core due to these features, then they should be counted as
“a random joint set” while evaluating Jn from Table 8.2. Rating of Jn is approximately
equal to square of the number of joint sets.

TABLE 8.2 Joint Set Number (Jn)

Condition Jn
A Massive, no or few joints 0.5–1.0

B One joint set 2

C One joint set plus random 3

D Two joint sets 4

E Two joint sets plus random 6

F Three joint sets 9

G Three joint sets plus 12
random

H Four or more joint sets, 15
random, heavily jointed,
“sugar cube,” etc.

J Crushed rock, earth-like 20

For intersections use (3.0 Á Jn). For portals use (2.0 Á Jn).
Source: Barton et al., 1974.