Page 261 - Introduction to Mineral Exploration

244 M.K.G. WHATELEY & B. SCOTT

TABLE 10.8 A summary of the description and classification of rock masses for engineering purposes. (Adapted
from Engineering Group Working Party 1977, Brown 1981.)
Descriptive indices of rock material
Consideration should be given to rock type, color, grain size, texture and fabric, weathering (condition),
alteration, and strength

Rock condition (weathering)
Fresh No visible signs of rock material weathering
Slightly weathered Discoloration indicates weathering of rock material and discontinuity surfaces
Weathered Half the rock material is decomposed or disaggregated. All rock material is
discolored or stained
Highly weathered More than half the rock material is decomposed or disaggregated. Discolored rock
material is present as a discontinuous framework. Shows severe loss of strength
and can be excavated with a hammer
Completely weathered Most rock material is decomposed or disaggregrated to a soil with only fragments
of rock remaining

Rock material strength
Extremely strong Rock can only be chipped with geological hammer. Approximate unconfined
compressive strength >250 MPa
Very strong Very hard rock which breaks with difficulty. Rock rings under hammer.
Approximate unconfined compressive strenth 100–250 MPa
Strong Hard rock. Hand specimen breaks with firm blows of a hammer, 50–100 MPa
Medium Specimen cannot be scraped or peeled with a strong pocket knife, but can be
fractured with a single firm blow of a hammer, 25–50 MPa
Weak Soft rock which can be peeled by a pocket knife with difficulty, and indentations
made up to 5 mm with sharp end of hammer, 5–25 MPa
Very weak Very soft rock. Rock is friable, can be peeled with a knife and can be broken by
finger pressure, 1–5.0 Mpa

Descriptive indices of discontinuities
Consideration should be given to type, number of discontinuities, location and orientation, frequency of
spacing between discontinuities, separation or aperture of discontinuity surfaces, persistence and extent,
infilling, and the nature of the surfaces
Rock quality designation
Sound RQD >100 Rock material has no joints or cracks. Size range >100 cm
Fissured RQD 90–100 Rock material has random joints. Cores break along these joints. Lightly broken.
Size range 30–100 cm
Jointed RQD 50–90 Rock material consists of intact rock fragments separated from each other by
joints. Broken. Size range 10–30 cm
Fractured RQD 1–50 Rock fragments separated by very close joints. Core lengths less than twice NX
(Table 10.7) core diameter. Very broken. Size range 2.5–10 cm
Shattered RQD <1 Rock material is of gravel size or smaller. Extremely broken. Size range less than
2.5 cm
Length of core in pieces 10 cm and longer
RQD in per cent = × 100
Length of core run
RQD diagnostic description
>90% Excellent
75–90 Good
50–75 Fair
25–50 Poor
<25% Very poor