Quantitative description of discontinuities in rock masses - Rock Slope Engineering: Civil and Mining - page 385

Table II.2 Grain size scale

Description Grain size

Boulders 200-600 mm (7.9-23.6 in)

Cobbles 60-200 mm (2.4-7.9 in)

Coarse gravel 20-60 mm (0.8-0.24 in)

Medium gravel 6-20 mm (0.2-0.8 in)

Fine gravel 2-6 mm (0.1-0.2 in)

Coarse sand 0.6-2 mm (0.02-0.1 in)

Medium sand 0.2-0.6 mm (0.008-0.02 in)

Fine sand 0.06-0.2 mm (0.002-0.008 in)

Silt, clay < 0.06 mm ( < 0.002 in)

Table II.3 Classification of rock material strengths

Grade Description Field identification Approximate

compressive
(MPa)

Range of
strength
(psi)

R6 Extremely strong rock Specimen can only be chipped with

geological hammer.

> 250 > 36,000

R5 Very strong rock Specimen requires many blows of

geological hammer to fracture it.

100-250 15,000-36,000

R4 Strong rock Specimen requires more than one

blow with a geological hammer to
fracture it.

50-100 7000-15,000

R3 Medium weak rock Cannot be scraped or peeled with a

pocket knife; specimen can be
fractured with single firm blow of
geological hammer.

25-50 3500-7000

R2 Weak rock Can be peeled with a pocket knife;

shallow indentations made by firm
blow with point of geological
hammer.

5-25 725-3500

R1 Very weak rock Crumbles under firm blows with

point of geological hammer; can be
peeled by a pocket knife.

1-5 150-725

R0 Extremely weak rock Indented by thumbnail. 0.25-1 35-150

S6 Hard clay Indented with difficulty by

thumbnail.

> 0.5 > 70

S5 Very stiff clay Readily indented by thumbnail. 0.25-0.5 35-70

S4 Stiff clay Readily indented by thumb but

penetrated only with great
difficulty.

0.1-0.25 15-35

S3 Firm clay Can be penetrated several inches by

thumb with moderate effort.

0.05-0.1 7-15

S2 Soft clay Easily penetrated several inches by

thumb.

0.025-0.05 4-7

S1 Very soft clay Easily penetrated several inches by

fist.

< 0.025 < 4