Characteristics of Geologic Materials and Formations - Geotechnical Engineering Investigation - page 375

City Building Code (Table 5.22) provides a relatively comprehensive nomenclature, but its
applicability is geographically limited to the local rock types, which are not deeply weath-
ered. (Building Code, City of New York, 1986)

Simple Classification Systems
Early workers in rock mechanics developed systems to classify joints according to spac-
ing, as given in Table 5.23 , and rock quality based on RQD and the velocity index, as given
in Table 5.24.

TABLE 5.22
Allowable Bearing Pressures for Rock a

Rock Material Class

Characteristic Hard, Sound Medium Hard Intermediate Soft

Q alt , tsf b,c 60 40 20 8

Rock type Crystalline: gneiss, Same as hard rock Same as hard to All rocks and

diabase,schist, marble, medium-hard uncemented

serpentinite rock, and sandstones

cemented
sandstones and
shales

Struck with pick Rings Rings Dull sound Penetrates

or bar
Exposure to air or Does not disintegrate Does not Does not May soften

water disintegrate disintegrate

Fractures Sharp, fresh breaks Cracks slightly Show weathered Contain weathered

Appearance weathered surfaces zones

Width 3 mm 6 mm Weathered zone Weathered zone to 30

to 25 mm width cm width

Spacing 1 m 60 cm Weathered zone Weathered zone filled

spaced as close with stiff soil

as 30 cm

Core recovery d 85% 50% 35% 35%

SPT 50 blows/ft

a

After New York City Building Code, 1968.

b

Allowable bearing value applies only to massive crystalline rocks or to sedimentary or foliated rocks where
strata are level or nearly level, and if area has ample lateral support. Tilted strata and their relation to adja-
cent slopes require special consideration.

c

Allowable bearing for hard to intermediate rock applies to foundations bearing on sound rock. Values can be
increased by 10% for each 30 cm of penetration into rock of equal or better quality, but shall not exceed twice
the basic values.

d

Rock cored with double-tube, diamond core barrel, in 5 ft (1.5 m) run.

TABLE 5.23
Classification of Joints Based on Spacing a

Description Joint Spacing Rock-Mass Designation

Very wide 3 m Solid

Wide 1 to 3 m Massive

Moderately close 30 cm to 1 m Blocky/seamy

Close 5 to 30 cm Fractured

Very close 5 cm Crushed

a

From Deere, D.U., Rock Mech. Eng. Geol . 1, 18-22, 1963. With permission and Bieniawski, Z. T., Proceedings of
3rd International Congress on Rock Mechanics , Denver, CO, Vol. IIA, 27-32, 1974. With permission.