TM 5-818-1 / AFM 88-3, Chap. 7
Table 2- 7. Engineering Classification of Intact Rock
On basis of strengh, σult :
a
I.
Class
Description
Uniaxial Compressive
A
Very high strength
Over 32,000
B
High strength
16,000 - 32,000
C
Medium strength
8,000 - 16,000
D
Low strength
4,000 - 8,000
E
Very low strength
Less than 4,000
On basis of modulus ratio, Et / σult :
a
II.
Class
Description
Uniaxial Compressive
H
High modulus ratio
Over 500
M
Average (medium) ratio
200-500
L
Low modulus ratio
Less than 200
a
Rocks are classified by both strength and modulus ratio, such as AM, BL, BH, and CM.
Modulus ratio = Et/σult, where Et = tangent modulus at 50 percent ultimate strength and σult = uniaxial compressive
b
strength.
(Courtesy of K. G. Stagg and O. C. Zienkiewiez, Rock
Mechanics in Engineering Practice, 1968, pp 4-5.
Reprinted by permission of John Wiley & Sons, Inc, New
York.)
and/or cemented clay particles. It is usually laminated
good rock. Compaction shales, being of an interme- diate
from the general parallel orientation of the clay particles
quality, will generally soften and expand upon exposure
as distinct from claystone, siltstone, or mudstone, which
to weathering when excavations are opened.
c. Dry unit weight of shale may range from
are indurated deposits of random particle orientation. The
about 80 pounds per cubic foot for poor-quality
terms "argillaceous rock" and "mudrock" are also used to
compaction shale to 160 pounds per cubic foot for high-
describe this type of rock. Shale is the predominate
quality cemented shale. Shale may have the appearance
sedimentary rock in the earth's crust.
b. Shale may be grouped as compaction
of sound rock on excavation but will often deteriorate,
during or after placement in a fill, into weak clay or silt, of
shale, and cemented (rock) shale. Compaction shale is a
low shear strength. Figure 2-5 may be used as a guide in
transition material from soil to rock and can be excavated
classifying shale for foundation use.
with usual large excavation equipment. Cemented shale
d. Compaction shales may swell for years after
generally requires blasting. Compaction shales have
a structure is completed and require special studies
been formed by consolidation pressure and very little
whenever found in subgrade or excavated slopes. The
cementing action. Cemented shales are formed by a
predicted behavior of shales cannot be based sofely
combination of cementing and consolidation pressure.
upon laboratory tests and must recognize local
They tend to ring when struck by a hammer, do not slake
experiences.
in water, and have the general characteristics of
2-14
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