Environmental Engineering Reference
In-Depth Information
TABLE 5.19
Engineering Classification of Intact Rock
a
(A) On the Basis of Uniaxial Compressive Strength
Class
Strength
Uniaxial
Point-Load
Rock Type
Compression (tsf)
Index
b
A
Very high
2200
95
Quartzite, diabase, denase
basalts
B
High
1100-2200
50-95
Majority of igneous rocks,
stronger metamorphics, well-
cemented sandstone, hard
shales, limestones, dolomites
C
Medium
550-1100
25-50
Shales, porous sandstones,
limestones, schistose
metamorphic rocks
D
Low
275-550
13-25
Porous and low-density rocks,
friable sandstones, clay-shales,
chalk, halite, and all altered
rocks
E
Very low
275
13
As for class D
(B) On the Basis of Modulus Ratio
c
Class
Modulus ratio
Value
Rock Fabric
H
High
500
Steeply dipping schistosity or foliation
M
Medium (average)
200-500
Interlocking fabric, little or no schistosity
L
Low
200
Closure of foliations or bedding planes affect deformation
a
After Deere, D.U.,
Rock Mechanics in Engineering Practice
, Stagg and Zienkiewiez, Eds., Wiley, New York, 1969.
Reprinted with permission of Wiley.
b
Point-load index values from Hock, E. and Bray, J.W.,
Rock Slope Engineering,
Inst. of Mining and Metallurgy,
London, 1974. With permission.
c
Modulus ratio: Defined as the ratio of the tangent modulus at 50% ultimate strength to the uniaxial compres-
sive strength (see
Figure 3.70).
Deere (1969) proposed an engineering classification system based on strength, which he
related to rock type (Table 5.19a), and the modulus ratio, which he related to rock fabric
(Table 5.19b), providing for anisotropic conditions caused by foliations, schistose, and bed-
ding. It is to be noted that very low strength rock is given as less than 275 tsf, which covers
the category of rocks composed of soft minerals as well as those in the decomposed state.
Suggested Classification Systems
Hardness classifications based on simple field tests and related to uniaxial compressive
strength ranges are given in
Table 5.20.
Weathering grade, class, and diagnostic features are given in
Table 5.21.
Rock-Mass Classification
General
Historically, rock-mass classification has been based on percent core recovery, which is
severely limited in value (see
Section 2.4.5).
Core recovery depends on many factors
including equipment used, operational techniques, and rock quality, and provides no
direct information on hardness, weathering, and defects. Even good core recovery cannot
