Environmental Engineering Reference
In-Depth Information
Average dispersion of
strength for most rocks
Rock density = 32 31
30
350
300
250
29
2
2
26
2
24
2
22
2
20
kN
±
200
±
8
±
100
200
±
60
150
±
40
10
90
80
70
60
50
40
30
±
20
20
Hammer vertical downwards
15
10
0
10
20 30
Schmidt hardness R,L hammer
40
50
60
FIGURE 3.40
Correlation chart for Schmidt L. hammer, relating rock density, uniaxial compressive strength, and rebound
number
R
(Schmidt hardness). Hammer vertical downward; dispersion limits defined for 75% confidence.
(
Note
: 100 MPa
6.3 pcf.) (From ISRM,
Rock Characterization
and Monitoring
, E. T. Brown, Ed., Pergomon Press, Oxford, 1981. With permission. After Deere, D. U. and
Miller, R. P., Technical Report AFWL-TR-65-116, AF Special Weapons Center, Kirtland Air Force Base, New
Mexico, 1966.)
14.5
10
3
psi
1021 tsf; 1 kN/m
2
Uniaxial Tensile Strength
Cable-Pull Test
Caps are attached to the ends of a cylindrical specimen with resins. The specimen is then
pulled apart by cables exerting tension axially (
Figure 3.37f)
.
The method yields the low-
est values for tensile strength, which generally ranges from 5 to 10% of the uniaxial com-
pression strength.
Point-Load Test (Broch and Franklin, 1972) (ASTM D5731-95)
Compressive loads
P
are applied through hardened conical points to diametrically oppo-
site sides of a core specimen of length of at least 1.4
D
until failure occurs. The equipment
is light and portable (
Figure 3.41)
and is used in the field and the laboratory.
Point-load index
is the strength factor obtained from the test, and is given by the empiri-
cal expression (Hoek and Bray, 1977)
I
s
P
/
D
2
(3.42)
where
D
is the diameter.
