Environmental Engineering Reference
In-Depth Information
FIGURE 3.41
Point load strength test apparatus.
Values for
I
s
are used to estimate
U
c
through various correlations as shown in Figure
3.42, and, for a core diameter of 50 mm,
U
c
24
I
s
(3.43)
Flexural Strength or Modulus of Rupture
Procedure
A rock beam is supported at both ends and loaded at midpoint until failure (
Figure 3.37h)
.
Data Obtained
The flexural strength is proportional to the tensile strength but is about three times as great
(Leet, 1960).
Uniaxial compressive strength U
c
, psi x 10
3
30
0
10
20
40
50
2500 3000 kg/cm
2
U
c
500
1000
1500
2000
20
200
kg/cm
2
P
Point-load strength
index = I
s
= P/D
2
L
15
150
Core
D
2000
L = 0.7 D(min)
Strong
Norite
Experimental
point range
10
100
Weak
Norite
1000
Quartzite
5
50
I
s
U
c
= 24 I
s
(for NX core)
500
Sandstone
U
c
50
100
150
200
250
300
MP
a
FIGURE 3.42
Relationship between point load strength index
I
s
and uniaxial compressive strength
U
c
. (After Bieniawski, Z.
T.,
Proceedings 3rd International Congress for Rock Mechanics,
International Society for Rock Mechanics, Vol. IIA,
Denver, 1974, pp. 27-32. Reprinted with permission of National Academy Press.)
