Page 158 - Rock Mechanics For Underground Mining

ROCK STRENGTH AND DEFORMABILITY

4 A series of laboratory tests on intact specimens of quartzite gave the following
mean peak strengths. The units of stress are MPa, and compression is taken as
positive.

1 ( 1 + 3 ) 1 ( 1 − 3 )
2 2
100 100
135 130
triaxial 160 150
compression 200 180
2 = 3 298 248
435 335

1 2 3
biaxial 0 0 −13.5
tension/ 0 −13 −13
compression 218 50 0
225 100 0
228 150 0
210 210 0

Developapeakstrengthcriterionforthequartziteforuseinundergroundexcavation
design. Experience has shown that the in situ uniaxial compressive strength of the
quartzite is one-half the laboratory value.

5 A series of triaxial compression tests on specimens of a slate gave the following
results:

Confining Peak axial Angle between
pressure stress cleavage and 1
3 (MPa) 1 (MPa) ◦
2.0 62.0 40
5.0 62.5 32
10.0 80.0 37
15.0 95.0 39
20.0 104.0 27

In each test, failure occurred by shear along the cleavage. Determine the shear
strength criterion for the cleavage planes.
6 In a further series of tests on the slate for which the data of Problem 5 were obtained,
it was found that, when failure occurred in directions other than along the cleavage,
the peak strength of the rock material was given by

1 = 150 + 2.8 3

where 1 and 3 are in MPa.
Construct a graph showing the expected variation of peak axial stress at a confining
pressure of 10 MPa, as the angle between the cleavage and the specimen axis varies
◦
◦
from 0 to 90 .
140