41 8 Questions 6.7-6.7 0: intact rock
Q6.6 A servo-controlled compression test has been conducted on a
weak soapstone such that the specimen length remained unchanged
throughout: as the axial stress, a,, was increased, so the confining
pressure, p, was increased so that no net axial strain resulted. A plot
of axial stress (vertical axis) against confining pressure (horizontal axis)
gave an initial straight line passing through the origin. At a critical
confining pressure of p = 85 MPa (when a, = 39.1 MPa), the slope of
the a,-p plot suddenly changed to 29” and remained constant for the
remainder of the test. This change in slope may be taken to represent the
onset of yield.
(i) Determine an elastic constant from the slope of the initial portion of
the 0a-P curve.
(ii) Assuming that the Mohr-Coulomb criterion is applicable, determine
a,, c and q5 for the rock.
46.7 Laboratory tests on specimens of a limestone have produced
unconfined compressive and tensile strengths of 80 MPa and 10 MPa,
respectively. Using the Hoek-Brown and plane Griffith criteria, estimate
the maximum principal stress at failure for two biaxial tests in which
02 = 20 MPa and a2 = 40 MPa. Which of these two criteria would best
predict peak strength under these conditions?46.8 Comment on the applicability of each of the Griffith, Mohr-
Coulomb, and Hoek-Brown criteria for the following triaxial test results
on quartzite:(al +a3)/2 -6.65 100 135 160 200 298 435 MPa
(al -a3)/2 6.65 100 130 150 180 248 335 MPa46.9 In 1926, in a paper on tensile tests for cotton yarns (Pierce, 1926),
Pierce stated, ”it is a truism, of which the mathematical implications are
of no little interest, that the strength of a chain is that of its weakest link”.
What is the relevance of this statement to the tensile strength of intact
rock?46.10 The two marble panels illustrated opposite are from the Greek
Parthenon frieze which depicts the four-yearly procession of the ‘Great
Panathenaia’. They were sculpted in light relief under the direction of
Pheidias and placed in position around 450 BC. One panel is from the
north side of the Parthenon and one panel is from the south side. Which
is which?