Engineering Rock Mechanics

72 Intact rock: deformability, strength and failure

Figure 6.1 Rock core obtained from a site investigation borehole provides samples of

intact rock for testing. Note also the fractures intersecting the borehole core.

A Compressive strength: maximum

of the curve

• load-bearing capability at the peak


• 
  

A

0-

the type of failure, in this

case semi-brittle

specimen stiffness for

Young’s modulus for
stress-strain axes.
IIIIIII
Displacement
Figure 6.2 Actual complete force-displacement curve obtained from a strain-controlled
uniaxial compression test on marble. The force axis is converted to stress through division
by the cross-sectional area of the specimen; the displacement axis is converted to strain
through division by the original specimen length.

This test was strain-controlled in a servo-controlled testing machine so
that strain was the independent variable, i.e. the test was programmed
so that the strain increased at a constant rate. Stress was therefore
the dependent variable (see Q and A5.10 in Chapter 5). Under these
conditions, the complete force-displacement curve shown in Fig. 6.2
was obtained. This curve represents the response of the intact,rock to
continued compressive displacement of the specimen ends, from initial

When the axes in Fig. 6.2 are scaled to stress and strain using the original specimen
area and length, the result is an ‘engineering’ stress-strain curve representing the struc-
tural breakdown of the specimen. Some researchers consider that the ’true’ stress-strain
curve for the rock material reduces less sharply after the peak than shown, because the
effective force-transmitting area of the specimen reduces after the peak, but this is not
accounted for in the calculation of the stress.