Part 2
STRESS
and
STRAIN
ASPECTS OF ROCK
• Unlike soils, pure rock (solid material
between joints) is an elasto-plastic material,
subject to elastic recovery AND permanent
deformation, as shown in this Stress vs
strain plot.
• Physical phenomena associated with
uniaxial compression tests of rock
cylinders, under near zero confinement.
• Unconfined compression tests on rock cylinders generally
exhibit significant propagation of extension cracks after peak
strength is exceeded, followed by friction and interlocking
between cracked sections of the sample.
• Example of a full-range stress-strain plot,
showing how post-failure stiffness is
estimated
• Physical phenomena associated with gradual
breakdown of rock cylinders on the post-failure
side of a stress-strain plot
• The stress strain behavior of a natural rock like
sandstone is a combination of its mineralogical
components, in this case: quartz and calcite
• Most rocks exhibit increasingly plastic behavior with
increasing levels of confinement, as shown in this
series of jacketed triaxial tests on a marble. So, the
most brittle behavior is typically exhibited under
conditions of the least confinement, at the Earth’s
surface.
Elastic
deformation
Plastic deformation
• Elasto-plastic behavior of Solenhofen
Limestone, as seen in measured relaxation in
percent axial strain after triaxial compression
under significant lateral confinement
• The most common physical attributes of
laboratory tests on intact rock are portrayed
here. The true elastic modulus can only be
determined by employing load cycles.
• Goodman’s concept of Modulus of Permanent Deformation, M,
was introduced in 1980. It is important to appreciate in porous
rocks which often exhibit “permanent set” with each loading
cycle, such as sandstone.
Mohr Circles are the most common technique used to describe the
failure envelope and the strength parameters friction and cohesion
Rock tends to exhibit a slightly curvalinear failure envelope with
low tensile strength, shown at upper right
• Be careful when evaluating elastic modulus values.
The terms used connote different kinds of calculations,
as shown here.
• The elastic modulus (E) varies with the stress
level and the induced strain