Rock Mechanics presentation for educatio

for education

1. STRESSSTRESS
and and
STRAINSTRAIN
ASPECTS OF ROCKASPECTS OF ROCK
Part 2Part 2

2. •Unlike soils, Unlike soils, pure rockpure rock (solid material (solid material
between joints) is an between joints) is an elasto-plastic materialelasto-plastic material, ,
subject to elastic recovery AND permanent subject to elastic recovery AND permanent
deformation, as shown in this Stress vs deformation, as shown in this Stress vs
strain plot.strain plot.

3. •Physical phenomena associated with Physical phenomena associated with
uniaxial compression tests of rock uniaxial compression tests of rock
cylinders, under near zero confinement.cylinders, under near zero confinement.

4. •Unconfined compression testsUnconfined compression tests on rock cylinders generally on rock cylinders generally
exhibit significant propagation of extension cracks after peak exhibit significant propagation of extension cracks after peak
strength is exceeded, followed by friction and interlocking strength is exceeded, followed by friction and interlocking
between cracked sections of the sample. between cracked sections of the sample.

5. •Example of a Example of a full-range stress-strain plotfull-range stress-strain plot, ,
showing how showing how post-failure stiffnesspost-failure stiffness is is
estimated estimated

6. •Physical phenomena associated with gradual Physical phenomena associated with gradual
breakdown of rock cylinders on the post-failure breakdown of rock cylinders on the post-failure
side of a stress-strain plot side of a stress-strain plot

7. •The stress strain behavior of a natural rock like The stress strain behavior of a natural rock like
sandstone is a combination of its mineralogical sandstone is a combination of its mineralogical
components, in this case: quartz and calcitecomponents, in this case: quartz and calcite

8. •Most rocks exhibit Most rocks exhibit increasingly plastic behavior with increasingly plastic behavior with
increasing levels of confinementincreasing levels of confinement, as shown in this , as shown in this
series of jacketed triaxial tests on a marble. So, the series of jacketed triaxial tests on a marble. So, the
most brittle behavior is typically exhibited under most brittle behavior is typically exhibited under
conditions of the least confinement, at the Earth’s conditions of the least confinement, at the Earth’s
surface.surface.

9. •Elasto-plastic behavior of Solenhofen Elasto-plastic behavior of Solenhofen
Limestone, as seen in Limestone, as seen in measured relaxationmeasured relaxation in in
percent axial strain after triaxial compression percent axial strain after triaxial compression
under significant lateral confinement under significant lateral confinement
Plastic deformationPlastic deformation
ElasticElastic
deformationdeformation

10. •The most common physical attributes of The most common physical attributes of
laboratory tests on intact rock are portrayed laboratory tests on intact rock are portrayed
here. The true elastic modulus can only be here. The true elastic modulus can only be
determined by employing load cycles.determined by employing load cycles.

11. •Goodman’s concept of Goodman’s concept of Modulus of Permanent Deformation, M,Modulus of Permanent Deformation, M,
was introduced in 1980. It is important to appreciate in porous was introduced in 1980. It is important to appreciate in porous
rocks which often exhibit “permanent set” with each loading rocks which often exhibit “permanent set” with each loading
cycle, such as sandstone. cycle, such as sandstone.

12. Mohr CirclesMohr Circles are the most common technique used to describe the are the most common technique used to describe the
failure envelopefailure envelope and the and the strength parametersstrength parameters frictionfriction and and cohesioncohesion
Rock tends to exhibit a slightly curvalinear failure envelope with Rock tends to exhibit a slightly curvalinear failure envelope with
low tensile strengthlow tensile strength, shown at upper right, shown at upper right

13. •Be careful when evaluating elastic modulus values. Be careful when evaluating elastic modulus values.
The terms used connote different kinds of calculations, The terms used connote different kinds of calculations,
as shown here. as shown here.

14. •The elastic modulus (E) varies with the stress The elastic modulus (E) varies with the stress
level and the induced strainlevel and the induced strain