Slide 1

baconossifiedΜηχανική

29 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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STRESS

and

STRAIN

ASPECTS OF ROCK

Part 2


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.


Elasto
-
plastic behavior of Solenhofen
Limestone, as seen in
measured relaxation

in
percent axial strain after triaxial compression
under significant lateral confinement

Plastic deformation

Elastic

deformation


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