Rock strength Flashcards
What structural features dictates the mechanical properties of rock?
Composition (strong/weak material)
Fabric/sedimentary features
Jointing/faulting
Folding
What does rock strength apply to?
intact (unfractured) rock
What is rock mass strength?
Applies to larger rock masses considering fractures and other properties
Not the same as intact rock
What are the several methods of rock failure?
Flexure
Shear failure
Direct tension
Compression/ crushing
What is flexure failure?
failure by bending - tensile cracks propagate (slope toppling)
What is shear failure?
displacement along rupture surface
- usually in soil-like, weak rocks
What is direct tension failure?
Act of pulling the rock apart
What is compression/ crushing failure?
rock is shortened
* complex (tensile cracks grow through flexure and shear)
What factors influence rock strength?
Weathering
Temperature (closer to ductile)
Specimen sample size (larger = less strong)
Sample shape
Sample structure
What is the Unconfined Compressive test and what does it derive?
Rock strength (MPa)
Uniaxial compression
Dry
Where will shear failure occur in the UNconfined compressive test?
~30* to comrpession
What is an in the field method of testing rock strength?
Schmidt hammer
What does the Schmidt hammer do and whats its limitations?
Hand held spring loaded hammer which measures rebound from rock surface
Done quickly - distingushing between diiferent stages of rock but only sampling whats on the surface
How is triaxial tetsing done for rock strength?
COmpressed in 1 dorection
AT higher stresses oil used as confining fluid
but otherwise the same as for soil
What is the dogbone specimen for tensile rock strength?
Rock accurately cut into dog bone shaped and pulled apart (problem is very hard to make sure set up correctly)
What is the Brizilian test (indirect tensile strength)?
Squeeze rock cylinder between two plates until it splits
quick and cheap but inaccurate without calibration
1/4 to 1/10 of compressive strength
Why is the point load test done?
To test capacity for supporting point loads
1/20 of UNconfined soil strength
WHat are the storngest types of rock based on lab measurements?
fine-grained igneous rocks and isotropic metamorphic rocks (granite, basalt, hornfels)
Then very well-cemented sedimentary and metasedimentary (greywacke, marble) and gneiss
Then well-cemented sedimentary (Carb limestone & sandstone)
What are the weakest rocks based on lab measurements?
friable weak rocks such as chalk, clay, shale, mudstone)
Why does the strength of schists and slate depend on direction?
Due to being very anisotropic
Up until what point will rock defomr elastically and what does this mean?
up until ~80% of peak strength and then it wont be able to rebound to original state
What happens to rock when its elastic strength is overcome?
Microcracks form when these connect the rock fails as peak stress has been met
WHat happens to a rock when it has reached peak stress?
A fault forms and stress required for strain is reduced
Once a rock has reached residual strength what is resistance to slip die to?
Friction
How does rock deformation relfect the quality and strength of the intial rock mass?
magnitude of the peak stress and the abruptness of the drop once a fault or fracture forms
What will a very good, an average and a poor quality rock deformation graph look like?
Very good - elastic brittle - steep increase and very sharp drop
Average - strain softening - more gradual increase with lesser drop off
Poor - elastic, plastic - gradual increase no decrease
What happens to the resdidual strength of rock when confiing pressre is increased?
increase the peak strength and influence the deformation behavior of sample
What law combines both amontons law and coulombs law for failure criteria?
Byerlees law
What does byerlees law show?
ANy rock below failure envelope is intact and abive is broken
WHat is Poissons ratio?
Ratio of transverse contraction strain to longitudinal extension strain in the direction of the stretching force
WHat are examples of elastic moduli?
Youngs modulus (linear elastic relation applied stress and resultant deformation)
Tangent youngs modulus (avgerage)
Initial tangent modulus (tangent modulus at initial point)
Secant modulus (slope of line from origin to peak)
What is mohr coulomb useful for and what does it miss?
Good for normal and shear stresses but want to assess shear stress and likelihhod of failure for principle compressive stresses
WHat stresses will be produced from uniaxial tension?
one non-zero stress is tensile
WHat stresses will be produced from confined compression?
1 tensile + 2 compressive stresses
WHat stresses will be produced from uniaxial compression?
one non-zero stress is compressive.
=> unconfined strength test
WHat stresses will be produced from triaxial compression?
one compressive stress larger than the
other two, all non-zero.
WHat will happen to the mohr circles and failure envelope as you move from uniaxial tension thrugh to triaxial compression?
Increases in size and migration to the right producing a curved failure envelope
What is the griffith law used for?
explains tensional strength and how rocks will be made weaker under tension and normal becomes negative
What does Mohr Coulomb not account for?
When rocks start to deform ductilely or under tension (Griffths)
What is Hoek-Brown criteria?
Rock strength plotted in terms of principle stresses best fitting curve not striaght line so departure from mohr behaviour
What will the mi be from Hoek-Brown be for strong and weak rocks?
Strong = 20-30
Weak = <10
