Lecture 8:
Goal:
Relate the crustal forces that result from each of the three types of plate
boundaries and their relationship to rock behaviour.
List and describe the various ties of folds, and examples of their
signicance with respect to engineering.
Identify the 3 types of faults and describe the forces and motion
responsible for their development.
Compare and contrast the characteristics and mechanical behaviour of
faults and joints, and how they affect rock mass strength.
ROCKBEHAVIOUR
STRESS-the force applied over a unit area (not tangible)
STRAIN-how that force manifests itself in the rock (tangible)
DIFFERENT STRESSES
compression-pushed together. vertical lengthening, horizontal
shortening
tensional-pulled apart
shear-pushed in opposite directions, twisting laterally
PLATE BOUNDARIES FORCES
-CONVERGENT- compression causes shortening of the rock layer
through folding and faulting
-TENSION-lengthens and causes faulting
STESS-STRAIN
elastic limit/yieldpoint- strain will not be recovered (property of rock)..
FOLDING-DUCTILE. FAULTING-BRITTLE
BRITTLE/DUCTILE DEFORMATION:
depends on:
1. kind of stress (CTS)
2.amount of stress
3.temperature: lower temperature requires more stress and less strain
percent for fracture
4.lithology of material being deformed (Strength)
5. strain rate. length of time for stress
6. conning pressure-depth higher pressure= higher stress
IN EARTH's CRUST: rocks display increasing ductile behaviour with
depth (increasing T&P)
most brittle on top, most ductile at the bottom.
Brittlle-ductile transition zone.. (6-15km). few rocks are brittle below
30km
Ductile-FOLDING FOLDING-result from horizontal COMPRESSION of rock layers by
internal forces of the earth along plate boundaries.
ANTICLINE-a fold that resamples an ARCH like structure with the rock
beds (or limps) dipping way from the centre of the structure. OLD-
middle. YOUNG-away from axis
SYNCLINE-fold that resamples a TROUGH, rocks are warped
downward. YOUNG-middle. OLD-away from axis.
anticline and syncline occur in pares with adjacent folds sharing limbs.
GREAT PRESSURE=INCLINED AND ASYMMETRICAL ANTICLINE
AND SYNCLINE (fold centre tilts away from middle)
RECUMBENT FOLDS TO THRUST FAULT. important in the alps
(structure is calles "nappes"). FOLD IS JUST FOLDING. FAULT IS
WHEN TOP OF FOLD REACHES GROUND.
BRITTLE DEFORMATION-FAULTING
FAULTING-rocks break and move along a fracture (elastic-brittle
deformation, fracturing-rock laters too rigid to bend/fold, happens in
seconds and connected with earthquakes)
TECTONIC FORCES & FAULTS
1NORMAL FAULTS-vertical motion along the fault plane so that the
hanging wall is boring DOWN relative to footwall (FROM TENSIONAL
STRESS). left to right drainage=canyon on footwall block and forms an
alluvial fan.
Nevada,USA==> regional tensional stress=series of normal faults.
ex. GRAND TETON MTS in Wyoming-high angle normal faults
erosional remnats.
(in a series of normal fault, HORSTS are up-lifted blocks that form
mountains, down dropped blocks that form valleys are called
GRABENS)
2REVERSE FAULTS-vertical motion along the fault plane so that the
hanging wall is moving up relative to the footwall (FROM
COMPRESSION STRESS). left to right drainage=lake on footwall block.
THRUST FAULTS-special class of reverse faults, low angle reverse
faults with dips less than 30 degrees. due to differences in regional
stress directions. EX. CANADIAN ROCKIES
3STRIKESLIP FAULTS-SHEAR STRESS-movement horizontal, thus
parallels to the strike of the fault. movement is classied to what is seen
by observer (father direction). EX. San Andreas
FAULT ZONE-engineering hazard represent major zones of weakness
(Los Angeles Aqueduct one week after lling killed 450 people because
the dam was built over a fault zone and failed)
-provide pathways for increased uid ow (groundwater)FAULTS AND JOINTS
Faults-involve fractures along which there has been observable
movement of one side relative to the other.
Joints-involve fractures along which thare has been NO observable
movement of one side relative to the other.
FORMATION of ROCk JOINTS. (rock mass may contain several JOINT
SETS)
-occur as JOINT SETS.
TECTONIC JOINS=tectonic stresses inducing TENSILE failure of the
rock
UNLOADING JOINTS- uplift and erosion removes the overlying rocks,
thereby reducing the compressive load and allowing the rock to expand
laterally.
COOLING JOINTS-cooling of hot magma or lava introduces stress
induce tensile failure
See ROCK BEHAVIOUR SLIDE
RQDrockqualitydesignation=overall quality of the rock mass cut by the
different fracture systems
BLOCKYROC=70RMR(good rock), lowstress regime, minimal but
systematic ground suppo
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