Download ROCK MECHANICS

ROCK MECHANICS
• The study of rock behavior in the solid state
under varying environmental and internal
conditions
• Main focus is how rocks respond to applied
stresses, especially those that naturally occur
due to:
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Gravity
Mantle Convection
Plate Tectonics
Diapiric Movements: magma, salt diapirs
Stress
• Same as Pressure
• = Force/Area
• Units in Earth Sciences are MPa = 106 Pa
= 10 bars =10 atm = 147 psi
STRAIN
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Response to or result of applied stress
Linear strain is defined as (L1 – L0)/L0
Units are ???? (unitless)
Volumetric strain = (V1-V0)/V0
• Units are ?????
• Areal Strain = (A1-A0)/A0
• Strain is usually expressed as a percent change
ENVIRONMENTAL CONDITIONS
affecting rock behavior
Stress Factors:
– Applied Stress
– Stress History
• Duration
• How stress applied over time
– Temperature
– Pore Fluids and Pressure
Lithospheric Pressure
• Aka Confining Pressure
• The vertical component can be calculated as Pc
= rgz,
• Where r = average density in g/cc,
• g= grav. constant,
• z = depth
• In the Lithosphere Pc increases at about 25-30
MPa/km
• In general Pc increases at r*10 MPa/km
Pressure vs. Depth in Earth
Geothermal Gradient
• T increases in the Lithosphere at 250C/km
• T continues to increase all the way to the
core, but at a lower rate
• The melting T of Quartz is
• The melting T of Feldspar
• The melting T of calcite is
573-870
>1100
910
0C
Temperature vs. Depth in the Earth
Horizontal Stresses in the Earth
do not equal vertical load,
rgz, but are about 0.25-0.5 of sv
depending on composition, porosity, pore
fluids
INTERNAL CONDITIONS
affecting rock behavior
• Composition (silicates and oxides most resistant to
strain)
• Texture/Fabric (phaneritic most resistant)
• Pores, cracks, flaws
• Previous stresses stored in grains
• Degree of Weathering (minerals that weather
from oxides and silicates to other compounds are
weakened)
• Presence of fluids in pores or cracks
Rock Texture/Fabric
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Grain size
Interlocking or grains in cement?
Sorting
Foliation/ non-foliated
Sedimentary layering
Textures
Note: Most Strain is not due to breaking bonds,
but is due to slip of grains against each other,
or between grains and cement
Porosity vs. Depth
Rigid and Brittle Behaviors
• Rigid:
• No strain, no response
• Most rocks show rigid behavior at room T and no
Pc, and at stresses below Pa
– Brittle:
• No strain before failure
• In ideal brittle behavior, if stress is removed before
rock fails, there will be no internal signs of strain