Lecture 2 Rock Mech

Rocks

Solid, dense aggregates of mineral grains

Soils

Anything that can be excavated by a shovel

Fluids

Water, magma, petroleum, natural gas, atmosphere

Voids

are always filled with some type of fluid - either liquid or gas

Porosity and void ratio

are parameters used to quantify relative amount of void space.

Permeability

rate at which fluids will move through a saturated material. Determined by size and connectedness of voids, and fluid properties

Intrinsic permeability, k (cm2 or darcys)

permeability defined by the property of the material

Hydraulic Conductivity, K (cm/s, m/s)

measure of the ability of a rock or soil to transmit water

Pressure

force per unit area applied to solid by a load

Stress

transmitted from the external face to an internal location, also force per unit area

Compressive

stresses of equal magnitude that act toward a point from opposite directions

Tensile

stresses of equal magnitude that act away from a point

Shear

stresses that are offset from one another and act in opposite directions

Mohr Circle:

Graphical representation of shear and normal stresses on inclined planes

Deformation

Response to Stress

strain

amount of deformation

Elastic

 Linear regression on a plot of stress vs strain

Viscosity

is slope of regression line in a stress strain rate plot

Plastic

No strain until some critical stress value has been reached; then continuous deformation

Brittle

respond in a mostly elastic fashion until failure 

Ductile

respond elastically until the elastic limit, then in plastic fashion until failure 

Compressive Strength

Failure of a brittle rock - point when the rock loses all resistance to stress and crumbles.

unconsolidated materials (e.g. dry sand)

resistance to failure under tensile stress

consolidated materials or cohesive soils

relationship is also linear, but there is inherent shear strength due to interparticle bonding (cohesion - C)

Tensile strength

resistance to failure under tensile stress

Confining Pressure

Weight of overlying rock applies pressure in all directions to given body of rock - confining pressure

Triaxial test

Confining pressure can be applied to better mimic depth conditions

•Effect of increasing confining pressure

Rocks change from brittle to ductile behavior; Strength of rock increases

Effect of increasing temperature

• Strength decreases; Ductile response occurs at lower pressures (stress) under higher temperatures

•Effect of time

Stress applied in geologic systems occurs over millions of years; Rock strength decreases with decreasing strain rate

•Intact Rock

• Strength classification is based on strength of the rock (compressive strength & modulus of   elasticity

Intact Rock Classification - Igneous rocks

Intrusive: high modulus of elasticity/medium modulus ratio

 Extrusive: greater variability

Clastic rocks

Depends upon grain size, grain size distribution, sorting, packing, cement type, lithification processes

•Nonclastic rocks

• Depends upon composition

  • Limestones/dolomites generally medium to high strength and modulus ratios

  • Evaporites tend to be much weaker

Intact Rock Classification - Metamorphic rocks

•Tend to increase strength due to recrystallization and compaction

  • Marbles may be weaker than limestones due to bigger grain size

Weakest link principle

overall strength of a rock not determined by bulk properties, but by strength of weakest link