Engineering Properties of Rocks

• Geology deals with physical, chemical and mineralogical characteristics of rocks.

• Engineering deals with the mechanics of rocks and other earth materials.

• Eg. The response of materials to applied load

ENGINEERING PROPERTIES OF ROCKS

Illustrated By Sabariah 2009

Rocks as Earth Materials

• Solid and aggregates of mineral grain

• Igneous: Interlocking minerals from crystallization of magma

• Metamorphic: Re-crystallization of an existing rock

• The strength depends on the degree of interlocking

• Sedimentary:

~Chemical precipitation (interlocking aggregate of minerals)

~Clastic (particle from pre-existing rocks), cemented


Porosity and Void Ratio



Properties of Porous Rock

for Fluid to Flow

1. Permeability: Indicates degree of

interconnection of voids

2. Intrinsic permeability, k : measured in

darcys or cm2

3. Hydraulic conductivity, K, (m/s) :

K = kρ g

µ


~Handouts~

1. Range of Values of Intrinsic

Permeability and Hydraulic

Conductivity

2. Mechanical Properties of Rock

and Other Materials


• The vertical stress acting on shallow horizontal planes in the earth is the sum of the unit weight of the material times the depth (h) and the atmospheric pressure (Pa)


• Vertical stress beneath a sequence of layers is the sum of the unit weight of each layer times its thickness

• Atmospheric pressure is neglected.


Question

• Calculate the vertical stress at a

depth of 8m at a location where a

5m bed of sandstone with a unit

weight of 25 kN/m3 overlies a

thick shale unit with a unit

weight of 27.5 kN/m3


Solution


Shear Stress

• To calculate shear stress use Oho Mohr (1835-

1918) graphical method.

σ1

σ3σ3

σ1

θ

Principal

Stress

Minimum

Stress

Below ground,

Min σ = Intermediate σ


Mohr’s Circle


To Plot Mohr’s Circle:

1. σ1 and σ3 are plotted on horizontal axis.

These two points represent normal

stress

2. Note: ~the circle of radius is

~shear stress on any plane is

represented by the circle at

angle θ from the principle

plane measured from σ3

σ1 - σ2

2


• General equations for shear and normal

stress derived from Mohr’s circle


Question

• Determine the normal and shear

stresses on a plane incline at 45º

to the principal plane if the

vertical and horizontal principal

stresses are 144 kN/m2 and 36

kN/m2 respectively.



Behavior of Rocks under

Stress

• Rock may deform under stress

• The amount of deformation is called strain

• In strain depends on σ , T

• How much rocks can deform under the loads of applied engineering structures?


Review on Important

Definition

• Rocks my behave

~Brittle or Ductile

Str

ess

Str

ess

Strain Strain

BrittleDuctile


Testing a Rock Sample

• E is called modulus of elasticity

• E is measured by the slope of segment 2

1

2

3

Point of Failure

Slope, E =

Stress σ

Strain ε


Rock Properties

Important properties for

engineering material

1. Density

2. Strength

3. Compressibility

4. Durability


Determination of Specific

Gravity

1. Weigh in air (A) after drying for 24

hours in oven

2. Weigh in air, saturated surface

dried (B)

3. Weigh in water (C)


SG x % Adsorption

SpG = Mass of Rock in air

Mass of Equal Volume

of Water

= Wa

Wa - Wb

% adsorption = B – A X 100

A


Rock Strength

• Rock can be subjected to three type

of stress:

~compressive, shear and tensile

Shear

Stress


Compressive Strength

• The compressive strength is the

compressive stress required to break the

rock specimen

• It ranges 1000 kPa to 280,000 kPa

• The unit is psi or N/m2 ,

1psi=6.895 kN/m2 = 6.895 kPa

• Rock’s strength

σ = ρ/A

ρ=failure load, A=cross-sectional area


Question

• A rock core of limestone is 3 in. in

diameter and 6 in. long. It was loaded

to failure in a confine testing

machine. If the failure load was 62150

lb, what is the unconfined

compression strength σ of the

sample?

Documents

Engineering Properties of Rocks