C4: ROCK MECHANICS AND THE PROPERTIES OF ROCKS (PHYSICAL, MECHANICAL, AND DYNAMIC PROPERTIES). Flashcards
is a discipline that uses the principles of mechanics to describe the behaviour of rocks under various physical forces.
ROCK MECHANICS
In 1963, the ___________ was founded and the term adopted the following definition
Rock Mechanics Committee of the American
National Academy of Science
the theoretical and applied science of
the mechanical behaviour of rock. It is the branch of mechanics concerned with the response of rock to the force fields of its physical environment.
Rock mechanics
TWO MAJOR BRANCHES OF ROCK MECHANICS:
- STRUCTURAL ROCK
MECHANICS - COMMINUTION
Typically continuous,
homogeneous, isotropic, and linearlyelastic (CHILE). Their
properties are well-known and controllable during
production.
Construction Materials
Are discontinuous, inhomogeneous, anisotropic, and non-linearly elastic (DIANE). The properties of rocks are often variable and difficult to control, making their behavior less predictable.
Rocks
are fractures or breaks in rocks that
affect their mechanical behaviour
Discontinuities
These include large, smooth
fractures that are often weathered or infilled, such as faults,joints, bedding, and fractures
Normal Discontinuities
These are smaller, rough, and often fresh fractures with no specific pattern, typically created by human activities.
Artificial Discontinuities
This refers to the
variation in rock properties at different locations, often
due to mineral composition or the presence of discontinuities.
Inhomogeneity of Rock Materials:
This is primarily caused by
the existence of discontinuities in the rock mass, which
significantly affect its mechanical behavior.
Inhomogeneity of Rock Masses
is defined as variation of properties with respect
to the directions concerned in design and analysis of rock
structures.
Anisotropy
is the force per unit area applied to a rock. It
can be classified into:
Stress
Stress that acts perpendicular to a surface.
Normal Stress
Stress that acts parallel to a surface.
Shear Stress
- may be known as Index Properties
- it describes the rock material and helps in classifying them
Physical Properties
The composition,structure, and texture of rock materials directly control their strength. While there are over 2,000 known minerals, only about nine play a significant role in rock formation.
Mineralogic Composition, Structure, and Texture
is the ratio of the density of the rock to the density of water. It helps determine whether the rock will float or sink.
Specific Gravity(G)
also known as specific weight, relates the rock’s size, mass, density, and force.
Unit Weight (Y)
refers to how compact the rock is, and It is related to its mineral composition.
Density
Is the ratio of the volume of voids to the volume of solids, and is related to porosity.
Void Ratio
is the proportion of the volume that is not solid, affecting water storage and permeability.
Porosity
influences physical properties such as weight and density.
Moisture Content
indicates how much moisture is present in the rock relative to its total void capacity.
Degree of Saturation
Describes the rock’s ability to transmit fluids. Highly porous rocks with well connected voids have high permeability.
Permeability
majority of rocks are dielectric, and measurements of dielectric constants are used to interpret data.
Electrical Properties
The increase in temperature also makes rock weaker due to the formation of cracks in the rock mass
Thermal Properties:
occurs when rocks absorb water and Increase in volume.
Swelling
refers to the condition where the properties of a rock mass vary depending on the direction.
Anisotropy
refers to how resistant rocks are to weathering and other forms of degradation overtime.
Durability
describe a rock’ s strength and its ability to resist Deformation under load.
Mechanical properties
The ability of a rock to
withstand axial loads without failure.
Compressive Strength:
is determined by dividing
The load at failure by the cross-sectional area
Of the rock.
Uniaxial compressive Strength
Tests like the uniaxial compression and triaxial compression tests directly measure compressive strength.
Direct Methods
Techniques like the Schmidt Hammer Test and Point Load Test estimate compressive strength without damaging the rock sample
Indirect Methods
The maximum tension a rock can withstand before failure. Rocks are generally weak in tension, but tensile strength is important for understanding failure
Tensile Strength
Involve applying uniaxial tensile stress.
Direct Methods
Techniques like the Brazilian Disc Test estimate tensile strength nondestructively.
Indirect Methods
This refers to the maximum resistance to deformation caused by shear stresses. It is essential for analyzing rock slopes and foundations.
Shear Strength:
Include laboratory tests to measure shear resistance along weakness Planes in the rock.
Shear Strength Tests
are generated when stress is applied to a rock, causing deformation that propagates as sound waves through the material. They are essential for understanding seismic activities.
Stress waves
Pressure experienced by rocks due to the weight of overlying materials.
Lithostatic Stress
Unequal stress experienced by rocks, often caused by tectonic forces.
Differential Stress
The deformation or change in shape that occurs in response to applied stress.
Strain
The rock returns to its Original shape after the stress is removed.
Elastic Deformation
Occurs under high temperature and pressure, causing the rock to flow rather than fracture.
Ductile Deformation
Characterized by rock fracturing when stress exceeds its yield strength.
Brittle Deformation
These constants describe the rock’s ability to return to its original shape after deformation
Elastic Constants
4 elastic constants
- modulus of elasticity (young’s modulus)
- shear modulus
- bulk modulus
- poisson’s ratio
Measures resistance to compression.
Modulus of Elasticity (Young’s Modulus
Measures resistance to shear deformation.
Shear Modulus
Measures resistance to volume
change under pressure.
Bulk Modulus
Describes the relationship between lateral strain and longitudinal strain underloading.
Poisson’s Ratio
