Engineering Properties of Rocks
Classification Systems
Rocks are classified based on their engineering properties using systems like:
- Rock Quality Designation (RQD)
- Rock Mass Rating (RMR)
- Q-system
- Geological Strength Index (GSI)
Index Properties
Key index properties include:
- Density and specific gravity
- Porosity and permeability
- Water content and absorption
- Durability and weathering characteristics
- Hardness and abrasiveness
Rock Strength Characteristics
Compressive Strength
The maximum axial compressive stress that a rock specimen can withstand before failure. Typically measured through uniaxial or triaxial compression tests.
Tensile Strength
The resistance of rock to failure in tension. Typically much lower than compressive strength. Measured through Brazilian test or direct tension tests.
Shear Strength
The resistance to failure along internal surfaces. Governed by Mohr-Coulomb criterion: τ = c + σ·tanφ, where c is cohesion and φ is friction angle.
Compression and Shear Tests
Uniaxial Compression Test
The most common test for determining rock strength. Cylindrical rock specimens are subjected to axial load until failure.
Procedure:
- Prepare specimen (typically L/D ratio of 2.5-3.0)
- Measure dimensions and weight
- Apply load at constant strain rate (0.5-1.0 MPa/s)
- Record stress-strain data until failure
- Calculate uniaxial compressive strength (UCS)
Triaxial Compression Test
Conducted under confined pressure to simulate in-situ conditions. Provides more comprehensive strength parameters.
Key Parameters:
- Confining pressure (σ₃)
- Peak strength (σ₁)
- Cohesion (c)
- Angle of internal friction (φ)
- Dilatancy angle
Shear Test Methods
Direct Shear Test
Rock specimen is sheared along a predetermined plane under constant normal stress.
Brazilian Test
Indirect tensile strength test where disk-shaped specimen fails in tension when compressed diametrically.
Point Load Test
Index test for estimating UCS by applying concentrated load to rock specimen.
Rock Deformation and Elastic Properties
Elastic Parameters
Young's Modulus (E)
Ratio of axial stress to axial strain in the elastic range. Typically 1-100 GPa for rocks.
Poisson's Ratio (ν)
Ratio of lateral strain to axial strain. Typically 0.1-0.4 for most rocks.
Bulk Modulus (K)
Resistance to uniform compression. Related to E and ν by K = E/[3(1-2ν)].
Shear Modulus (G)
Resistance to shear deformation. G = E/[2(1+ν)].
Stress-Strain Behavior
Typical stages in rock deformation:
- Elastic deformation (reversible)
- Crack initiation and stable crack growth
- Unstable crack propagation (damage accumulation)
- Peak strength and post-peak behavior
- Residual strength
Significance in Engineering
Tunnel Design
Deformation characteristics determine support requirements and excavation methods.
Slope Stability
Understanding deformation helps predict failure mechanisms in rock slopes.
Foundation Design
Elastic properties affect settlement calculations for structures on rock.
Rock Failure Mechanisms
Coulomb's Failure Criterion
The simplest and most widely used failure criterion for rocks:
τ = c + σ·tanφ
Where:
- τ = shear strength
- c = cohesion
- σ = normal stress
- φ = angle of internal friction
Factors Affecting Rock Strength
Water Content
Water reduces rock strength through pore pressure effects and chemical weathering. Strength reduction up to 50% possible when saturated.
Temperature
High temperatures generally reduce strength, while freezing can increase strength temporarily.
Loading Rate
Higher loading rates typically show higher apparent strength (strain rate effect).
Scale Effects
Strength generally decreases with increasing specimen size due to greater probability of defects.
Advanced Failure Criteria
Hoek-Brown Criterion
Empirical criterion considering intact rock properties and rock mass conditions.
Griffith Theory
Based on crack propagation in brittle materials under tensile stress.
Drucker-Prager
Smooth approximation of Mohr-Coulomb in principal stress space.