Rock mechanics testing determines the physical and mechanical properties of intact rock and rock masses. These tests are essential for foundation design on rock, tunnelling, slope stability in rock cuts, mining, and quarry assessments.
Why Rock Testing?
Rock testing provides the engineering parameters needed to:
- Determine the bearing capacity of rock for foundations
- Design rock cuts and slopes
- Assess excavatability (rippability vs blasting)
- Design tunnel support systems
- Evaluate rock quality for construction materials
- Assess rock mass behaviour under dynamic loading
Laboratory Test Types
Unconfined Compressive Strength (UCS)
The most fundamental rock strength test. A cylindrical rock core is loaded axially to failure.
| Standard | Method |
|---|---|
| AS 4133.4.1.1 | Determination of uniaxial compressive strength |
| ASTM D7012 | Standard test method for compressive strength |
| ISRM Suggested Method | International standard |
- Sample: NQ or HQ core, L:D ratio = 2:1 to 2.5:1
- Result: UCS in MPa
Typical UCS Values:
| Rock Type | UCS (MPa) | Classification |
|---|---|---|
| Granite | 100–250 | Very strong |
| Basalt | 100–300 | Very strong to extremely strong |
| Sandstone | 20–150 | Weak to very strong |
| Shale | 5–50 | Very weak to medium strong |
| Limestone | 30–100 | Medium to very strong |
| Marble | 50–150 | Medium to very strong |
Point Load Strength Index (Is₅₀)
An index test that correlates with UCS, suitable for field or laboratory use on irregular rock pieces or core.
$$ UCS = k \times I_{s(50)} $$Where k = 20–25 (typical correlation factor).
| Standard | Method |
|---|---|
| AS 4133.4.2.1 | Determination of point load strength index |
| ASTM D5731 | Standard test method for point load index |
- Sample: Core pieces or irregular lumps
- Test types: Axial, diametral, irregular lump
- Result: Is₅₀ in MPa
Indirect Tensile Strength (Brazil Test)
Measures the tensile strength of rock by diametral compression of a disc specimen.
$$ S_t = \frac{2P}{\pi D t} $$- Standard: AS 4133.4.2.2, ASTM D3967
- Result: Tensile strength typically 5–15% of UCS
Modulus and Poisson's Ratio
Determined during UCS testing using strain gauges or LVDTs.
| Parameter | Description | Typical Range for Rock |
|---|---|---|
| Young's Modulus (E) | Stress/strain ratio | 5–100 GPa |
| Poisson's Ratio (ν) | Lateral/axial strain ratio | 0.15–0.35 |
Standards: AS 4133.4.1.1 (with deformability measurement), ASTM D7012
Direct Shear Test (Rock)
Measures the shear strength of intact rock or rock discontinuities.
| Test Type | Application |
|---|---|
| Intact rock shear | Rock mass strength parameters |
| Discontinuity shear | Joint, bedding plane, or fault shear strength |
| Residual shear | Post-peak strength after large displacement |
- Standards: AS 4133.4.3.1, ASTM D5607
- Parameters: Peak shear strength (c, φ), residual shear strength
Slake Durability Index
Measures the resistance of rock to weathering and disintegration after two cycles of drying and wetting.
$$ I_{d2} = \frac{\text{Mass retained after 2 cycles}}{\text{Initial mass}} \times 100\% $$| Classification | Id₂ (%) |
|---|---|
| Very high durability | > 98 |
| High durability | 95–98 |
| Medium durability | 85–95 |
| Low durability | 60–85 |
| Very low durability | < 60 |
- Standard: AS 4133.4.5.1, ASTM D4644
- Application: Assessing suitability as construction material, tunnel wall stability
Porosity and Density
| Parameter | Standard | Significance |
|---|---|---|
| Dry density | AS 4133.2.1.1 | Rock quality indicator |
| Saturated density | AS 4133.2.1.1 | Buoyancy calculations |
| Porosity | AS 4133.2.1.1 | Strength and durability indicator |
Swelling Pressure Index
Measures the swelling potential of clay-bearing rocks (e.g., mudstone, shale) under zero-volume-change conditions.
- Standard: AS 1289.7.1.1 (adapted for rock), ISRM suggested method
- Application: Assessing heave potential in tunnels and foundations in argillaceous rocks
Field Rock Testing
Point Load Test (Field)
Portable device for rapid strength assessment in the field.
- Requires minimal sample preparation
- Can test irregular rock pieces
- Results correlated to UCS
Schmidt Hammer Rebound
Non-destructive index test for rock surface hardness.
| Rebound Value (R) | Rock Hardness |
|---|---|
| > 55 | Very hard |
| 40–55 | Hard |
| 25–40 | Medium hard |
| < 25 | Soft |
Rock Quality Designation (RQD)
A measure of rock mass quality from drill core.
$$ RQD = \frac{\text{Sum of intact core pieces} \geq 100 \text{ mm}}{\text{Total core run length}} \times 100\% $$| RQD (%) | Rock Quality |
|---|---|
| 90–100 | Excellent |
| 75–90 | Good |
| 50–75 | Fair |
| 25–50 | Poor |
| < 25 | Very poor |
Australian Standards for Rock Testing
| Standard | Title |
|---|---|
| AS 4133 Series | Methods of testing rock |
| AS 4133.4.1.1 | UCS (uniaxial compressive strength) |
| AS 4133.4.2.1 | Point load strength index |
| AS 4133.4.2.2 | Indirect tensile strength (Brazil test) |
| AS 4133.4.3.1 | Direct shear strength |
| AS 4133.4.5.1 | Slake durability index |
| AS 4133.2.1.1 | Rock porosity and density |
| AS 1726-2017 | Geotechnical site investigations (rock description) |
Interpreting Rock Test Results
| Parameter | Foundation on Rock | Rock Cut Slope | Tunnelling |
|---|---|---|---|
| UCS | Bearing capacity | Stand-up time | Excavation method |
| E (Modulus) | Settlement | Deformation | Lining design |
| Shear strength | Sliding resistance | Slope stability | Wedge stability |
| RQD | Rock mass quality | Support requirements | Rock mass classification |
| Slake durability | Weathering resistance | Long-term stability | Swelling potential |