Soil & Rock Triaxial Shear Testing
Triaxial shear testing is a laboratory procedure used to determine the mechanical properties of soil and rock specimens. It is the industry-standard laboratory method for determining the strength and deformation characteristics of soil and rock under simulated in-situ stress conditions. By enclosing a cylindrical specimen in a confining cell and incrementally increasing axial stress, we can measure shear strength parameters that govern stability, settlement and deformation behaviour of foundations, slopes, tunnels and retaining structures.
The data obtained from triaxial tests is crucial for safe and economical geotechnical design. It allows engineers to predict how soil or rock will behave under different loading scenarios, which is fundamental for assessing stability and deformation in structures such as foundations, slopes, retaining walls, and embankments.
Types of Triaxial Tests We Offer
Atlas Engineering and Technology Group offers a range of triaxial tests to simulate various field drainage conditions and loading rates. The choice of test depends on the soil type and the specific engineering problem being addressed.
Consolidated Undrained (CU) Test
In the CU test, the soil specimen is first consolidated under an all-around confining pressure, allowing full drainage. After consolidation is complete, the drainage lines are closed, and the specimen is sheared under undrained conditions by increasing the axial load. Pore water pressure changes are monitored during shearing. This test is commonly used to determine effective stress shear strength parameters ( $c'$ and $\\phi'$ ) and can also provide total stress parameters.
Consolidated Drained (CD) Test
For the CD test, the specimen is consolidated as in the CU test. However, during the shearing phase, drainage is permitted, and the axial load is applied slowly enough to ensure that no excess pore water pressure develops within the specimen. This test directly measures the effective stress shear strength parameters ( $c'$ and $\\phi'$ ) and is particularly relevant for long-term stability analyses of cohesive soils and for granular soils.
Unconsolidated Undrained (UU) Test
In the UU test, no consolidation or drainage is allowed at any stage. The specimen is subjected to a confining pressure, and then sheared to failure rapidly, with no drainage permitted. This test is typically performed on cohesive soils to determine the undrained shear strength ( $s_u$ ) and is relevant for assessing short-term stability where loading is applied quickly, and the soil does not have time to drain.
Summary of Common Triaxial Test Types
| Test Type | Drainage During Consolidation | Drainage During Shear | Typical Parameters Determined |
|---|---|---|---|
| Consolidated Undrained (CU) | Allowed | Prevented | Effective stress parameters ( $c'$ , $\\phi'$ ), Total stress parameters ( $c_u$ , $\\phi_u$ ) |
| Consolidated Drained (CD) | Allowed | Allowed | Effective stress parameters ( $c'$ , $\\phi'$ ) |
| Unconsolidated Undrained (UU) | Prevented | Prevented | Undrained shear strength ( $s_u$ ) |
Key Geotechnical Parameters Determined
Triaxial shear tests provide essential parameters for geotechnical analysis and design, including:
- Effective Cohesion ( $c'$ ): Represents the cohesive component of shear strength in terms of effective stress.
- Effective Angle of Internal Friction ( $\\phi'$ ): Represents the frictional component of shear strength in terms of effective stress.
- Undrained Shear Strength ( $s_u$ ): Represents the shear strength of a soil under undrained conditions, crucial for short-term stability analysis of cohesive soils.
- Stress-Strain Behaviour: Provides information on the deformation characteristics of the soil, including stiffness moduli like Young's Modulus ( $E$ ).
- Pore Water Pressure Parameters (e.g., Skempton's A and B parameters): Helps in understanding the soil's response to changes in stress.
Applications of Triaxial Test Data
The results from triaxial shear testing are fundamental for a wide range of geotechnical engineering applications:
- Design of shallow and deep foundations (bearing capacity and settlement).
- Stability analysis of natural slopes, excavations, and embankments.
- Design of retaining walls and other earth-retaining structures.
- Analysis and design of earth dams and levees.
- Characterisation of ground conditions for tunnelling and underground construction.
- Evaluation of liquefaction potential in sandy soils.
- Assessment of the effectiveness of ground improvement techniques.
- Pavement design and analysis.