The unconfined compression test (UCS) in Rockhampton uses a load frame with a 50 kN capacity and a proving ring or electronic load cell. A cylindrical specimen of undisturbed cohesive soil is compressed axially without confining pressure. The test measures the unconfined compressive strength (qu) of the sample, which is then halved to obtain the undrained shear strength (su). This method is standard for saturated clays and silts where the natural moisture content is near the liquid limit. The procedure follows AS 1289.6.3.1, a national standard derived from AS 1289.6.4.1. For projects in Rockhampton, we complement the UCS with densidad-cono-arena field density data and permeabilidad-laboratorio when seepage is a concern. The test is rapid, cost-effective, and provides a direct estimate of bearing capacity for foundation design in the city's clayey soils.
The unconfined compression test provides a rapid estimate of undrained shear strength for saturated clays, directly applicable to foundation bearing capacity in Rockhampton.
Scope of work
The key characteristics of the test include:
- Specimen height-to-diameter ratio of 2:1, typically 76 mm high by 38 mm diameter.
- Axial strain rate of 1% per minute, controlled by the loading frame.
- Failure defined at 15% axial strain or at peak load, whichever occurs first.
- Results expressed as qu (kPa) and su (kPa).
Area-specific notes
The main risk in Rockhampton is the contrast between the soft alluvial clays of the Fitzroy River floodplain and the stiff residual clays of the surrounding hills. A UCS test on a sample from the floodplain may show qu values below 50 kPa, indicating very low bearing capacity. In contrast, residual clays from the Berserker Range can exceed 200 kPa. Using a single UCS result without considering this spatial variability can lead to foundation failure. Differential settlement between these zones is a known issue. A proper site investigation should include UCS tests at multiple depths and locations to capture the range of su values. Ignoring this can result in over-designed foundations in one area and under-designed in another.
Standards used
AS 1289.6.3.1, AS 1289.6.4.1, AS 1726-2017
Linked services
Standard UCS Laboratory Testing
Conventional unconfined compression test on undisturbed tube samples. Performed in our NATA-accredited laboratory. Results include qu, su, stress-strain curve, and failure mode classification. Suitable for foundation design, slope stability, and earthworks quality control.
Field UCS Testing (Portable Frame)
On-site UCS testing using a portable load frame. Ideal for remote sites in Rockhampton where sample transport is difficult. Provides immediate qu results for fast decision-making. Mobile laboratory setup meets AS 1289.6.3.1 requirements.
Typical parameters
FAQ
What is the unconfined compression test (UCS) used for in Rockhampton?
The UCS test measures the undrained shear strength of saturated clays. In Rockhampton, it is commonly used to estimate bearing capacity for shallow foundations on alluvial soils near the Fitzroy River. It also helps assess slope stability in the Berserker Range residual clays.
How much does a UCS test cost in Rockhampton?
The typical cost for a single UCS test in Rockhampton ranges from AU$570 to AU$790. This includes sample preparation, testing, and a report with stress-strain data. Volume discounts apply for multiple samples from the same project.
What sample quality is required for a UCS test?
Undisturbed tube samples are required. Samples must be extruded carefully to avoid disturbance. For Rockhampton clays, the sample should be stored in a humidity-controlled environment to prevent moisture loss before testing. Disturbed samples produce unreliable qu values.
Can UCS be used for stiff clays in Rockhampton?
Yes, but with caution. For stiff clays (qu > 300 kPa), the UCS test may underpredict strength due to tensile cracking at the specimen ends. In such cases, we recommend using the unconfined compression test in conjunction with field vane shear or triaxial testing for more accurate su values.
What is the difference between UCS and triaxial test?
The UCS test applies no confining pressure, while the triaxial test does. This makes UCS faster and cheaper but only valid for saturated clays. For Rockhampton's alluvial clays, UCS is adequate for preliminary design. For residual soils with partial saturation, a triaxial test is preferred.