The direct shear test setup we deploy across Bunbury sites uses a 60 mm square shear box mounted on a Wykeham Farrance frame, with displacement controlled at 0.5 mm/min. We prepare specimens from undisturbed tube samples or remoulded bulk material depending on the target density. The load cell records peak and residual shear stress while a dial gauge tracks horizontal displacement. For projects near the Leschenault Estuary, where soils can be saturated silty sands, we run the test submerged to replicate field conditions. Before interpreting results we always cross-check against density with sand cone data to confirm compaction consistency and use Atterberg limits when the fines content suggests plasticity.
Peak friction in Tamala sands averages 33 degrees; residual values drop to 28 degrees, which controls long-term slope behaviour.
Methodology and scope
Local considerations
A common oversight among developers in Bunbury is relying solely on SPT N-values from the coastal sands to estimate friction angle without a direct shear test. The SPT correlation (Peck, Meyerhof) overestimates strength in loose, saturated sands because it does not account for the pore-pressure response during rapid loading. We have seen retaining wall designs in the Marlston Hill precinct fail to mobilise the expected passive resistance precisely because the design friction angle was taken from a table, not from a direct shear test. The consequence is wall movements of 20 to 30 mm within the first year, requiring expensive underpinning. Running a direct shear test on reconstituted samples at the target density eliminates that guesswork.
Applicable standards
AS 1289.6.2.2 – Soil strength and consolidation tests: Determination of the shear strength of a soil by direct shear, AS 1289.6.2.2 – Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions, AS 4678-2002 – Earth-retaining structures
Associated technical services
Consolidated Drained (CD) Direct Shear
Standard test for slow-draining soils like the silty clays found near the Bunbury Outer Ring Road corridor. The sample is consolidated under normal stress before shearing at 0.5 mm/min. Results include effective cohesion and effective friction angle.
Remoulded / Compacted Direct Shear
Used for fill quality control on subdivisions in the Dalyellup and Glen Iris estates. Bulk samples are compacted to 95% standard Proctor density, then sheared to verify that design shear parameters are achieved. Ideal for earthwork specifications.
Multi-Stage Direct Shear
Single specimen sheared sequentially at three normal stresses. Economical for projects with limited sample availability, such as deep boreholes from the Bunbury port reclamation area. Reduces testing time by 40% compared to three separate tests.
Typical parameters
Frequently asked questions
What is the difference between peak and residual shear strength in a direct shear test?
Peak strength is the maximum shear resistance mobilised before the soil structure begins to break down, typically occurring at 2 to 5 mm of horizontal displacement. Residual strength is the constant shear stress maintained after large displacement (usually beyond 10 mm) once particles have realigned or clay platelets have oriented parallel to the shear plane. For Bunbury's coastal sands, the residual friction angle can be 3 to 5 degrees lower than the peak value.
How much does a direct shear test cost in Bunbury?
The typical cost for a single direct shear test in Bunbury ranges from AU$860 to AU$1,370, depending on whether the sample is undisturbed or remoulded and whether a multi-stage or single-stage procedure is used. Volume discounts apply for 10 or more tests from the same project. A formal quote with turnaround time is provided after sample receipt.
Can you test saturated samples from below the water table?
The reference range for this service in Bunbury is AU$860 - AU$1.370. The final price depends on the project scope and volume.
How many specimens are needed for a reliable strength envelope?
A minimum of three specimens tested at different normal stresses is required to define a Mohr-Coulomb failure envelope. For Bunbury projects where soil variability is high — such as the mixed sand-clay profiles in the College Grove area — we recommend testing five specimens to capture the heterogeneity. Each specimen should be as identical as possible in density and moisture content.