We run the direct shear test on a Wykeham Farrance shear box apparatus at our Calgary lab. The sample sits inside a split square box, and we apply a horizontal force at a constant strain rate while the vertical load stays fixed. This gives us the cohesion and friction angle of the soil. For projects in the Bow River valley or glacial till deposits around Calgary, those two numbers define whether your footing holds or slides. The test works best for sands, gravels, and stiff clays — materials we see daily in this region. Before testing we check the sample against ASTM D3080-18 criteria, then run three or four specimens under different normal stresses to build a clean failure envelope.
A single direct shear test on Calgary till can show cohesion values between 5 and 20 kPa, depending on the gravel content.
Methodology and scope
Calgary grew fast after the 1970s oil boom, and the city spread over everything from glacial till plains to the Paskapoo sandstone bedrock. That variety means the shear strength of the ground changes block by block. The direct shear test captures that variability because we test intact samples from the actual depth of your foundation. We pair the shear results with a granulometria distribution to understand the soil fabric, and with limites-atterberg on the fines to predict how the clay fraction behaves under load. The procedure itself is simple but precise: we saturate the sample, set the normal stress, and shearing takes about 10 to 15 minutes per stage. We run three stages minimum to get a linear Mohr-Coulomb envelope.
Local considerations
Calgary sits at the edge of the Rocky Mountain Foothills, so the groundwater table fluctuates wildly between spring melt and dry summers. That seasonal change affects the effective stress in the soil, and the direct shear test must reproduce those field conditions. If we test a sample at the in-situ moisture content from a dry August but the footing will be poured in April during snowmelt, the shear strength might be 30% lower. We account for this by running the test both at natural moisture and under saturated conditions. Another risk is the presence of cobbles in the local till — they can jam the shear box and produce erratic readings. Our lab protocol includes a pre-screening step to identify oversized particles before mounting the sample.
Peak and residual cohesion (c'), friction angle (φ')
Typical test duration
2 – 4 hours for a 3-stage set
Standards
ASTM D3080-18, AASHTO T-236, CSA A23.3 reference
Associated technical services
01
Standard direct shear (undrained)
Three-stage test at constant moisture content. Run on undisturbed tube samples or remoulded specimens. Ideal for rapid embankment fills and temporary excavations where pore pressure dissipation is minimal.
02
Consolidated drained direct shear
Each stage consolidates under the normal stress before shearing. We measure the volume change during consolidation. Used for long-term slope stability and retaining wall design where drainage is expected over time.
03
Residual shear strength test
We shear the sample back and forth multiple times to reach the residual state. Critical for reactivated landslides and existing slip surfaces in the Paskapoo claystone formations west of Calgary.
Applicable standards
ASTM D3080-18: Standard Test Method for Direct Shear Test of Soils, CFEM (Canadian Foundation Engineering Manual): Standard Practice for Classification of Soils for Engineering Purposes, CSA A23.3-19: Design of Concrete Structures (references shear strength parameters for foundation design)
Frequently asked questions
What is the difference between a direct shear test and a triaxial test?
The direct shear test forces failure along a horizontal plane, while the triaxial test lets the sample fail along its natural weak plane. Direct shear is faster and cheaper, but the triaxial test gives more reliable effective stress parameters for saturated clays. For Calgary till with gravel, direct shear often gives more consistent results because the large particles don't cause membrane punctures.
How much does a direct shear test cost in Calgary?
A standard three-stage direct shear test on undisturbed samples ranges from CA$890 to CA$1.250 per test set. The price includes sample preparation, three shear stages, and a report with the Mohr-Coulomb envelope. Remoulded samples or residual tests cost extra. Volume discounts apply for sets of six or more.
When should I request a direct shear test instead of a triaxial test?
Use direct shear for granular soils with gravel, for thin clay seams in till, or when you need a quick estimate of the residual strength. It works well for shallow foundations on stiff soils and for slope stability in layered deposits. For deep foundations in soft clay or for critical seismic analyses, a triaxial test is usually more appropriate.
