The slow shear test, also known as the consolidated drained (CD) direct shear test, is a fundamental geotechnical laboratory method used to determine the effective shear strength parameters of soil, namely effective cohesion (c') and effective internal friction angle (φ'). By shearing a consolidated specimen at a sufficiently slow rate to allow complete pore water pressure dissipation, the test accurately simulates long-term drained conditions in the field. This article provides a comprehensive overview of the slow shear test principle, sample requirements, testing procedures, applicable standards such as GB/T 50123 and ASTM D3080, and its practical applications in engineering design.
Technical Overview
The slow shear test is a consolidated drained direct shear test performed to measure the effective shear strength parameters of soils. In this test, a soil specimen is first confined in a shear box and consolidated under a specified vertical (normal) stress until primary consolidation is complete. A horizontal shear force is then applied at a controlled, deliberately slow displacement rate so that excess pore water pressure fully dissipates throughout the shearing process. Because pore water pressure remains at or near zero, the applied normal stress equals the effective normal stress, allowing the test to yield the effective stress shear strength envelope.
The test results are expressed through the Mohr-Coulomb failure criterion, τ = c' + σ' tan φ', where τ is the shear strength at failure, σ' is the effective normal stress, c' is the effective cohesion, and φ' is the effective internal friction angle. By testing multiple identical specimens under at least four different normal stresses, a failure envelope can be constructed and the strength parameters determined through linear regression.
Commonly referenced standards for slow shear testing include GB/T 50123 (Standard for Geotechnical Testing Method), ASTM D3080 (Standard Test Method for Direct Shear Test of Soils Under Consolidated Drained Conditions), and BS 1377-7 (Methods of Test for Soils for Civil Engineering Purposes, Shear Strength Tests). These tests are routinely performed by accredited third-party laboratories and geotechnical testing centers.
Testing Samples
Cohesive Soils
Undisturbed or remolded specimens of clay, silty clay, and loess are the most common materials tested. For cohesive soils, the slow shear rate is critical because low permeability requires very slow shearing to guarantee full drainage. Undisturbed samples are typically recovered using thin-wall samplers or block sampling to preserve the natural structure and stress history of the soil.
Cohesionless Soils
Sands, gravels, and sand-gravel mixtures can be tested under drained conditions, and because of their high permeability, a faster displacement rate may be permitted by the applicable standard. Specimens are usually prepared by compaction or pluviation (dry raining) into the shear box to achieve a target relative density.
Compacted and Fill Materials
Engineered fills, embankment materials, and stabilized soils are frequently tested in remolded form. Specimens are compacted to the design dry density and moisture content to evaluate the effective strength parameters that will govern long-term performance of the constructed facility.
Specimen Dimensions and Quantity
- Standard circular specimen: 61.8 mm diameter with 20 mm height (typical for strain-controlled direct shear apparatus).
- Square or circular specimens of other dimensions may be used when permitted by the standard and equipment specifications.
- A minimum of four specimens is required, each consolidated under a different normal stress, to define a reliable failure envelope.
Testing Items
The slow shear test determines the following key parameters and results:
- Effective cohesion (c'), in kPa.
- Effective internal friction angle (φ'), in degrees.
- Peak shear stress and, where applicable, residual (ultimate) shear stress at each normal stress level.
- Shear stress versus horizontal displacement curves for each specimen.
- Vertical displacement (dilatancy or contraction) versus horizontal displacement behavior.
- The Mohr-Coulomb failure envelope in terms of effective stresses.
Testing Methods
The slow shear test is conducted in the following general sequence, in accordance with GB/T 50123, ASTM D3080, or equivalent standards:
- Specimen preparation: The soil specimen is trimmed and installed into the shear box, with porous stones and filter paper placed on the top and bottom surfaces to permit drainage. For remolded specimens, the soil is compacted to the specified density and moisture content.
- Application of normal stress: A vertical load is applied to produce the target normal stress (commonly 50, 100, 200, and 400 kPa for the four specimens).
- Consolidation stage: The specimen is allowed to consolidate under the applied normal stress. Consolidation is monitored via vertical displacement readings, and the consolidation curve is used to estimate an appropriate shear rate.
- Rate selection: The shear (displacement) rate is selected to be slow enough that the specimen undergoes negligible excess pore pressure build-up. For clays, rates as low as 0.02 mm/min or slower may be required; the rate is commonly derived from the coefficient of consolidation obtained during the consolidation stage.
- Shearing stage: Horizontal shear displacement is applied at the selected constant rate until failure is observed, or until the shear stress stabilizes at a residual value, or until a specified maximum displacement (typically 6 to 10 mm) is reached. Shear force and displacement readings are recorded at regular intervals.
- Determination of failure stress: The peak or plateau shear stress from the shear stress-displacement curve is taken as the shear strength at the applied normal stress.
- Plotting and regression: Shear strength values from all specimens are plotted against their corresponding normal stresses, and a best-fit straight line is drawn. The intercept of the line gives c' and the slope gives tan φ'.
- Post-test examination: The shear plane is inspected, and the water content of the specimen is measured to document the final condition.
Quality control measures include verifying apparatus calibration, ensuring proper seating of the shear box halves, preventing soil extrusion at the shear plane gap, and confirming that the selected shear rate satisfies the drainage criterion of the governing standard.
Applications
Slow shear test results represent drained, long-term strength conditions and are applied in numerous geotechnical design and analysis scenarios:
- Slope stability analysis for natural slopes, cut slopes, and embankments under long-term (drained) conditions.
- Earth dam and levee design, where steady seepage and long-term strength govern stability.
- Bearing capacity evaluation for foundations on cohesive soils where drained failure conditions control.
- Lateral earth pressure calculations for retaining walls and basement structures under long-term service.
- Stability assessment of excavations and open cuts well after construction is completed.
- Design of landfill liners, waste fills, and tailings dams where effective stress parameters are required.
- Research and forensic analysis of landslides, including the use of residual strength parameters for pre-existing slip surfaces.
FAQ
What is the difference between the slow shear test, the consolidated quick shear test, and the quick shear test?
- Slow shear test: The specimen is consolidated and then sheared slowly enough to allow full drainage, yielding effective strength parameters (c', φ'). It represents long-term drained conditions.
- Consolidated quick shear test: The specimen is consolidated, then sheared rapidly so that drainage cannot occur during shear; results approximate consolidated undrained conditions.
- Quick shear test: No consolidation and no drainage are permitted; the specimen is sheared immediately after load application, approximating undrained, unconsolidated short-term conditions.
Why must the shearing rate be so slow?
The slow rate ensures that pore water can drain freely from the shear zone as it deforms, keeping excess pore water pressure at essentially zero. This is what allows the measured normal stress to be interpreted as effective stress, which is essential for obtaining c' and φ' rather than total stress parameters.
How many specimens are required for a valid test?
At least four specimens of the same soil are tested, each under a different normal stress, so that a statistically acceptable failure envelope can be established by linear regression.
Which standards govern slow shear testing?
Primary references include GB/T 50123 (Standard for Geotechnical Testing Method), ASTM D3080 (direct shear under consolidated drained conditions), and BS 1377-7. Accredited third-party laboratories select the applicable standard based on project specifications and regional requirements.
When should slow shear parameters be used in design instead of quick shear parameters?
Slow shear (drained) parameters should be used whenever long-term stability governs, such as slope stability after construction, embankments under steady seepage, and retaining structures in service. Quick shear parameters are appropriate for short-term, end-of-construction conditions in low-permeability soils.