Soil shear strength parameters, primarily cohesion and the internal friction angle, are fundamental inputs for geotechnical design, governing foundation bearing capacity, slope stability, and earth pressure calculations. This article presents a comprehensive technical overview of laboratory and field methods for determining soil shear strength parameters, including direct shear, triaxial compression (UU, CU, CD), unconfined compression, and vane shear testing, with reference to ASTM, ISO, and GB standards.
Technical Overview
Shear strength is the maximum resistance a soil mass can mobilize against shearing stress before failure occurs along a slip surface. In geotechnical engineering, shear strength is most commonly expressed by the Mohr–Coulomb failure criterion, in which the shear strength at failure is a linear function of the normal stress acting on the failure plane. Under the total stress formulation, the criterion is expressed as the shear strength equal to the cohesion intercept plus the product of total normal stress and the tangent of the internal friction angle. Under the effective stress formulation, the normal stress is replaced by effective stress, yielding the effective stress parameters, which describe the long-term or fully drained strength of the soil.
The two governing shear strength parameters are cohesion, expressed in kilopascals, and the internal friction angle, expressed in degrees. For clays, cohesion reflects interparticle bonding and electrochemical attraction, while for sands and gravels, strength is derived almost entirely from interparticle friction and dilation. The measured undrained shear strength is also a key parameter for short-term stability analysis of saturated fine-grained soils.
Accurate determination of these parameters is essential for bearing capacity analysis of shallow foundations, slope stability evaluation, earth pressure computation for retaining structures, excavation and shoring design, embankment and dam construction, and landslide investigation. Testing is performed in accredited third-party laboratories or in the field in accordance with recognized standards, including GB/T 50123, GB 50021, ASTM D3080, ASTM D2850, ASTM D4767, ASTM D7181, ASTM D2166, ASTM D2573, and the ISO 17892 series.
Testing Samples
Undisturbed Cohesive Soil Samples
Undisturbed samples are required whenever the in-situ structure, fabric, stress history, and strength of the soil must be represented. Cohesive soils are typically sampled using thin-walled tube samplers with a low area ratio, piston samplers, or block sampling in test pits. Samples must be sealed with wax and protective caps, stored in moisture-tight containers, transported without shock or vibration, and tested promptly to preserve natural moisture content and structural integrity. Sample quality is classified in accordance with GB 50021 or ISO 22475, and only quality grades suitable for strength testing should be used for parameter determination.
Remolded and Recompacted Samples
Remolded or reconstituted specimens are used to evaluate the strength of engineered fill, compacted embankment materials, and backfill, as well as to determine soil sensitivity by comparing the strength of remolded and undisturbed specimens. These samples are prepared at target dry density and moisture content, typically on the compaction curve at specified compaction energy, to simulate field placement conditions.
Cohesionless Granular Soil Samples
Sands and gravels are extremely difficult to sample undisturbed, so shear strength testing of granular soils is usually performed on reconstituted specimens prepared at the in-situ relative density inferred from in-situ tests. Direct shear apparatus and large-scale triaxial or large direct shear equipment may be used for coarse materials containing particles beyond standard specimen dimensions, with oversize particles handled by scalping or scalping-and-replacement procedures.
Specimen Dimensions and Preparation
Typical direct shear specimens are circular with a diameter of 61.8 millimeters and a height of 20 millimeters, or square specimens of 50 to 100 millimeters. Triaxial specimens are cylindrical, commonly 39.1, 61.8, or 101 millimeters in diameter, with a height-to-diameter ratio between 2.0 and 2.5 to minimize end restraint effects. Specimens are trimmed with a wire saw or soil lathe, and their mass, dimensions, and moisture content are measured before testing to verify density uniformity.
Testing Items
The principal measured and derived quantities in shear strength parameter determination include:
- Total stress cohesion and total stress internal friction angle
- Effective stress cohesion and effective stress internal friction angle
- Undrained shear strength of saturated fine-grained soils
- Unconfined compressive strength
- Deviator stress at failure and axial strain at failure
- Stress-strain and volumetric strain behavior
- Pore water pressure at failure and Skempton pore pressure coefficient B
- Residual shear strength parameters for pre-sheared or slip surfaces
- Soil sensitivity, defined as the ratio of undisturbed to remolded undrained strength
Testing Methods
Direct Shear Test
The direct shear test, performed in accordance with ASTM D3080, ISO 17892-10, or GB/T 50123, confines a specimen in a rigid split box and shears it along a predetermined horizontal plane while a constant normal stress is applied vertically. A series of at least three, and preferably four to five, identical specimens is tested at different normal stress levels, typically 50, 100, 200, and 300 kilopascals, selected to bracket the design stress range. Shear stress is plotted against normal stress at failure, and a best-fit straight line through the failure points yields the cohesion as the intercept and the internal friction angle as the slope of the envelope.
Three test modes are distinguished by drainage conditions. The quick test, corresponding to unconsolidated-undrained conditions, applies shear immediately after the normal load. The consolidated quick test, corresponding to consolidated-undrained conditions, allows consolidation under the normal load before rapid shearing. The slow shear test, corresponding to consolidated-drained conditions, shears the specimen slowly enough to ensure full drainage. The direct shear test is simple, fast, and well suited to sands, but it imposes the failure plane, produces non-uniform stress distributions, and does not permit measurement of pore water pressure or precise drainage control.
Triaxial Compression Test
The triaxial compression test, conducted per ASTM D2850, ASTM D4767, ASTM D7181, ISO 17892-8, ISO 17892-9, and GB/T 50123, is the most rigorous laboratory method for determining shear strength parameters. A cylindrical specimen encased in an elastic membrane is subjected to a cell pressure as the minor principal stress, while an axial deviator load increases the major principal stress until failure. At least three specimens are sheared under different confining pressures, and the family of Mohr circles at failure defines the strength envelope.
The unconsolidated-undrained triaxial test applies cell pressure without allowing drainage and shears at a constant strain rate, yielding the undrained shear strength used in short-term, total stress analysis of saturated clays. The consolidated-undrained triaxial test first saturates the specimen using back pressure until the B-value reaches at least 0.95, consolidates it isotropically or under anisotropic stress conditions, and then shears undrained while measuring pore water pressure, allowing simultaneous determination of total stress and effective stress parameters. The consolidated-drained triaxial test shears at a strain rate slow enough to maintain zero excess pore pressure, providing effective stress parameters directly, with test durations that may extend to several days for low-permeability clays. Failure is defined at the peak deviator stress, the peak principal effective stress ratio, or 15 percent axial strain if no peak is reached.
Unconfined Compression Test
The unconfined compression test, performed in accordance with ASTM D2166, ISO 17892-7, or GB/T 50123, applies an axial load to a cylindrical cohesive specimen without lateral confinement until failure or a defined strain limit. The unconfined compressive strength is the maximum axial stress recorded, and for saturated clays under short-term conditions, the undrained shear strength is taken as one half of the unconfined compressive strength, reflecting the assumption of a zero internal friction angle in undrained analysis. The test is rapid and economical but is applicable only to cohesive soils that can stand unsupported and is highly sensitive to sample disturbance.
Field Vane Shear Test
The field vane shear test, standardized under ASTM D2573 and the ISO 22476 series, is an in-situ method for measuring the undrained shear strength of soft, saturated clays. A four-bladed rectangular vane is pushed to the target depth and rotated at a controlled rate, and the measured torque at peak resistance is converted to undrained shear strength. Continued rotation provides the remolded strength and hence the sensitivity of the clay. Measured vane strengths are commonly corrected for plasticity effects in stability design, and the test is particularly valuable in very soft deposits where high-quality sampling is impractical.
Residual and Ring Shear Testing
For landslide slip surfaces, bedding planes, and pre-sheared fissured clays, the residual strength is of primary importance because it represents the minimum strength available after large displacement. Multiple reversal direct shear tests or dedicated ring shear apparatus are used to impose large cumulative shear displacements and measure the residual friction angle, which is essential for reactivation analysis of existing landslides.
Data Interpretation and Reporting
Laboratory reports should include full specimen identification, sampling method and sample quality class, index properties, initial and post-test dry density and moisture content, consolidation pressures and B-values, stress-strain curves, pore pressure response, Mohr circles with the fitted failure envelope, and the derived cohesion and friction angle values with the regression quality. Any anomalies, signs of sample disturbance, curvature of the envelope, or deviations from standard procedure must be clearly documented so that designers can apply appropriate judgment.
Applications
Determined shear strength parameters are applied throughout geotechnical design and construction, including:
- Bearing capacity of shallow and deep foundations, in which cohesion and friction angle enter the bearing capacity factors in Terzaghi, Meyerhof, and Hansen-type solutions
- Slope stability analysis using limit equilibrium methods such as Bishop, Janbu, and Morgenstern-Price, employing undrained strengths for rapid drawdown and construction stages and effective stress parameters for long-term conditions
- Earth pressure calculation for retaining walls, basement walls, and sheet pile structures using Rankine or Coulomb theory, in which lateral pressure coefficients are functions of the friction angle and wall-soil interface friction
- Excavation and braced cut design, including base heave and overall stability checks governed by undrained shear strength
- Embankment and earth dam design, including staged construction control using undrained strength and long-term stability using effective stress parameters
- Landslide investigation and mitigation design using residual shear strength parameters along slip surfaces
- Ground improvement verification and comparison of pre-treatment and post-treatment strength
- Axial capacity of piles in clay using total stress alpha methods based on undrained shear strength profiles
FAQ
What is the difference between the direct shear test and the triaxial compression test?
The direct shear test is simpler and faster, shears the specimen along a fixed horizontal plane, and does not allow measurement of pore water pressure, whereas the triaxial test applies controlled confining pressure, permits saturation and consolidation control, measures pore pressures, and defines the failure plane naturally within a more uniform stress field. Triaxial testing is preferred when reliable effective stress parameters or undrained strengths are required; direct shear remains widely used for sands, fills, and routine projects due to its economy.
How do I choose between UU, CU, and CD triaxial tests?
Select the test mode according to the drainage conditions anticipated in the field. Use UU tests for short-term stability of saturated clays loaded rapidly, CU tests with pore pressure measurement when both total and effective stress parameters are needed for general purposes, and CD tests when long-term, fully drained strength parameters govern, such as in slow slope failures or drained foundations on free-draining soils.
How many specimens are required to determine the strength envelope?
A minimum of three specimens tested at different normal or confining stresses is required by most standards to define a straight-line envelope, while four to five points are recommended in practice to improve regression reliability and reveal envelope curvature. Specimens in one series should be identical in density, moisture, and stress history.
When should total stress parameters be used and when should effective stress parameters be used?
Total stress parameters and undrained shear strength apply to short-term conditions immediately after loading in saturated fine-grained soils, before consolidation-induced dissipation of excess pore pressure. Effective stress parameters apply to long-term drained conditions, free-draining granular soils, and any analysis in which pore pressures can be estimated independently.
How does sample disturbance affect measured strength?
Disturbance destroys natural soil structure, reduces the measured undrained shear strength, and may artificially alter cohesion and friction angle. Poor sampling, transport vibration, delayed testing, and careless trimming can lower measured strengths significantly and produce unconservatively low or misleading parameters, which is why sample quality classification and careful handling are mandatory.
Which standards govern shear strength parameter determination in China and internationally?
In China, GB/T 50123 specifies laboratory direct shear, triaxial, and unconfined compression procedures, while GB 50021 governs sampling and in-situ testing requirements. International practice commonly follows ASTM D3080, D2850, D4767, D7181, D2166, and D2573, or the ISO 17892 series for laboratory testing and ISO 22476 for field testing. Accredited third-party laboratories select the applicable standard based on project specifications and jurisdiction.
