The triaxial shear test is one of the most widely used laboratory methods for determining the shear strength parameters of soils, including cohesion and the angle of internal friction. This comprehensive guide explains the testing principle, specimen requirements, test types (UU, CU, CD), standard procedures, and practical engineering applications of the triaxial compression test.
Technical Overview
The triaxial shear test, also known as the triaxial compression test, is a fundamental laboratory testing method in geotechnical engineering used to evaluate the shear strength characteristics of soil and soft rock specimens. During the test, a cylindrical soil specimen is enclosed in a flexible rubber membrane, placed inside a pressure chamber, and subjected to a constant all-around confining pressure while an additional axial (deviator) stress is applied at a controlled rate until the specimen fails.
The test is based on the Mohr-Coulomb failure criterion, which defines shear strength as a linear function of normal stress acting on the failure plane. By conducting at least three (typically three to four) identical specimens at different confining pressures, engineers can construct a series of Mohr stress circles at failure. The envelope tangent to these circles yields the two essential shear strength parameters: cohesion (c) and the internal friction angle (φ). When pore water pressure is measured during undrained testing, the effective stress parameters (c′ and φ′) can also be determined, providing a more rigorous basis for stability analysis.
Compared with the direct shear test, the triaxial test offers significant advantages, including better control over drainage conditions, complete stress state measurement, uniformly distributed stresses on the failure plane, and the ability to measure pore water pressure and volume change. These benefits make it the preferred method for critical projects requiring reliable strength parameters. The test is standardized internationally under ASTM D4767 (consolidated undrained), ASTM D2850 (unconsolidated undrained), BS 1377 Part 7 and Part 8, and ISO/TS 17892-9, as well as Chinese standard GB/T 50123.
Testing Samples
Undisturbed Soil Specimens
Undisturbed samples are recovered from the field using thin-wall samplers, piston samplers, or block sampling techniques to preserve the in-situ structure, density, moisture content, and stress history of the soil. High-quality undisturbed specimens are essential for evaluating the natural strength of cohesive soils such as clays and silts. Specimens must be carefully trimmed, protected from moisture loss, and tested as soon as possible after sampling.
Remolded and Compacted Specimens
Remolded specimens are prepared in the laboratory by compacting or statically pressing soil at a target dry density and moisture content. These specimens are commonly used for fill materials, embankment design, research programs, and quality control of engineered fills. Remolded samples allow systematic study of the influence of density, gradation, and molding water content on shear strength.
Specimen Dimensions and Preparation
Standard triaxial specimens are cylindrical, with common diameters of 35 mm, 38 mm, 50 mm, 61.8 mm, or 100 mm, and a height-to-diameter ratio of 2.0 to 2.5 to minimize end restraint effects. The maximum particle size should generally not exceed one-tenth of the specimen diameter. Preparation steps include:
- Trimming the specimen to the required dimensions with a vertical accuracy within permitted tolerances
- Weighing and measuring the specimen to calculate initial density and void ratio
- Determining moisture content using trimmings from the same sample
- Enclosing the specimen in an impermeable rubber membrane sealed with O-rings at the top cap and base pedestal
- Saturating the specimen using back pressure, if required by the test type
Testing Items
The triaxial shear test program is tailored to the drainage conditions and stress paths relevant to the engineering problem. The principal test types and measured parameters are listed below:
- Unconsolidated Undrained (UU) Test: The specimen is sheared without consolidation and without drainage. It provides total stress parameters (ccu, φcu) and is used for short-term stability analysis of saturated clays, where the undrained shear strength (Su) is often approximately constant.
- Consolidated Undrained (CU) Test: The specimen is first consolidated under the confining pressure and then sheared without drainage while pore water pressure is recorded. This test yields both total stress parameters and effective stress parameters (c′, φ′), making it the most versatile and commonly specified triaxial test.
- Consolidated Drained (CD) Test: The specimen is consolidated and then sheared slowly enough to allow full drainage, so excess pore pressure remains zero. It provides effective stress parameters (cd, φd) for long-term stability analysis of sands and clays.
- Unconfined Compression (UC) Test: A special case of the UU test with zero confining pressure, used to quickly estimate the unconfined compressive strength (qu) of cohesive soils, where Su equals qu divided by two.
- Supplementary Measurements: Pore water pressure, volume change during consolidation, stress-strain behavior, and failure strain characteristics.
Testing Methods
The general procedure for a consolidated undrained triaxial test with pore pressure measurement, the most widely used variant, is described below:
- Step 1 – Specimen Assembly: Prepare, measure, and membrane-seal the specimen, then mount it on the base pedestal of the triaxial cell with saturation-checked drainage lines and pore pressure transducers.
- Step 2 – Saturation: Apply increments of cell pressure and back pressure to dissolve air bubbles in the pore fluid, typically until the Skempton B-value exceeds 0.95, confirming full saturation.
- Step 3 – Consolidation: Apply the target effective confining pressure while allowing drainage until the primary consolidation is complete, as verified by volume change and pore pressure dissipation curves. A minimum of three specimens is consolidated at different effective stresses.
- Step 4 – Shearing: Close the drainage valves and apply axial load at a constant strain rate selected to ensure uniform pore pressure distribution (commonly 0.05% to 1.0% strain per minute depending on soil permeability). Continuously record axial load, axial deformation, and pore water pressure.
- Step 5 – Failure Identification: Continue shearing until the deviator stress peaks and then drops, or until 15% to 20% axial strain is reached if no distinct peak occurs. Record the failure mode, which may be barrel-shaped bulging in plastic soils or a distinct shear plane in brittle soils.
- Step 6 – Data Reduction: Apply area correction for barrel deformation, compute the principal stresses at failure for each confining pressure, draw the Mohr circles, and fit the strength envelope to determine cohesion and friction angle in both total and effective stress terms.
- Step 7 – Reporting: Present specimen descriptions, density and moisture data, stress-strain curves, pore pressure responses, Mohr circle diagrams, strength parameters, and failure observations in a complete test report.
Applications
The triaxial shear test provides the fundamental strength input for a wide range of geotechnical analyses and designs:
- Slope stability analysis of natural hillsides, cut slopes, and open-pit mines for both short-term and long-term conditions
- Bearing capacity evaluation for shallow foundations and pile design, where undrained and drained strength parameters govern capacity calculations
- Earth pressure calculations for retaining walls, sheet pile walls, and underground structures
- Embankment and dam design, including staged construction analysis involving consolidation-induced strength gain
- Tunneling and deep excavation support in soft ground, where undrained behavior controls immediate stability
- Liquefaction susceptibility assessment and stress-path testing for seismic design of sands
- Ground improvement verification, comparing strength before and after treatment such as densification or grouting
- Research on constitutive soil behavior, including critical state soil mechanics and anisotropy studies
FAQ
What is the difference between the UU, CU, and CD triaxial tests? The UU test permits neither consolidation nor drainage and gives total stress parameters for rapid loading conditions; the CU test allows consolidation before undrained shearing and provides both total and effective strength parameters; the CD test allows full drainage throughout shearing and yields effective stress parameters for long-term analysis.
Why is the triaxial test preferred over the direct shear test? The triaxial test controls drainage precisely, measures pore water pressure, applies stresses uniformly on known principal planes, and allows failure to occur along the weakest orientation rather than a predetermined horizontal plane, resulting in more reliable strength parameters.
How many specimens are required for one triaxial test series? At least three identical specimens must be tested at different confining pressures to define the Mohr-Coulomb failure envelope; four or more specimens improve the statistical reliability of the derived cohesion and friction angle.
What does the B-value check indicate? The Skempton B-value measures pore pressure response to an increment of cell pressure. A B-value of 0.95 or higher confirms that the specimen is sufficiently saturated, which is essential for valid pore pressure measurements and effective stress analysis.
How is the shearing rate selected? The axial strain rate must be slow enough to ensure uniform pore pressure within the specimen during undrained shearing or complete drainage during CD tests. Rates typically range from 0.003% to 1.0% per minute depending on soil permeability, specimen size, and drainage path length.
