ASTM D2850: Triaxial Compression Testing of Soils — Method & Applications
What Is Triaxial Compression Testing?
Triaxial compression testing is a fundamental geotechnical laboratory test that measures the shear strength and deformation behaviour of soil specimens under controlled confining pressure and axial loading. Unlike simple unconfined compression tests, triaxial tests apply a confining pressure (σ₃) that simulates the in-situ stress state of the soil in the ground, providing shear strength parameters far more representative of actual field conditions.
ASTM D2850 — Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils — is the primary ASTM standard for the most commonly performed triaxial test variant in geotechnical practice.
Why Triaxial Testing Is the Gold Standard for Soil Shear Strength
Soil shear strength — the fundamental parameter governing slope stability, bearing capacity, and earth retaining structure design — depends critically on the drainage conditions and consolidation state of the soil at the time of loading. Triaxial testing can replicate three different drainage/consolidation scenarios:
UU Test — Unconsolidated-Undrained (ASTM D2850)
The specimen is not allowed to consolidate under the applied cell pressure, and drainage is prevented during shear. This gives the undrained shear strength (Su) relevant for rapid loading of saturated clay — the basis for end-of-construction stability analysis.
CU Test — Consolidated-Undrained (ASTM D4767)
The specimen is fully consolidated under the cell pressure before shearing without drainage. This provides effective stress friction angle (φ’) and cohesion (c’) parameters relevant for long-term stability and most earthwork design.
CD Test — Consolidated-Drained
Full consolidation and full drainage during shear — provides drained shear strength parameters for long-term, slow loading conditions.
ASTM D2850 Test Procedure
Specimen Preparation
Specimens are trimmed from undisturbed tube samples (Shelby tubes) or prepared from disturbed material to 38 mm or 70 mm diameter, with a height-to-diameter ratio of 2:1. The cylindrical specimen is encased in a thin rubber membrane sealed to the apparatus base pedestal and cap with O-rings.
Cell Pressure Application
The specimen is placed in the triaxial cell, which is filled with water. Cell pressure (confining pressure, σ₃) is applied through the water pressure — values typically range from 50 to 400 kPa depending on the expected in-situ stress range.
Axial Loading
A constant strain rate (0.3–1% per minute for UU tests) is applied through the loading ram until the specimen fails. Force and axial displacement are recorded continuously. The deviator stress at failure (σ₁ − σ₃) and the corresponding axial strain define the failure condition.
Mohr-Coulomb Analysis
Multiple specimens are tested at different cell pressures (minimum three). Mohr circles of total or effective stress at failure are plotted, and the Mohr-Coulomb failure envelope (c and φ) tangent to the circles defines the shear strength parameters.
Industrial and Engineering Applications
In foundation engineering, triaxial shear strength parameters determine the bearing capacity of shallow and deep foundations supporting infrastructure, industrial facilities, and energy installations. In slope-stability analysis, CU and CD triaxial parameters inform factor-of-safety calculations for embankment dams, highway cuts, and natural slopes. In retaining-structure design, shear strength governs the lateral earth pressure coefficients.
Why Choose Infinita Lab for Triaxial Soil Testing?
Infinita Lab provides ASTM D2850 (UU), ASTM D4767 (CU), and CD triaxial testing for cohesive and granular soils through our nationwide accredited geotechnical testing laboratory network. Our geotechnical specialists deliver accurate, standards-compliant shear strength data for foundation and slope engineering.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is the difference between UU, CU, and CD triaxial tests? UU (unconsolidated-undrained, ASTM D2850): no consolidation, no drainage — gives undrained shear strength Su for rapid loading. CU (consolidated-undrained, ASTM D4767): consolidation allowed, drainage prevented during shear — gives effective stress c' and φ'. CD (consolidated-drained): full consolidation and drainage throughout — gives drained parameters for slow loading.
What soil types are most appropriately tested by ASTM D2850 UU triaxial? . ASTM D2850 UU testing is most applicable to saturated cohesive soils (soft to stiff clays, silts) where undrained behaviour governs short-term loading stability — immediately after embankment construction, for example. It is not appropriate for sands or gravels (which drain freely) or for long-term stability analyses.
Why are at least three specimens tested at different cell pressures in triaxial testing? Three specimens at different confining pressures provide three Mohr circles at failure. The common tangent to these circles defines the Mohr-Coulomb failure envelope, from which both cohesion intercept (c) and friction angle (φ) are determined. A single specimen provides only one point on the failure envelope — insufficient to separate c and φ contributions.
What is the significance of the undrained shear strength (Su) in geotechnical design? Undrained shear strength (Su) — obtained from UU triaxial or unconfined compression tests — is the fundamental parameter for short-term stability analysis of saturated clay foundations, embankments, and excavations. It directly determines bearing capacity (Nc × Su for strip footings), pile skin friction, and short-term slope stability factor of safety.
How does specimen quality affect triaxial test results? Sample disturbance — from tube sampling, transportation, trimming, and membrane application — reduces the effective stress and structure of the in-situ soil, typically lowering measured undrained shear strength and stiffness. High-quality Sherby tube samples with minimum disturbance are essential for reliable triaxial results. Block samples provide the best quality for sensitive soils.
