Undrained Shear Strength Determination: Testing Methods, Standards, and Applications

Undrained shear strength determination is a fundamental requirement in geotechnical investigation, providing the key design parameter for short-term stability, bearing capacity, and earth pressure analysis of cohesive soils. This article explains how undrained shear strength (Su) is measured in the laboratory and in the field, covering unconsolidated-undrained (UU) triaxial testing, unconfined compression, consolidated-undrained (CU) triaxial testing, vane shear testing, and CPT-based correlations in accordance with ASTM, BS, and ISO standards. Engineers, laboratory managers, and consultants can use this guide to select the appropriate test, interpret the results correctly, and apply them confidently to foundation, excavation, embankment, and slope design.

Technical Overview

Undrained shear strength, commonly denoted as Su or Cu, is the shear resistance that a saturated cohesive soil mobilizes when it is loaded faster than excess pore water pressure can dissipate. Because clays and plastic silts have very low permeability, most construction loads in these materials produce an undrained response in the short term, making Su the governing strength parameter for end-of-construction analysis. In total stress analysis this behavior is represented by the phi-equals-zero concept, in which the undrained strength envelope is treated as horizontal and Su is independent of the applied total confining stress within the range of engineering interest.

Undrained shear strength determination is performed using both laboratory tests on recovered samples and in-situ tests that measure strength directly in the ground. The principal standardized methods include the unconsolidated-undrained (UU) triaxial test, the unconfined compression test, the consolidated-undrained triaxial test with pore pressure measurement, the field and laboratory vane shear tests, and empirical correlations with cone penetration test (CPTu) results. Each method has specific applicability, limitations, and correction requirements, and high-quality undisturbed sampling is a prerequisite for reliable laboratory results.

• ASTM D2850 — Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils
• ASTM D2166 — Unconfined Compressive Strength Test of Cohesive Soil
• ASTM D4767 — Consolidated-Undrained Triaxial Compression Test for Cohesive Soils with Pore Pressure Measurement
• ASTM D2573 — Field Vane Shear Test in Saturated Fine-Grained Soils
• BS 1377 Parts 7 and 8 — Laboratory vane shear and triaxial testing procedures
• ISO 22476-1 and ASTM D5778 — Cone penetration testing used for Su correlation

Testing Samples

Undisturbed Natural Samples

High-quality undisturbed samples are essential because sampling disturbance reduces the measured strength and can make results unconservative. Cohesive samples are typically recovered using thin-walled Shelby tubes (ASTM D1587), stationary piston samplers, or carefully hand-cut block samples, targeting sample quality classes 1 to 2. Tubes are sealed on site, kept vertical, protected from vibration and temperature change, and tested as soon as practicable. Specimens are extruded and trimmed to the required diameter-to-height ratio, with visible disturbed or fissured material discarded and the moisture content, density, and degree of saturation recorded for every specimen.

Remolded and Reconstituted Samples

Remolded specimens are prepared by thoroughly kneading or compacting the soil at its natural or specified moisture content without changing the water content. Remolded strength is required to calculate sensitivity, an index of how much strength is lost when a sensitive clay is disturbed, and reconstituted samples are used in research and in the calibration of correlations between index properties and strength.

Field Test Locations

For in-situ methods, testing positions are selected within boreholes or CPTu soundings distributed across the investigation area and through the full depth of the critical strata. Test depths are logged precisely so that the measured Su can be paired with the correct effective overburden stress, which allows normalization of the strength profile and comparison with regional experience.

Testing Items

Depending on the project requirements and the selected method, an undrained shear strength determination program may report the following items:

• Peak undrained shear strength Su of the intact soil at each test depth
• Unconfined compressive strength qu, with Su taken as qu divided by 2
• Remolded undrained shear strength and sensitivity St, defined as the intact strength divided by the remolded strength
• Undrained strength at multiple confining pressures to verify the saturated phi-equals-zero condition
• Total stress parameters cu and phi-cu and effective stress parameters c-prime and phi-prime from CU triaxial testing
• Pore pressure coefficients and the normalized strength ratio Su divided by effective overburden stress
• Strength anisotropy, comparing compression, direct simple shear, and extension modes where relevant
• Consistency classification based on Su, for example soft for 12.5 to 25 kPa, firm for 25 to 50 kPa, stiff for 50 to 100 kPa, and very stiff for 100 to 200 kPa

Testing Methods

Unconsolidated-Undrained (UU) Triaxial Test

In the UU triaxial test, a sealed specimen is placed in a triaxial cell, a confining pressure is applied without allowing consolidation, and the specimen is sheared at a rapid axial strain rate, typically on the order of 1 percent per minute, so that the test is completed before any meaningful drainage occurs. For a saturated specimen the failure envelope in terms of total stress is horizontal, and Su equals one half of the deviator stress at failure. Testing identical specimens at several confining pressures should produce essentially the same Su for saturated soil; significant scatter indicates gas in the voids, sampling disturbance, or fissuring. The UU test is the reference laboratory method for soft to firm saturated clays in short-term design.

Unconfined Compression Test

The unconfined compression test is a special case of the UU test in which the confining pressure is zero. An intact cylindrical specimen is loaded axially at a rapid rate until failure or a defined strain limit, giving the unconfined compressive strength qu, and Su is taken as qu divided by 2. The method is quick and economical and is widely used as an index of consistency, but it is appropriate only for cohesive soils stable enough to stand unsupported, and it is not suitable for fissured, varved, organic, or very soft and disturbed samples where the result may significantly underrepresent the field strength.

Consolidated-Undrained (CU) Triaxial Test with Pore Pressure Measurement

The CU test first saturates the specimen using back pressure until a B-value of at least 0.95 is achieved, consolidates it under an isotropic or anisotropic stress state representative of the field, and then shears it undrained while continuously measuring pore water pressure. Shearing rates are slow enough, commonly well below 1 percent axial strain per hour, to ensure equalization of pore pressure within the specimen. The test yields both effective stress parameters and the undrained strength at the consolidation stress, making it the preferred method when both short-term and long-term analyses are required and when a normalized strength profile Su divided by effective consolidation stress is needed.

Field and Laboratory Vane Shear Test

The field vane test (ASTM D2573) advances a four-blade rectangular vane to the test depth, allows a short rest period, and rotates the vane at a slow standard rate while recording torque. The undrained strength is computed from the peak torque using the vane geometry, with Su equal to T divided by the quantity pi times d squared times the quantity h over 2 plus d over 6 for a standard rectangular vane. Continuing rotation to the residual condition provides the remolded strength and therefore the sensitivity. Vane results in plastic clays are customarily reduced by the empirical Bjerrum correction factor, which depends on plasticity index, and the method is most reliable in soft to firm homogeneous clays. Laboratory vane, fall cone, torvane, and pocket penetrometer tests provide rapid supplementary strength measurements on tube samples and extruded material.

In-Situ Correlations with Cone Penetration Testing

The CPTu provides a continuous profile from which Su is estimated as the net cone resistance divided by the cone factor Nk, with Su equal to qt minus the total overburden stress, all divided by Nk. The cone factor, typically in the range of 10 to 20 and commonly near 15, is calibrated against local laboratory or vane data. CPTu correlations are especially valuable in soft, layered, or heterogeneous deposits where sample recovery is difficult, and they allow statistical treatment of spatial variability over the full depth of interest.

Applications

Undrained shear strength determination underpins a wide range of geotechnical design and construction activities:

• Short-term bearing capacity of spread footings and mats on clay, using a bearing capacity factor Nc of approximately 5.14 for a surface footing multiplied by Su
• Stability of excavations, supported cuts, and trenches, including base heave checks against uplift of soft strata
• Short-term slope stability analysis and landslide evaluation in saturated cohesive soils
• Axial pile capacity in clay using total stress methods, with shaft friction estimated as alpha times Su
• Stage construction of embankments on soft ground, with predicted and monitored strength gain during consolidation
• Design of diaphragm walls, secant piles, and tunnels in clay under undrained loading conditions
• Verification of ground improvement works such as preloading, vacuum consolidation, and soil mixing
• Offshore and marine investigations, where strength profiles from CPTu and laboratory testing feed into platform, pipeline, and anchor design

FAQ

What is the difference between drained and undrained shear strength? Drained strength applies when excess pore pressures have fully dissipated and is expressed in terms of effective stress, whereas undrained strength applies to rapid loading of low-permeability soil in which pore pressure cannot dissipate. Short-term loading of saturated clay is analyzed with Su, while long-term conditions are analyzed with effective stress parameters.

Which test should be selected for undrained shear strength determination? For soft saturated clays, the UU triaxial test and field vane test are the standard choices, supplemented by CPTu profiling. Where both short-term and long-term parameters are needed, the CU triaxial test with pore pressure measurement is preferred. The unconfined compression test serves as a rapid index method for intact, non-fissured samples.

How is Su obtained from an unconfined compression test? The unconfined compressive strength qu is halved to give Su, because the total stress failure circle for an unconfined specimen has a radius equal to qu divided by 2 under the phi-equals-zero assumption.

Why do vane shear results sometimes need correction? Field vane strengths in plastic and overconsolidated clays tend to overestimate the operative field strength, so the Bjerrum correction, which decreases with increasing plasticity index, is applied before the values are used in stability analysis.

What is soil sensitivity and why is it measured? Sensitivity St is the ratio of intact to remolded undrained strength. Values below about 8 indicate low to moderate sensitivity, while highly sensitive or quick clays can lose most of their strength upon disturbance, which is critical for excavation, slope, and construction planning in such deposits.

How does sample quality affect the results? Sampling and handling disturbance reduce the measured undrained strength and stiffness, typically producing unconservative design values. Thin-walled or piston sampling, careful sealing, upright transport, minimal storage time, and strict trimming quality control are required for representative results.

What rate of loading is used and why? Undrained tests are sheared rapidly, on the order of 1 percent strain per minute for UU and unconfined testing, so that no significant drainage occurs during shear, while CU testing is sheared slowly to allow pore pressure measurement. Because Su is rate dependent, standardized rates must be followed for comparable results.