Undrained Shear Strength (Su): Meaning, Units, and Tests

Practical Overview

Undrained shear strength, $s_u$, is the short-term shear resistance of saturated fine-grained soil when loading occurs faster than excess pore-water pressure can dissipate. It is commonly expressed in kPa, psf, or tsf and is central to construction-stage stability of clays, short-term bearing checks, embankment loading, excavations, and slope evaluation.

This article is written for preliminary engineering understanding, study, field review, and calculation checking. It should not be used as a substitute for project-specific subsurface exploration, local code requirements, or review by a licensed professional engineer.

Su, Cu, and the Total-Stress Model

The symbols $s_u$ and $c_u$ are both used for undrained shear strength. The notation $s_u$ is clearer because undrained strength is not a permanent effective-stress cohesion intercept. For saturated clay in an idealized total-stress analysis, the undrained friction angle is often taken as zero and failure strength is written:

This is a short-term model. Long-term stability is normally evaluated using effective-stress parameters $c^{\prime}$ and $\phi^{\prime}$ with an appropriate groundwater and drainage model. Undrained and effective-stress parameters should not be mixed casually in one strength envelope.

How Undrained Shear Strength Is Measured

No test produces a universally correct $s_u$. The selected value should match the loading mode, stress history, direction of shearing, strain rate, sample quality, and design problem.

For an unconfined compression test, the idealized relationship is:

Worked Unconfined Compression Example

Suppose an intact saturated clay specimen fails in unconfined compression at $q_u=120\ \text{kPa}$. Under the idealized interpretation:

The result suggests stiff consistency by a commonly used descriptive correlation. It does not automatically become the design value. Sample disturbance, fissuring, anisotropy, strain rate, stress history, and the governing shearing mode still need review.

Typical Su Ranges and Soil Consistency

The following ranges are useful only as broad descriptive context. Classification boundaries vary among references, and design must use project-specific measurements and judgment.

Strength Variation With Depth and Stress History

Normally consolidated clay often shows increasing undrained strength with effective overburden stress. Overconsolidated crusts may be stronger near the surface and weaker below. A single average value can therefore misrepresent both shallow and deep failure mechanisms.

Plot measured and interpreted $s_u$ against elevation or depth. Distinguish intact peak, remolded, and residual behavior where sensitivity or progressive failure matters. For sensitive clay, strength sensitivity may be described as:

The purpose of the example is not to create a universal design value. It shows how to organize the calculation, keep units visible, and interpret whether the result is controlled by strength, serviceability, water, construction, or uncertainty.

Field Checks And Practical Clues

• Check whether the soil is saturated or nearly saturated.
• Review $s_u$ variation with depth rather than using one value for the full profile.
• Consider anisotropy and sample disturbance.
• Use remolded or residual strength where progressive failure or sensitivity is a concern.
• Check whether compression, extension, or direct-simple-shear behavior is most relevant to the mechanism.
• Reconcile laboratory results with field vane, CPTu, boring observations, and sample recovery.

Common Mistakes

• Using unconfined compression results without considering sample disturbance or fissures.
• Ignoring strength loss in sensitive clay.
• Applying a single su value across layers with different depositional histories.
• Mixing undrained and drained parameters in one calculation.
• Treating a pocket-penetrometer reading as a precise design strength.
• Selecting only the highest test results and overlooking weak layers or disturbed zones.

How To Use This On Civil Geo Tools

The related infinite-slope and bearing-capacity calculators can be used as transparent screening worksheets after the drainage condition and strength model are understood. Do not enter $s_u$ as effective-stress cohesion while also using an effective friction angle unless a defensible model specifically supports that combination.

Engineering Interpretation

For real projects, the calculation is only one part of the decision. Review the boring logs, groundwater observations, lab data, construction sequence, drainage, loading, and consequence of poor performance. Where uncertainty is high, sensitivity checks are often more useful than a single polished number.

References And Further Reading

• Das, B. M. Principles of Foundation Engineering.
• Coduto, Yeung, and Kitch. Geotechnical Engineering: Principles and Practices.
• FHWA geotechnical engineering manuals and design circulars for soft-ground and foundation applications.
• ASTM D2166/D2166M for unconfined compressive strength of cohesive soil.
• ASTM D2850 for unconsolidated-undrained triaxial compression testing.
• ASTM D2573/D2573M for field vane shear testing.

FAQ

Can this article be used for final design?

No. It is educational and useful for preliminary screening, but final design requires project-specific data, applicable standards, and professional judgment.

Is undrained shear strength the same as cohesion?

It is sometimes labeled $c_u$, but it should not be confused with effective cohesion $c^{\prime}$. Undrained strength is a total-stress parameter tied to loading, drainage, stress history, and test conditions.

Why do different tests give different Su values?

Different tests impose different stress paths, confinement, strain rates, drainage conditions, and sample disturbance. The engineer selects or adjusts the value to represent the mechanism being analyzed.

Can Su be estimated from SPT or CPT data?

CPT and CPTu correlations are widely used with an appropriate cone factor and local calibration. SPT-based estimates are generally more approximate, especially in soft fine-grained soils.