Module 12 · Lesson 12.3

Strength tests

Shear box, triaxial UU/CU/CD, and in-situ tests — what each yields.

Why this matters

Strength is measured by shearing a sample and watching it fail. How the sample is drained during shearing decides whether you get the undrained strength $s_{u}$ or the effective parameters $c^{'}$ , $φ^{'}$ — so the triaxial test type is chosen to match the field drainage condition of Module 11.

The direct shear (shear box) test forces failure on a horizontal plane and gives $φ^{'}$ (and $c^{'}$ ) straightforwardly for drained conditions — simple, but the drainage and stress state are not well controlled.

The triaxial test is the workhorse: a cylindrical sample under a cell pressure, sheared axially, with drainage and pore pressure controlled and measured. Its three variants map onto the field cases:

UU (unconsolidated–undrained) — no drainage at any stage; gives the undrained strength $s_u$ ( $φ_{u} = 0$ ) for the short-term condition.
CU (consolidated–undrained), with pore-pressure measurement — consolidated first, then sheared undrained while measuring $u$ ; gives the effective parameters $c^{'}$ , $φ^{'}$ and the undrained strength, efficiently.
CD (consolidated–drained) — consolidated, then sheared slowly enough to stay drained; gives $c^{'}$ , $φ^{'}$ directly, but is slow for clays.

In-situ tests avoid sampling disturbance and test a larger volume of ground:

the field vane gives the undrained strength $s_{u}$ of soft clay directly;
the SPT and CPT (Module 3) are correlated with density and strength, giving continuous profiles fast.

Each in-situ test trades a controlled stress path for realism and volume — a different balance from the laboratory.

Check yourself

You need the effective strength parameters c′ and φ′ for a long-term slope analysis, from clay that would take weeks to shear drained. Which triaxial test is most efficient?

Practice

A field vane in soft clay records a shear strength directly. If a UU triaxial on the same clay fails at σ1 = 180 kPa and σ3 = 100 kPa, what undrained shear strength s_u does it give, in kPa?

Summary

Shearing a sample measures strength; the drainage during shear decides whether you get s_u or c′, φ′.
Triaxial: UU → s_u (short term); CU with u → c′, φ′ and s_u (efficient); CD → c′, φ′ directly (slow on clay).
In-situ tests (vane, SPT, CPT) trade a controlled stress path for realism and a larger tested volume.
Procedures and correlations are standard-dependent — verify against the current edition.

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This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint