Soil Mechanics / Laboratory and field testing
What a test must reproduce
Drainage, stress path and representativeness — why the right test matters more than a precise one.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- A geotechnical reviewer must check laboratory interpretation, classification conventions and the limits of drained, undrained and one-dimensional idealisations.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
Every design number — a strength, a settlement, a permeability — comes from a test, and a test is only useful if it puts the soil through the same conditions the ground will experience. A beautifully precise measurement of the wrong thing is worse than useless: it is misleading. Choosing and interpreting tests is where the whole course comes together.
What you should already know
- Effective stress and drainage (Modules 8, 9)
- Consolidation parameters Cc, Cr, cv (Module 10)
- Shear strength: c′, φ′, su and drained/undrained (Module 11)
Two questions decide whether a test represents the field:
- Does it impose the right drainage condition? A clay loaded quickly in the field behaves undrained; a test that lets it drain measures the wrong strength. The test's rate relative to the soil's permeability must match the field.
- Does it follow the right stress path? A soil beneath a foundation is loaded; behind a wall it is unloaded laterally; under an excavation it is unloaded vertically. Different paths reach failure differently (Module 9), so the test path should resemble the field path.
On top of these sits representativeness: the sample's quality (disturbance, Module 3), its size relative to the fabric (fissures, Module 2), and whether a handful of specimens speaks for a whole deposit.
Tests divide into two families:
- Classification (index) tests — grading, Atterberg limits, water content, specific gravity (Modules 4, 5). They identify and classify the soil and can be run on disturbed samples. They tell you what the soil is.
- Performance tests — consolidation (oedometer), strength (triaxial, shear box) and permeability. They measure how the soil behaves under stress and drainage, and generally need undisturbed samples or in-situ testing. They tell you what it will do.
Summary
- A test is meaningful only if it reproduces the field drainage condition and stress path.
- Classification tests identify the soil (disturbed samples); performance tests measure behaviour (undisturbed / in-situ).
- Representativeness — sample quality, scale relative to fabric, number of specimens — limits every result.
- Choose the test from the design question, not the other way round.