Soil Mechanics / Shear strength of soils

What gives soil its strength

Friction on the effective stress, interlocking and dilation, and the peak–critical–residual sequence.

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.
Stress-path and critical-state illustrations simplify stress anisotropy, total vs excess pressure, loading history, drainage, structured soils, strain softening and cyclic behaviour. Full specialist review remains necessary; selected corrections do not certify the models.

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 notes

Why this matters

A slope stands, a foundation holds and a wall stays up only because the soil can resist shear. Unlike steel, soil has almost no true cohesion — its strength is frictional, and friction acts on the stress carried by the grain contacts, which is the effective stress. That single idea, that strength is friction on effective stress, is the foundation of the whole subject and the reason water pressure is so dangerous.

What you should already know

Effective stress σ′ = σ − u, and that soil behaviour is governed by σ′ (Module 8)
The idea of shear stress on a plane, and normal vs shear stress
Void ratio and dense vs loose packing (Module 5)

Push one block across another and it resists with a friction force proportional to the normal force pressing them together: $T = \mu N$ . Soil is the same, grain on grain. The resistance to sliding on any plane is proportional to the effective normal stress on that plane — the part of the stress actually carried by the grain skeleton, not the porewater. Raise the porewater pressure and the effective stress falls, the grains are pressed together less firmly, and the available friction — the strength — drops. This is why a rise in water pressure can trigger a landslide with no change in the total load at all.

There is a second source of resistance in a dense soil: interlocking. Tightly packed grains cannot slide past one another without riding up and over — the soil must expand to shear, a behaviour called dilation. Lifting the grains against the confining stress takes extra work, so a dense soil shows an extra, peak strength above its basic frictional value. A loose soil does the opposite: it contracts as it shears, and shows no peak.

Shear a soil far enough and both dense and loose samples arrive at the same condition — shearing continuously at constant volume and constant stress. This is the critical state, and its strength is the critical-state friction angle $\varphi'_{cs}$ , a true material property. The behaviour goes:

Peak strength — the maximum, reached early; present only in dense/overconsolidated soils, from friction plus dilation.
Critical-state strength — the steady value at large strain, at constant volume; friction alone.
Residual strength — for platy clays only, an even lower value reached at very large displacement once the clay particles align on a polished slip surface. It governs long-term stability of old landslides.

Summary

Soil strength is frictional and acts on the effective stress — raising porewater pressure lowers strength.
Dense soils dilate (expand) to shear and show an extra peak strength; loose soils contract and do not.
At large strain both reach the critical state: constant volume, constant stress, friction angle φ′cs.
Peak > critical-state > residual; design on a strength you can rely on, often the critical-state value.