Module 11 · Lesson 11.1
What gives soil its strength
Friction on the effective stress, interlocking and dilation, and the peak–critical–residual sequence.
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: . 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 , 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.
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