Soil Mechanics / module 8
Effective stress and stress in the ground
Total stress, porewater pressure and Terzaghi's effective-stress principle — the single idea that governs soil strength and settlement.
What you will understand
- State Terzaghi's effective-stress principle and explain what effective stress physically is
- Compute total stress, porewater pressure and effective stress down a layered profile
- Predict how a change in groundwater or a surface load changes effective stress
- Explain upward seepage, the seepage force and the quick (boiling) condition
- Recognise the limitations of the elastic stress-distribution models used for applied loads
Lessons
Total stress, porewater pressure and effective stress
The one principle that everything else in soil mechanics rests on.
Stress profiles with a water table and a surface load
Build the three-line σ, u, σ′ diagram down a layered ground, and see what a surcharge does.
Seepage, the seepage force and the quick condition
When water flows upward, it lightens the soil — and can float it entirely.
Module summary
- σ′ = σ − u is the master principle: effective stress governs strength and settlement.
- Build stress profiles by accumulating unit weights, with hydrostatic pore pressure below the water table.
- Raising the water table or setting up upward seepage lowers effective stress; a surcharge raises it.
- At the critical gradient the effective stress vanishes and a cohesionless soil boils.
Teaching basis and review status
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 notesCheck your understanding
Practice
A uniform saturated clay has γsat = 20 kN/m³ and the water table is at the surface. What is the effective stress at 5 m depth? (kPa, γw = 9.81.)
Check yourself
Which change increases the effective stress in a soil layer?