Module 08
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 this module covers
- 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
The one principle that everything else in soil mechanics rests on.
Start lesson →Build the three-line σ, u, σ′ diagram down a layered ground, and see what a surcharge does.
Start lesson →When water flows upward, it lightens the soil — and can float it entirely.
Start lesson →
Module checkpoint
Check what you have taken in
2 questions
Question 1
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.)
Question 2
Which change increases the effective stress in a soil layer?