Soil Mechanics / module 5
Phase relationships and soil states
Solids, water and air — the void ratio, water content and saturation that describe a soil's state, and the unit weights that follow from them.
What you will understand
- Draw and label the three-phase model of a soil
- Define void ratio, porosity, water content, degree of saturation and specific gravity
- Derive n = e/(1+e) and the Se = wGs identity from the phase model
- Derive the bulk, dry, saturated and submerged unit weights
- Compute a soil's state and unit weights from measured quantities
- Place a fine soil on the plasticity scale using the liquidity index
Lessons
The three-phase model
Volumes and masses of solids, water and air, and the ratios that describe them.
The Se = wGs identity and unit weights
Derive the master identity and the four unit weights, then compute a real soil state.
Consistency limits and soil state
Atterberg limits, plasticity and the liquidity index that places a clay on its state scale.
Module summary
- The three-phase model turns simple measurements into void ratio, porosity, water content and saturation.
- n = e/(1+e) and Se = wGs are derived, not memorised.
- Bulk, dry, saturated and submerged unit weights all follow from the phase diagram; use γ′ below the water table.
- Atterberg limits and the liquidity index describe the state of a fine soil.
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 saturated soil has w = 30% and Gs = 2.72. What is its void ratio?
Check yourself
Which unit weight should be used for effective stress below the water table?