Soil Mechanics / module 7
Water in soil and Darcy's law
Heads and hydraulic gradient, Darcy's law, seepage velocity and layered permeability — how water moves through the ground.
What you will understand
- Define elevation, pressure and total head, and explain why flow follows total head
- State and apply Darcy's law for velocity and flow rate
- Distinguish the Darcy velocity from the true seepage velocity
- Compute the equivalent permeability of layered soils, parallel and normal to the layers
- Explain what a constant-head and a falling-head test measure
Lessons
Heads and hydraulic gradient
Water flows from high total head to low — not from high pressure to low.
Darcy's law and seepage velocity
The linear law for flow through soil, and why the water moves faster than it looks.
Layered soils and permeability tests
Flow along the layers versus across them, and how k is measured.
Module summary
- Flow follows total head (elevation + pressure head), driven by the hydraulic gradient i = Δh/L.
- Darcy's law q = kiA; the seepage velocity vs = ki/n is larger than the Darcy velocity v = ki.
- Layered permeability: horizontal favours the permeable layer, vertical is throttled by the least permeable.
- Constant-head and falling-head tests measure k for permeable and finer soils respectively.
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 constant-head test gives Q = 120 cm³ in t = 60 s, sample L = 10 cm, area A = 20 cm², head h = 5 cm. What is k in cm/s?
Check yourself
Why is the seepage velocity larger than the Darcy velocity?