Drainage and Filter Materials: Selection Criteria and Terzaghi's Rule
A drainage layer must pass water and hold back soil. Learn Terzaghi's filter criteria, material cleanliness and how to place French drains and geotextiles.
4.75–12.5 mm
Drainage & Filtration
Drainage and filtration
Single-size gravels pass water and hold back fine particles.
The role of materials in Drainage and filtration
In drainage layers the aim is to pass water and stop fine particles migrating. This takes materials with enough voids that stay stable in contact with water.
Filter grading is designed from the adjacent soil, so that permeability is kept and the layer does not clog over time.
Selection criteriaWhat matters
Three deciding criteria
- 1
Permeability
A narrow size range to preserve the voids between particles.
- 2
Cleanliness
No fines that could block the path of water.
- 3
Stability
Sound particles that resist breaking down in contact with water.
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