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.
Industrial applications
Base and Subbase: How Granular Layers Decide Pavement Life
Base and subbase layers spread traffic loads into the subgrade. Learn how the two layers differ, what their materials need and the key points of placing them.
The life of a pavement does not depend only on its asphalt or concrete surface. What lies beneath, the base and subbase layers, spreads traffic load gradually into the natural subgrade. Weakness in these layers usually shows up on the road surface as cracking, settlement and rutting.
The layered structure of a pavement
In a flexible pavement, wheel stress falls with depth. This is why the best materials sit in the upper layers and materials with lower requirements sit below.
| Layer | Main role | Key material property |
|---|---|---|
| Surface | Direct traffic loading | Abrasion resistance and stability |
| Base | Load bearing and stress distribution | High fractured content and continuous grading |
| Subbase | Load distribution and drainage | Permeability and good compaction |
| Subgrade | Final support | Compaction and stability |
Properties of base material
Base material has to carry high stress. Three properties matter particularly:
- Continuous grading: having every size present makes high density achievable.
- Fractured particles: broken faces increase interlock and the layer’s shear strength.
- Low plasticity: clayey fines sharply reduce bearing capacity in the presence of moisture.
The role of subbase
Besides spreading load, subbase stops subgrade fines entering the base and contributes to drainage. Requirements for this layer are more flexible than for base, which helps make economical use of regional materials.
Technical references
In Iran the specifications for these layers are mainly set by Publication 101 (General Technical Specifications for Roads), Publication 234 (Iran Highway Asphalt Paving Code) and each project’s particular specification. Among international standards, ASTM D1241 is one reference for soil-aggregate materials for these layers.
Placement notes
Final quality depends on placement as much as on materials:
- Prevent segregation during loading, dumping and spreading.
- Keep material moisture close to optimum during compaction.
- Choose the thickness of each lift to suit the roller’s capacity.
- Check the compaction of each layer before placing the next.
The full requirements of this field are on the Road construction page.
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