Desirable properties of subgrade soil

The subgrade is the top 500 mm of natural or compacted soil that supports the pavement. Its properties decide the pavement thickness, because all the load finally reaches it.

Desirable properties

1. Adequate strength and stability - a high CBR (design CBR measured at the field density and moisture after 4 days of soaking; NRS 2070/IRC:37 use the CBR to find the pavement thickness; the minimum subgrade CBR is about 5%, and 8-10% or more is good).
2. Low volume change - low swelling and shrinkage, which means low plasticity (low liquid limit and plasticity index).
3. Good drainage - permeable soil that does not retain water; a high water table is avoided.
4. Resistance to frost action - non-frost-susceptible soil in cold areas.
5. Ease of compaction - well-graded soil that reaches a high dry density (maximum dry density and optimum moisture content) with ordinary rollers, giving good compaction (at least 97% of MDD, modified Proctor, by IRC/NRS).
6. Low compressibility - low settlement and consolidation, so the pavement does not deform.
7. Durability - strength retained in wet conditions; the soil does not soften under rain or traffic.
8. Uniformity - uniform soil along the length, so that the pavement has no differential settlement.
9. Free of organic matter and deleterious material.
10. Good modulus - a high resilient modulus, which gives less deflection under repeated loads.