CBR and subgrade

The subgrade is the foundation of every road. Its strength decides how thick the pavement above it must be: a weak subgrade needs thick, costly layers; a strong one needs much less. In Indian practice, subgrade strength is described mainly by the California Bearing Ratio (CBR), and in mechanistic design by the resilient modulus derived from it.

The CBR test pushes a 50 mm plunger into compacted soil in a mould at a standard rate and compares the load at 2.5 mm and 5 mm penetration with the load needed on a standard crushed stone. For design, specimens are compacted at the expected field density and moisture and usually soaked for four days to represent the worst wet condition. Typical values range from 2 to 3 percent for soft clays, 5 to 10 for silts and sandy clays, and 15 or more for good granular soils.

Designers choose a design CBR from several tests along the alignment, often a high percentile of values so that most of the road is adequately designed. Where the CBR is very low, options include replacing the top 500 mm with better fill, stabilising with lime or cement, using geosynthetics, or raising the embankment above the water table. The subgrade must be compacted to a high relative density, since its strength in service depends on density and moisture control. In mechanistic design, the resilient modulus is related to CBR by empirical equations, with different forms below and above about 5 percent.