Highly plastic soils, organic materials, very soft or saturated soils, soils with high shrink-swell potential, expansive slag, and low subgrade strength (California Bearing Ratio or resilient modulus) are examples of materials that must be identified to provide proper subgrade design. Developing a suitable subsurface exploration and laboratory testing program is essential to evaluating and recommending the proper subgrade design parameters and treatment methods.
The use of subgrade undercuts and backfilling with properly compacted soil or aggregate is a typical method of providing treatments to improve subgrade performance. Depending on the conditions (very soft or saturated soils), it is possible that the subgrade material can be aerated and re-compacted to the recommended density. The use of chemical stabilization (lime or cement) is also an alternative that reduces or replaces the amount of subgrade undercutting. Geosynthetic materials can also be used to improve the strength of the pavement section to allow for less subgrade treatment.