subbase

Subbase is a layer in pavement structures that lies between the subgrade and the base course. It is typically composed of granular material or stabilized soil and is designed to provide structural support, improve drainage, and protect the subgrade from frost action and excessive deformation. The subbase distributes loads from the pavement to the subgrade, reduces settlement, and helps maintain pavement integrity during construction and service.

Materials used for subbase vary by region but commonly include natural granular aggregates such as crushed stone, gravel, and sand. In many projects an unbound subbase is used, while others employ stabilized subbases in which cement, lime, or other binders are added to create a cemented or lime-treated layer. Recycled materials, such as crushed concrete or asphalt millings, may also be used if they meet performance criteria for gradation, strength, and contaminants.

The subbase performs several functions: it provides a medium for drainage to prevent water accumulation in the pavement structure; it distributes traffic loads to the subgrade; it reduces frost-related heave in cold climates; and it offers a working platform during construction. In some designs, a bound subbase reduces permeability and increases stiffness, while an unbound subbase relies on compaction and material strength.

Construction and design considerations include thickness, which depends on traffic loads, climate, subgrade strength, and the properties of the subbase material. Materials must be free of organic matter, properly graded, and compacted to a specified density. Field verification may include compaction tests, stability or CBR tests, and permeability checks.