What is a sub-base?
A sub-base is a compacted layer of aggregate material, typically MOT Type 1, laid directly onto prepared ground beneath paving to distribute weight and prevent movement.
The sub-base is the lowest structural layer in a paved surface. It sits directly on prepared ground and is usually made from MOT Type 1 aggregate, a mix of crushed stone and binding dust that compacts firmly when rolled. This layer typically measures 100-200mm thick depending on the site's soil conditions and expected traffic loads.
Its primary job is to spread the weight of vehicles or foot traffic evenly across the ground beneath, preventing concentrated stress that would cause cracking or ruts in the paving above. Without a proper sub-base, movement in the underlying soil transmits straight up to the surface, causing uneven settling, potholes, and premature failure of the asphalt or concrete finish.
In Cheltenham, where clay soils and water drainage vary across properties, a well-constructed sub-base protects against the ground heave and subsidence that poor drainage can trigger. Contractors preparing a driveway or car park must compact the sub-base in layers to achieve the density needed to prevent movement over time. The quality of this foundation work often determines how long a surface will last, making it a critical (though hidden) part of any paving job.