Lightweight Drainage Aggregates

Lightweight aggregates, such as expanded or balled clay, expanded slate or shale, or other ceramic products, are an attractive alternative to drainage stone because they are much lighter. The consistency in particle size allows selection of grade or mix of grades for a  specific purpose such as allowable or required rate of compaction or rate of flow of water through the aggregate. Disadvantages are that they typically are much more costly than crushed stone or drainage mats or panels, and the composition, fabrication, and availability of the material can vary locally. The higher cost of the lightweight aggregate may need to be balanced against other factors such as the increased costs of the structure to support a heavier drainage stone or the implications of a thinner allowable planting or paving profile.

Both stone and lightweight drainage aggregates can be used to store water for recharge and later use, similar to a natural aquifer. Lightweight aggregates are fabricated according to standard grades and are more consistent in composition and the distribution of particle size. The appropriate grade must be specified; ASTM currently broadly categorizes the grades as fine, medium, or large, and grade is determined by the size and distribution of particles within an allowable range.