single-sized aggregate

The trench would then be back-filled with a porous material such as a single-sized aggregate, forming a French drain.

'Single-sized aggregate' without any binder is the most permeable paving material in existence, and the least expensive.

Single-sized aggregate without any binder, e.g. loose gravel, stone-chippings, is another alternative.

Similar related work on coarse, single-sized aggregates recommended for drainage layers in highway pavements is reviewed to illustrate the trade-offs between support compressibility and quality of aggregate defined by the aggregate index value.