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For advice on decorative surface aggregates see [[Stone]]
Gravel for underdrains and underground infiltration structures should be 20 - 50 mm, uniformly-graded, clean (maximum wash loss of 0.5%), crushed angular stone that has a void ratio of 0.4 (40%)<ref>Porosity of Structural Backfill, Tech Sheet #1, Stormtech, Nov 2012, http://www.stormtech.com/download_files/pdf/techsheet1.pdf accessed 16 October 2017 </ref>.
The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the void ratio).
The clean wash
is to prevent rapid accumulation of fines in the base of the reservoir.
Gravel with structural requirements should also meet the following criteria: