Particle void ratio. png|thumb| The highest void ratio is found in uniformly graded aggregate, as there are no smaller particles to occupy the inter-particle pores. <ref name = Judge/>]] [[File:Particle permeability.png|thumb|Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity. <ref name = Judge/>]] For advice on decorative surface aggregates see [[Stone]]
Gravel used for [[underdrains]] in [[bioretention]], [[infiltration trenches]] and [[infiltration chambers|chambers]], and [[exfiltration trenches]] 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<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 clean wash to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir. The uniform grading and the angularity are important to maintain pore throats and clear voids between particles. (i.e. achieve the
void ratio). Porosity and permeability are directly influenced by the size, gradation and angularity of the particles <ref name = Judge>Judge, Aaron, "Measurement of the Hydraulic Conductivity of Gravels Using a Laboratory Permeameter and Silty Sands Using Field Testing with Observation Wells" (2013). Dissertations. 746. http://scholarworks.umass.edu/open_access_dissertations/746</ref>
Gravel with structural requirements should also meet the following criteria:
*Minimum durability index of 35
*Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions
Standard specifications for the gradation of aggregates are maintained by ASTM [https: //www. astm. org/ Standards/ D2940.htm ASTM D 2940].