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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 or 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>. See [[jar test]] for on-site verification testing protocols.
Gravel with structural requirements should also meet the following criteria:
File:Particle void ratio.png|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|Higher permeability is found in larger, angular, uniformly graded aggregate. This is due to larger pore sizes and lower tortuosity. <ref name = Judge/>