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For advice on decorative surface aggregates see [[Stone]]
[[File: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.]]
[[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.]] For advice on decorative surface aggregates see [[Stone]]
Gravel used 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<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>.
Gravel used 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<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 to prevent rapid accumulation of fines from the aggregate particles in the base of the reservoir.
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>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:
Gravel with structural requirements should also meet the following criteria: