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| − | Gravel for underdrains and underground infiltration structures should be 20-50 mm, uniformly-graded, clean, 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>. | + | For advice on decorative surface aggregates see [[Stone]]<br> | 
|  | + | Gravel for underdrains and underground infiltration structures should be 20 - 50 mm, uniformly-graded, clean, 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>. | 
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|  | + | Gravel with structural requirements should also meet the following criteria: | 
|  | + | *Maximum wash loss of 0.5% | 
|  | + | *Minimum durability index of 35 | 
|  | + | *Maximum abrasion of 10% for 100 revolutions and maximum of 50% for 500 revolutions | 
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|  | [[category:materials]] |  | [[category:materials]] |