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Design Ecology has listed below the critical design criteria you must consider <br />Guidelines when designing permeable pavement. Local governments can adopt <br />alternative design criteria, as long as it does not conflict with the criteria <br />listed below. For modeling guidance of permeable pavements, refer to <br />Appendix III-C of Volume III, and the 2012 WWHM User Manual. <br />You can find additional guidance for permeable pavement design in the <br />LID Technical Guidance Manual for Puget Sound (2012). <br />Note that the LID Technical Guidance Manual. or Puget Sound (2012) is <br />for additional informational purposes only. You must follow the guidance <br />within this manual if there are any discrepancies between this manual and <br />the LID Technical Guidance Manual for Puget Sound (2012). <br />Subgrade <br />Compact the subgrade to the minimum necessary for structural <br />stability. Two guidelines currently used to specify subgrade <br />compaction are "firm and unyielding" (qualitative), and 90- 92% <br />Standard Proctor (quantitative). Do not allow heavy compaction due to <br />heavy equipment operation. The subgrade should not be subject to <br />truck traffic. <br />• To prevent compaction when installing the aggregate base, the <br />following steps (back -dumping) should be followed: 1) the aggregate <br />base is dumped onto the subgrade from the edge of the installation and <br />aggregate is then pushed out onto the subgrade; 2) trucks then dump <br />subsequent loads from on top of the aggregate base as the installation <br />progresses. <br />• Use on soil types A through C. <br />Separation or Bottom Filter Laver (recommended but ontionall <br />A layer of sand or crushed stone (0.5 inch or smaller) graded flat is <br />recommended to promote infiltration across the surface, stabilize the <br />base layer, protect underlying soil from compaction, and serve as a <br />transition between the base course and the underlying geotextile <br />material. <br />Base material <br />Local governments should adopt their own minimum base material <br />requirements as they see necessary for support of flexible pavements. <br />Many design combinations are possible. The material must be free <br />draining. The municipality should determine and publish estimates of <br />the void space for each standard base material allowed in their <br />jurisdiction. <br />Volume V — Runoff Treatment BMPs — December 2014 <br />5-22 <br />