In response to last month's article, Challenging the Myths of Permeable Pavement, David M. Quinn, LEED AP BD+C, technical representative for Angelus Block Co. Inc., sent the following letter to the editor "to clarify that the "myths' and inconsistencies written of are clearly addressed in the many studies and research papers that are available on PICPs.

It would be recommended that readers and editors of Landscape Online view these resources (found at INSERT) as a first step to dispelling confusion. All of these references document structural support provided by concrete pavers and permeable jointing and bedding aggregates over a compacted, open-graded aggregate base (typically 4 inches of ASTM No. 57) and subbase (typically ASTM No. 2, 3 or 4) for vehicular applications. The thickness of the subbase varies with the structural and hydrologic design requirements for the site. For residential and driveway applications, ICPI recommends not using a subbase and simply thickening the base to accommodate structural support and water storage requirements.

The reason why ICPI recommends a 4-inch base layer of ASTM No. 57 aggregate is to prevent the smaller bedding stone from entering the large voids in the subbase. ICPI recommends using larger sized aggregates for the subbase in vehicular areas due to its stability during construction and use compared to the smaller No. 57 aggregates. Geotextiles on the subgrade are specified (or not) at the discretion of the designer. If they are specified, selection should be based on the methods presented in AASHTO M-288.

Interlock is the inability of a concrete paver to move independently of its neighbors. Interlock is the means by which wheel loads are spread to reduce stresses that delay rutting on flexible pavements. Interlock within PICP interlock is not as great as that within interlocking concrete pavement (ICP) constructed with sand joints and bedding typically over a dense-graded aggregate base. Nonetheless, there is some load-spreading interlock within PICP as noted in the above references. With a correctly designed structure, PICP provides support for heavy trucks as well as for automobiles.

Unlike ICP, PICP is built to withstand rutting over soils saturated for days at a time. For that reason, PICP is designed to take less traffic over its life than ICP, which is not subject to such saturation. Mr. Baloglou's question of which jointing aggregate size (No. 8, 89, or 9) provides more interlock, or lateral transfer of vertical loads, to reduce stress and rutting is not a pressing one. Most rutting in PICP occurs in the soil subgrade or the base/subbase. Therefore, (the) extent of subbase and base compaction, as well as that on the permeable pavers and bedding aggregates, is significantly more important in reducing rutting than jointing stone selection."

Read the article and other comments at www.landscapearchitect.com/research/article.php/27723