June/July 2008

Stormwater Management Technical Article

What I learned in Paver School

The role of permeable paver systems as a stormwater management technique

By Michelle DeLaria, M.S., Meza Construction Company

Within the LEED rating system there are many Low Impact Development (LID) suggested for stormwater quanity and quality control: grass swales, buffers, porous landscape detention, etc. While such techniques are attractive, they have limited use and effectiveness in significantly reducing runoff volume in existing, highly impervious environments.

Last fall, I attended a training course in permeable paver systems at the School for Adanced Segmental Paving in Wisconsin. The school was established by several leaders in the segmental paver industry who are dedicated to establishing standard methods and practices in this field. The class I attended included the history of paver systems, applications, benefits and a demonstration in the practice area.

While the words permeable, porous and pervious are often interchanged and Urban Drainage and Flood Control District includes pavers under the porous pavement section of best management practices, in this article they will be used as follows:  The term “permeable’ will refer to water moving through openings between pavers and aggregate.  “Porous” refers to the material and how water moves through it as is the case with porous concrete and asphalt that has voids in the material because the fines are removed. "Pervious" refers to the ability of the surface
Photo: Herringbone pattern permeable of the material to accept water.
paver project for Denver's Ritz Carlton
courtesy Meza Construction Company

Permeable pavers and porous pavement are techniques that can substantively reduce stormwater runoff volume AND provide detention capacity in highly urbanized areas. Urban Drainage and Flood Control District includes guidelines on modular block paving and other porous pavement techniques in Volume 3 of the Technical Criteria Manual. There are also several successful installation of these materials in Colorado. However, comments and concerns linger about cost, cold climate function, maintenance and plowing. If fact, at the beginning of June the Urban Drainage and Flood Control District requested a moratorium on porous concrete installations, due to rapid deterioration of porous concrete wearing surfaces at several sites.

Communities along the Front Range are increasingly regulated for the effects of excess urban runoff, and are collecting millions of dollars in fees annually to manage hundreds of millions of dollars in backlogged waterway stabilization and water quality needs. Permeable pavers and porous pavements can be part of the solution. This article addresses some of the misperceptions and how paver systems and porous pavements can used in our communities to produce a more functional, sustainable urban environment.

The first thing I learned at the paver school is that permeable paver installations with open graded aggregate systems are based on 2000 year-old Roman road construction techniques. Romans would excavate a trench and fill it with a layer of large rock on the bottom, then smaller rock, followed by a setting bed. They would then fit large stones on top of the aggregate layers for the travel surface or a "wearing course". Some sections of Roman roads are still used today with a new asphalt wearing course, although many sections have been preserved as historical remnants throughout Europe. Current road construction is based upon Roman road construction. But instead of pavers, asphalt or concrete is used as the wearing
Image: Roman Road in Pompeii course.
Photo courtesy Paul Vlaar

Pavers with Open Graded Aggregate

The open-graded aggregate system consists of aggregate layers placed and compacted to provide a stable sub-surface for heavy vehicles and point loads.  Pavers with the full 18” depth of open graded aggregate also provide detention.  The layers from bottom to top are as follows:

• Twelve inches of 1 ½ inch size, all-fractured face aggregate.  This is called #4 aggregate and is compacted in 4” to 6” lifts.
• Four inches of ¾ inch all-fracture face aggregate.  This is called #67 aggregate.  It is also called a choker course because this size rock remains on top of the 1 ½ inch aggregate and “chokes” off the top of the larger aggregate, while allowing water to flow downward into the void space of the 1 ½ inch aggregate.  The smaller rock does not sift into the voids.  This layer is compacted.
• Two inches of 3/8th inch granite “chip” material is then placed on top of the base course.  This is the setting bed.  It is important that this material be all-fracture face and not  rounded “pea gravel”.  The setting bed is not compacted. 
• Pavers are placed on top of the setting bed by hand or machine.  The joints between the blocks are filled with ¼  inch material and a compactor is run over the blocks to vibrate and “lock in” the blocks. 

Photo Left: Open-graded aggreggate base course layer.

Photo Right: Screen boards used to lay a level setting bed
Courtesy Michelle DeLaria .

Fabric, or other means of separating the aggregate layers, is not used in this system. Current research indicates that separating aggregate l ayers in infiltration systems introduces a clogging layer and caused more rapid degradation of the system. Depending on the subsoil, a geogrid may be used between the soil and base course aggregate to increase stability

Photo Above:One quarter inch aggreggate
is swept into the paver joints
Courtesy Michelle DeLaria

 

CONTINUED: Cost Benefit

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CBG /June.July 2008