On a Permeable Paving Project

Permeable paving is the term used to describe several types of surface treatments that provide a durable and load bearing surface while allowing storm water to permeate the ground. Permeable paving is an important strategy in a low impact development program for any city. Permeable pavements form a suitable surface for walking or driving while providing spaces for runoff to infiltrate into the underlying soil and recharge the groundwater.

We had an area in our backyard that we wanted to convert to permeable paving. We wanted a permeable surface that we could drive across but that was also aesthetically attractive and cooling for the south side of our property. We started with an investigation of materials and contractors on line.

The product is a concrete paving mat that is permeable, flexible, and plantable. Pre-manufactured holes and cracks allow for root penetration through the mat into the sub soil, anchoring it firmly into place. The mats are 2 feet x 2 feet x 1.5 inches square and weigh about 45 pounds each (see picture of a mat). Once laid on a prepared bed of aggregate and sand, the only part of the mat that is visible are the tops of the individual 'muffins' or 'biscuits' of the mat. A variety of materials can be used to fill the spacesbetween the muffins, such as sand, gravel, small plants, and grass. This product is also available for the do-it-yourselfer at Home Depot. The company has a brochure that describes a number of uses for their product http:// www.soilretention.com/downloads/ DRIVE_Singles_2010.pdf

It was surprising difficult to locate a contractor that would work on a small residential project and who was familiar with the concept of permeable paving. With a lot of searching, we were finally able to locate a contractor that was willing to try this installation. The project was completed over 4 days. We chose to fill the spaces in the paving mats with grass seed since an irrigation system was already present in the project area and we wanted the cooling effect of the grass. We were very happy with the final result. Our backyard now has more useable play space and we also have an area for the entry and egress of our cars into the backyard. A brochure that describes the step by step process of installation can be viewed at http://www.soilretention.com/ articles/dg-installation.pdf.

Infiltration areas will have the following characteristics:

• They will be on slopes no greater than 20 percent.
• They will not sit uphill of a steep or moderately steep slope (anything greater than 20%).
• They will be easy to access and maintain.
• They will provide a safe exit for overflowing water.
• They will allow the water to infiltrate. Water should be gone within 36 hours of the rain; any longer and the area is not suitable for infiltration.