Where are you buying structural backfill these days? Is what you need readily available, or are you looking outside your area and/or waiting an extended time for delivery? With available land space continuing to decrease, stormwater detention/retention is commonly being designed underground. The current construction boom and increasing detention volume requirements have made rock scarce and expensive in many parts of the country.

Rock, gravel, and other aggregates suitable for utilization as structural backfill require extraction, transportation, and availability in the area of the job site, placing increased pressure on quarries near developing areas that may struggle to meet the demand. Engineers, developers and contractors are all asking, “How can we eliminate or reduce the amount of rock required on underground detention systems?”

Contech’s answer to these questions is based on our design flexibility. This answer lies in our ability to custom manufacture our CMP Underground Detention Systems. Contech can fully maximize the pipe size for the given depth and footprint limitations on the site and take advantage of the most cost-effective component of underground detention - void space.

Ideally, we will design to fit a solid wall system to the footprint that stores the total detention volume within the system and allows for standard, less expensive backfill. We are also not limited by standard pipe sizes – meaning we can manufacture the CMP in 1” increments if needed to take advantage of all the depth on site.

This design begins at the flow line. The pipe is sized so that only 6” of structural backfill is required between the crown of the pipe and the bottom of the rigid pavement to achieve, in most cases, a minimum height of cover of 12” to withstand traffic loads. Maximum storage volume in the pipe equals less backfill material needs.

If an open or perforated system is required to fit the footprint, a Contech still utilizes a pipe that runs from the flowline to 6” below the bottom of the concrete paving. Based on this design, the only rock required is in the space between the barrels and around the system's perimeter. This approach does not account for any storage volume above or below the pipe, reducing the overall rock quantity/costs and eliminating the need for an underdrain system.

So the answer to GOT ROCK is maybe.

Maybe you don’t need it at all.