If more than one type of infiltration system meets your project requirements, your decision may be cost driven. It’s important to maximize the storage efficiency in the available space at the lowest possible cost.

Underground, Open-Bottomed Chambers

This example highlights the need to evaluate multiple options and configurations to find the most appropriate infiltration system for your site. Figure 4a is an example of a given site designed with a composite underground chamber system for infiltration.

In this example, site constraints require that the underground retention system fit into a 100-foot by 300-foot rectangular footprint while being able to temporarily detain as much as 88,660 cubic feet of runoff. Additionally, the site has only 5 feet of available vertical depth. Cost estimates are based on best available information in today’s market.

Perforated CMP

An alternative design to the above uses fully perforated 42-inch diameter, 16-gage CMP (Figure 4b). In the alternative design, a cost savings of 16.5% is achieved while requirements around site constraints are kept intact.

Value engineering in this example was achieved by maintaining the same volume of storage in the CMP system as the chamber system while keeping the footprint and burial depth the same. If site redesign is allowed, the storage volume of the CMP system could easily be increased by using minimum spacing requirements, therefore allowing you to fit more pipe in at 100 percent void space, instead of utilizing the stone at 40 percent void space.

This redesign would maximize the storage efficiency in the available space at the lowest possible cost. This example highlights the need to evaluate multiple options and configurations to find the most appropriate infiltration system for your site.