A Box with Rocks – New Underground Detention System is a Winning Design

Civil engineers are constantly looking for alternatives for site improvements that are cost effective to construct and maintain. When underground stormwater detention is needed for efficient site development, it can be costly. What could be more cost effective than a hole in the ground with a lid on top?

That is essentially the concept of the patented Stormwater Detention Systems by the GeoStorage Corporation, a company formed in 2008, to provide a cost effective alternative to traditional underground detention systems.

This system takes a page from the FHA’s integrated bridge system for single-span bridges.  It utilizes geosynthetics, stone, and reinforced concrete.  And in addition to the open chamber, the design takes advantage of the voids in the stone bearing walls for water storage .

From the GeoStorage Corporation’s sales brochure:  “A geotextile or geomembrane liner system is installed within an excavation.  Walls are constructed around the perimeter with geosynthetic reinforcement backfilled with open-graded stone to create a large underground chamber. Inlet and outlet pipes extend through the liner and wall into the open chamber.  Precast lids (or a cast in place reinforced concrete roof) is set over the chamber, supported by the perimeter walls.  The roof is precast or cast in place reinforced concrete planks that also function as a drivable surface (HS-20).  Precast appears to be the cheapest choice.  A liner system is installed over the stone surface of the perimeter walls before installing fill and/or pavement to finish grades.  The walls and internal supports are geogrid reinforced open graded stone.  Maintenance access to and silt removal from the detention volume is easy.  Site conditions, local regulations and regional commodity prices influence the relative cost of all detention systems.  However, some generalizations regarding the cost effectiveness of the GDS system can be made.  The concrete roof and stone comprise the major material cost items.  Given the stone porosity’s contribution to storage capacity, the concrete roof cost becomes even more significant. ”

The GeoStorage system offers a potentially significant cost reduction. At depths of 6-8 feet (1.83-2.44 m) GDS material costs are roughly 60% of pipe and pipe arch systems before accounting for excavation and installation costs. As the available profile on a project increases, the relative cost advantage of the GDS system improves.

Additional benefits of this system include:

  • it contributes to LEED points by reducing truck traffic to the construction site, reducing the amount of excavation and potential trucking away of excavated material
  • it can provide a reusable water supply with a water tight system. (The floor of the storage chamber is a geomembrane, permeable or impermeable.)

We are excited about this design – especially in densely populated areas like downtown Ann Arbor, Michigan.   Given new rules for stormwater detention in Washtenaw County, Midwestern Consulting will consider the GeoStorage underground storage system on all our Washtenaw County development projects.

Earl Ophoff is a registered landscape architect in the state of Michigan.  He is a senior associate and project manager at Midwestern Consulting, Ann Arbor, Michigan.  Contact Earl at 734.995.0200 or efo@midwesternconsulting.com

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