In the Field: After a Surprise City Requirement, Civil Engineers Pivot on Stormwater
Midway through renovation work on Kelly Elementary School, the project was thrown an unexpected twist. The City of Carlsbad requested the school apply for a grading permit, including strict requirements for stormwater retention and water quality treatment.
“The city unexpectedly required we provide stormwater detention to reduce the post construction flow rate and meet the local hydromodification requirements.” says LPA civil engineer Rob Richardson, “The storm drain system would still discharge the same volume of water, however a detention system would need to be implemented to reduce the post construction stormwater flow rate”.
Typically, an efficient and cost-effective solution might be a detention pond or basin, but there was no land available on the dense site. Building an underground storm water storage area under a parking lot was a possible alternative, but the designers were confronted with another surprise. The Project Geotechnical Report documented groundwater at about 18 feet below the surface; instead, when the contractor’s crew began excavation, they found groundwater at 4 feet. “That’s a big difference,” Richardson says. “The system of plastic arched chambers that we proposed was no longer viable because it would float and damage the parking surface,” Richardson says.
The team pivoted to an alternate solution that consisted of a system of subterranean precast concrete vaults installed under the school’s parking lot.
Dewatering system to draw down the high ground water.
Soil spoils from excavation. (See basketball hoops as height reference... It was a lot of dirt)
The campus drainage network was redesigned to incorporate this new storm water detention system. An impermeable liner wraps the system to keep groundwater out of the chambers.
Excavation for vaults.
The heavier concrete vaults provided enough weight to resist the buoyancy from the higher groundwater, while storing more than 70,000 gallons.
Placement of vaults via mobile crane.
Completed system.
The immediate pivot in strategy efficiently addressed the unexpected stormwater requirements in the smallest possible footprint, with little disturbance to the renovation project’s larger goals. The only evidence of the extensive stormwater detention system within the finished campus are manhole covers in the parking lot.
“Most people will never know it’s there,” Richardson says.
