Fluvanna County High School, Stormwater Detention System
The new Fluvanna County High School included four buried, stackable crate-type, stormwater detention systems under deep cover. Shortly after installation in 2010, the contractor observed that the backfill material over two of the detention systems had settled several feet. SGH investigated the cause of failure.
SGH conducted a field and laboratory investigation to observe and document the condition of the failed polypropylene structures. At exploratory test pits, we observed structural failures of the crate-type units, characterized by buckling of the plate elements and vertical collapse of the units.
We performed structural calculations to evaluate the as-built loads on the system, conducted short-term buckling and creep tests to determine the capacity of the units, and completed material testing to understand material and mechanical properties. Based on our laboratory testing, we concluded that failures initiated by out-of-plane bowing of the inner lateral plates, followed by similar deformation to the end and side plates, resulting in buckling, connection failure, and ultimate collapse of the units.
SGH determined that the short-term compression strength of the units was substantially less than that reported by the manufacturer and supplier. We concluded that even at the manufacturer’s reported strength, the units were inadequate to safely support long-term loads with an appropriate safety factor. Through our material testing, we demonstrated that the recycled plastics used in the units exhibited highly variable material and mechanical properties, further compromising the structural capacity of the units.
Based on our findings, the failed structures were completely removed and replaced with an alternate system.