A recent series of articles by the American Society of Civil Engineers (ASCE) Committee on Claims Reduction and Management warn engineers about underground plastic stormwater detention systems and cited four failures across the country. (1, 2)
The ASCE articles cite a failed polypropylene stormwater detention system under a church parking lot that occurred four months after the installation of the full overburden. Thermoplastic creep was suggested as the initiating factor in the structural collapse. It is believed that out-of-plane deflection increased over a four-month period until the sides of the polypropylene modular boxes buckled, which resulted in the settlement of the overburden and parking lot. (1)
Expert witnesses cited numerous construction deviations from the polypropylene modular unit manufacturer’s installation specifications as the likely contributing factors to the failure. However, the court concluded that the contractor’s deviations were immaterial, and it accepted the contractor’s argument that the civil engineer specified an unsuitable product for the project. (2)
Image 1: Drone video reveals the carnage of the plastic panels and columns from the collapse. (Sept. 20, 2018. Pennlive.com. Drone footage of massive sinkhole at Tanger Outlets in Lancaster County. https://www.youtube.com/watch?v=nnI4MFJGzCM)
Currently, there is no structural design method available in the United States for buried plastic crate systems. A work item has been initiated to develop a structural design method as an ASTM standard. Professional engineers are encouraged to participate in the ASTM standards process if they wish to consider using plastic crate-style stormwater storage systems.
Plastic Stormwater System Collapses, Replacement Burns in Massive Inferno
On August 10, 2018, the parking lot of the Tanger Outlets in East Lampeter Township, Pennsylvania subsided, and the depression expanded over four days. No injuries were cited, although one person was rescued and eight vehicles in total were impacted or sank into the subsidence. (3, 4)
Upon an investigation by third party engineers, “a whole array of issues” factored into the parking lot collapse of one of the two separate underground stormwater management systems, according to the shopping center manager. Engineers decided to replace both systems rather than risk a similar parking lot collapse in the second system. (4, 5)
To further complicate matters, eleven months after the original plastic stormwater system collapse and near completion of the replacement system, on July 1, 2019, the second plastic “crate” stormwater storage system caught on fire. Thirty fire units, including the Lancaster County Foam Task Force, were required to finally extinguish the flames. The intense heat caused substantial damage to many store fronts in the Tanger Outlets. (5, 6)
Images 2 - 3: Images from local fire photographer Edmunds Photography capture the scale of the plastic stormwater storage system fire. (Source: uly 1, 2019. Edmunds Photography albums. 7-1-19 Sinkhole-plastic fire in East Lampeter TWP. https://www.facebook.com/media/set/?set=a.2272844139459494&type=3 )
The state police fire marshal could not rule out smoking or the improper use of an acetylene torch as possible cause of the fire. However, the official cause remains undetermined. (7)
FDOT-Prohibited Recycled PP Materials Typically Used in Modular Crate Systems
Manufacturer literature indicates that many modular polypropylene stormwater storage systems are made from recycled polypropylene. Although not many plastic modular crate systems are installed in Florida, a Jacksonville, Florida-based residential project was permitted in 2003 to allow a recycled propylene modular tank stormwater detention system. It is imprtant to note that current Florida Department of Transportation Standard Specifications for Road and Bridge Construction (SSRBC) Section 948 prohibits the use of post-consumer recycled plastics (PVC, HDPE, and PP). Although these crate-style systems are typically used on private projects, like churches, schools, parks, and residential projects, engineers and agencies generally prefer to use FDOT-compliant construction products.
Sources:
- Tawresey, John. September 30, 2024. Warning: Underground plastic stormwater detention systems. American Society of Civil Engineers. https://www.asce.org/publications-and-news/civil-engineering-source/article/2024/10/01/warning-underground-plastic-stormwater-detention-systems .
- Tawresey, Johm. October 1, 2024. Responsibility when specifying engineered products. American Society of Civil Engineers. https://www.asce.org/publications-and-news/civil-engineering-source/article/2024/10/01/responsibility-when-specifying-engineered-products.
- Geli, Alex. August 11, 2018. Massive Sinkhole expanded overnight at Tanger Outlets parking lot in East Lampeter Twp. com. Massive sinkhole expanded overnight at Tanger Outlets parking lot in East Lampeter Twp. | Local News | lancasteronline.com.
- Stauffer, Heather. February 7, 2019. Tanger crater was not a sinkhole; rebuilding plan OK’d.com. Tanger crater was not a sinkhole; rebuilding plan OK’d | Local News | lancasteronline.com.
- Benscoter, Jana. August 30, 2019. Lancaster outlet mall hopes to finish repairing huge parking lot hole before Christmas shopping season. com. https://www.pennlive.com/news/2019/08/tanger-outlets-hopes-to-finish-repairing-collapsed-parking-lot-before-christmas-shopping-season.html.
- July 2, 2019. Large Pit Fire at Tanger Outlets. Lafayette Fire Company. https://www.lafayettefire.com/large-pit-fire-at-tanger-outlets/.
- July 4, 2019. Update: Cause of Tanger Outlets fire deemed ‘undetermined.’ https://www.wgal.com/article/fire-reported-at-tanger-outlets-in-lancaster-county-parking-lot-sinkhole/28251609.