Moncton moves to underground stormwater detention system

A geotextile fabric was installed over the entire excavated surface to form an envelope around the system. Fill was crushed stone.

New Brunswick’s largest city adds downtown entertainment complex and remedies overloading sewer system.

To meet the city’s Zero Net Increase (ZNI) requirement for stormwater runoff at the new Moncton Downtown Centre (MDC), engineers designed an underground system that would solve a number of problems. Facing the city’s in-house design team were several obstacles, including little usable land, possible contaminates in the soil, and a high water table.

Opened in August 2018, the $104 million MDC was constructed on the former site of the Highfield Square shopping center adjacent to the CN/VIA Rail Station in the heart of downtown Moncton, New Brunswick. To prepare for this redevelopment, a stormwater detention management plan and system was required.

To restrict the flow of stormwater leaving private property and avoid overloading storm sewers, several years ago the City of Moncton adopted regulations that require post-construction stormwater volumes remain equal to the pre-construction amount. To meet regulatory requirements, proposed solutions usually include construction of an open-air or underground detention system.

Originally, two open-air basins were considered to handle runoff at the new complex, but this type of structure requires a large footprint, while the space allocated was already limited and was planned for other surface developments, including an ice rink and music pavilion. In addition, due to past industrial land use and the high water table at that location, installation of a watertight detention system was an important criterion in the selection of the solution.

“Urban redevelopment tends to replace natural pervious surfaces with impervious surfaces, including rooftops, parking lots, and plazas,” said Darryl Bonhower, P.Eng., project engineer/ingénieur de projets for the City of Moncton. “Adding impervious surfaces increases both the peak flow and volume of stormwater runoff. ZNI for stormwater requires that post-development peak flows do not exceed pre-development peak flows for a wide range of rainfall events from the two-year storm to the 100-year storm event. ZNI requirements indicated that approximately 1,600 cubic meters of onsite detention storage was required for this project.”

Bonhower decided to use an underground watertight detention system that would use high-density polyethylene (HDPE) pipe in diameters up to 48 inches. From the design stage, the diameter of the pipe was adapted to the soil and its characteristics, making it possible to construct a tailor-made underground detention system that complies with the ground topography.

“Initially, conventional detention basins were considered for the MDC,” he said. “The volume of onsite detention storage, the required footprint area, and public safety concerns, however, indicated that an underground detention system (UDS) would be more appropriate for the location.”

The City of Moncton evaluated several UDS technologies and products available, but at this particular site, high groundwater elevations were a concern. Recognizing the previous use of the site, there was a possibility that local soils and groundwater could contain contaminants.

“We determined that the UDS had to be a watertight system so that the impacted groundwater could not migrate into the UDS and be conveyed to the storm outfall.” Bonhower said. “This watertight requirement eliminated a number of UDS technologies and materials. In order to provide [56,503 cubic feet] of onsite stormwater detention storage, the UDS was constructed with approximately [4,400 feet] of [48-inch-diameter] HDPE pipe in 18 rows complete with headers, inspection ports, and inlet/outlet connections.”

Dan Currence, P.E., director of engineering for the Plastics Pipe Institute, Inc. (PPI) corrugated plastics pipe association division, said the HDPE pipe was the correct solution for the situation. PPI is the major North American trade association representing all segments of the plastic pipe industry.

“Pipe-grade HDPE is a highly engineered compound that is designed for long-term service in critical applications and put through a series of rigorous testing procedures before it is available for manufacturing,” he said. “For this project, that need was paramount. Whatever product would be used would need to able to stand up to any corrosive contaminants in the soil and also be strong enough to meet the load bearing requirements from above. And another significant challenge was how to keep pollutants in the surrounding soil from entering the system. The HDPE pipe would provide the flow characteristics, stand up to the harsh environment, and have a gasket that would securely seal the pipe joints. This same project built out of concrete would have required about 2.3 times more joints, increasing the risk of possible leaks.”

For the underground watertight detention system, Bonhower elected to use Solflo Max HDPE pipe from Soleno, Inc. (Saint-Jean-Sur-Richelieu) that has a bell and spigot with an O-ring gasket. During the spring of 2017, the subcontractor, Carter Excavating Ltd., installed more than 4,750 linear feet of 48-inch-diameter pipe in a trench that was 124 feet wide by 264 feet long and 12.6 feet deep. This system will store the 423,000 gallons of water during heavy rain events.

More than 40 custom-made fittings and accessories were used to construct this system in eight weeks. It includes four access chimneys attached to the pipe for inspection and maintenance, two of which will be used solely for the complex; a third will serve the plaza. A geotextile fabric was installed over the entire excavated surface to form an envelope around the system. Fill was crushed stone.

The Solflo Max pipe was manufactured locally at Soleno’s McAdam, New Brunswick plant. The corrugated HDPE pipe meets AASHTO M294 and ASTM F2306 material standards and complies with Canadian Standards Association, CAN/CSA B182.8. Fabrication of the fittings was done at Soleno’s McAdam and Saint-Jean-Sur-Richelieu plants.

As with any watertight system in high groundwater conditions, buoyancy forces causing uplift of the UDS were a concern. Bonhower worked closely with the engineering support provided by Soleno to ensure that adequate cover was provided over the UDS. Recognizing that the UDS was located beneath the MDC Plaza was another concern. A refrigerated ice surface for skating and a soundstage were located directly above the UDS. Once again, the city worked closely with engineering support provided by Soleno to ensure that the UDS could withstand the dead load.

The bedding and compaction of flexible pipe is integral to ensure the structural integrity of the UDS in order to prevent deflection of the flexible pipe. Additionally, upon installation and bedding, each joint of the UDS was inspected to ensure that there was no immediate deflection due to installation. Upon completion and backfilling, each joint of the UDS was once again inspected to ensure there was no short-term deflection during installation.

“Moncton’s first HDPE detention system, which is watertight, allows the city to ensure the durability of its infrastructure and control stormwater runoff,” said Tony Radoszewski, CAE, president of PPI. “HDPE pipe is a sustainable and environmentally responsible choice that will serve generations to come. It is ecologically friendly during manufacturing and provides peak protection from contamination during service. Strong, durable, lightweight, and flexible, HDPE pipe systems require significantly less energy to fabricate, transport, and install than metal or concrete alternatives. With superior resistance to corrosion and abrasion, plastic piping systems also supply long service life [and] excellent joint performance, adding up to exceptional value. That is why PPI can emphatically state that this was the right choice for the City of Moncton’s Downtown Centre.”

Information provided by the Plastics Pipe Institute (www.plasticpipe.com).