Downtown Centre Realization, Moncton New Brunswick

The Context

A storage solution reducing the hydrological impacts of urbanization

As part of the Centre’s new facilities in downtown Moncton, New Brunswick, the general contractor, Bird Construction, was to have a detention system built without groundwater replenishment (watertight) under the plaza of the complex, which was to open in the fall of 2018. This type of system allows for the temporary detention of large amounts of water to minimize the discharge in municipal storm sewer networks during heavy rain events, prior to discharge to an outlet. It was the advantages of the detention system using unperforated Solflo Max pipe that convinced Darryl Bonhower, an engineer with the City of Moncton, to rely on Soleno’s polyethylene products and expertise.

The Solution

Bonhower therefore opted for an underground watertight detention system, composed of Solflo Max HDPE pipes with bells with integrated O-ring gaskets. In the spring of 2017, the sub-contractor, Carter Excavating Ltd. installed over 1,450 linear metres of 1,200-mm (48 in) diameter pipe in a trench 38 m (124 ft) wide by 3.85 m (12.6 ft) deep. This system, divided into 18 rows 80.6 m (264 ft) long, will store 1,600 m³ (56,503 ft³) of water for heavy rain events. More than 40 custom-made fittings and accessories were used to construct this system in a mere 8 weeks.

A first in New Brunswick!

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City of Moncton's Challenge

By Darryl Bonhower, P. Eng.

Background

The Moncton Downtown Center (MDC) was constructed on the former site of the Highfield Square shopping center adjacent to the CN/VIA Rail station in the heart of Moncton’s downtown. In order to prepare for this redevelopment, downstream sewer systems had to be improved. Additionally, stormwater management in the form of detention storage was required in order to meet Zero Net Increase (ZNI) for stormwater runoff requirements.

Zero Net Increase for Stormwater Runoff

Urban redevelopment tends to replace natural pervious surfaces with impervious surfaces including rooftops, parking lots, and plazas. Increasing impervious surfaces increases both the peak flow and volume of stormwater runoff. Zero Net Increase (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 2-year storm to the 100-year storm. ZNI requirements indicated that approximately 1,600 m³ of on-site detention storage was required.

In order to meet the ZNI requirement, on-site detention storage is typically used. Initially, conventional detention basins were considered for the MDC. However, the volume of on-site detention storage, the required footprint area, and public safety concerns indicated that an underground detention system (UDS) would be more appropriate for the location.

Underground Detention System

The City of Moncton evaluated a number of UDS technologies and products available. However, for this particular site, high groundwater elevations were a concern. Recognizing the former use of the site, there was a possibility that local soils and groundwater could be impacted by with contaminants. In recognition of this, it was determined that the UDS had to be a watertight system so that impacted groundwater could not migrate into the UDS and be conveyed to the storm outfall. This watertight requirement eliminated a number of UDS technologies and materials. In order to provide 1,600 m³ of on-site detention storage, the UDS was constructed with approximately 1.5 kilometers of 1,200mm diameter HDPE pipe in 18 rows complete with headers, inspection ports, and inlet/outlet connections.

UDS System Design

As with any watertight system in high groundwater conditions, buoyancy forces causing uplift of the UDS were a concern. The City 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 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.

UDS Installation

The City recognizes that the bedding and compaction of flexible pipe is integral to ensure the structural integrity of the UDS system in order to prevent deflection of the flexible pipe. The City provided full-time construction inspection. Stantec provided full-time geotechnical inspection and testing. Soleno provided inspection of materials and installation as required throughout the UDS installation. Upon installation and bedding, each joint of the UDS system was inspected to ensure that there was no immediate deflection due to installation. Upon completion and backfilling, each joint of the UDS system was once again inspected to ensure there was no short-term deflection due to installation.

Closing

In recognition of the challenges to provide 1,600 m³ of onsite stormwater detention in an urbanized location where available footprint is limited and public safety is paramount; and in recognition of high groundwater conditions and the possibility of conveying contaminants off-site; a watertight UDS system constructed of HDPE pipe was the chosen to be the most appropriate technology to meet the site specific requirements.

Darryl Bonhower, P. Eng.

Project Engineer, City of Moncton