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Solution de stockage réduisant les impacts hydrologiques de l’urbanisation
Dans le cadre des nouvelles installations du Complexe au centre-ville de Moncton, Nouveau-Brunswick, l’entrepreneur général, Bird Construction, devait faire aménager un bassin de rétention sans recharge de la nappe phréatique (étanche) sous l’esplanade de ce complexe entrant en fonction à l’automne 2018. Ce type de bassin permet de retenir temporairement des quantités d’eau afin de réduire au minimum l’apport aux systèmes d’égouts pluviaux municipaux en période de pointe, avant le rejet vers un exutoire. Ce sont les avantages du bassin de rétention en conduites Solflo Max non perforé qui ont motivé le choix de M. Darryl Bonhower, ingénieur à la ville de Moncton, de faire confiance aux produits en polyéthylène et à l’expertise de Soleno.
Étude de cas complète
M. Bonhower a donc opté pour un bassin de rétention étanche sous chaussée, composé de conduites en PEHD de type Solflo Max avec cloches à garniture de joint torique. Au printemps 2017, l’entrepreneur sous-traitant, Carter Excavating Ltd., a installé plus de 1 450 mètres linéaires de conduites de 1 200 mm (48 po) de diamètre dans une tranchée de 38 m (124 pi) de largeur par 3,85 m (12,6 pi) de profondeur. Ce bassin, réparti en 18 rangées de 80,6 m (264 pi) de long, permettra de stocker 1 600 m3 (56 503 pi3) d’eau pour les épisodes de forte pluie. Plus d’une quarantaine de raccords et d’accessoires fabriqués sur mesure ont permis d’aménager ce bassin en 8 semaines.
Installation d'un bassin de rétention en SolFlo Max composé de 1 471 mètres de conduites de 1 200 mm (48 po).
Les conduites en PEHD ont l’avantage de pouvoir être soudées ensemble afin d’obtenir des longueurs étanches et ainsi réduire au minimum le nombre de joints lorsque les contraintes du projet l’exigent. Ce même projet réalisé en béton aurait nécessité environ 2,3 fois plus de joints, augmentant les risques de fuites possibles.
- Philippe Losier, ingénieur chez Soleno
Philippe Losier, ingénieur, accompagné de Robert Burtt, ingénieur et directeur d’usine à McAdam, ont offert une présence et un support remarquable pendant la conception et tout au long de l’installation.
- M. Darryl Bonhower, ingénieur à la ville de Moncton
Par Darryl Bonhower, ingénieur
(version anglaise seulement)
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
