The Saint-Louis Forts and Chateaux historic site is Quebec City, Quebec, Canada, has a new storm water retention system that uses corrugated high-density polyethylene (HDPE) pipe and will provide erosion protection for the next 100 years. The project involved installation in a sensitive area of the city, one that dates back to the 17th century, with minimal disturbance to the inhabitants and commerce. The use of the HDPE pipe system enabled the project engineers to overcome a number of challenges, including a difficult cliff-face terrain and the demands inherent in the preservation of centuries-old buildings. The project, located under Dufferin Terrace in Quebec City near the Chateau Frontenac, was finished in early 2011.
Back in time
Four forts and two chateaux were constructed in upper Quebec City between 1620 and 1834. The site itself has three elements: the forts, the chateaux and gardens. The first of the forts is made up of a few wooden buildings surrounded by a palisade; it was constructed by Samuel de Champlain in 1620 after his 1608 discovery of the area. Champlain later built St. Louis fort, in 1626. Charles Huault de Montmagny, who built over Champlain’s second fort during a 24-year construction project that included the first chateau, constructed a third fort in 1636. The fourth fort was built in 1693 and was followed by a new chateau the following year. Dufferin Terrace was added to the complex in 1879.
In 2005 an archaeological research project in the area uncovered significant artefacts from the 1620s. Parks Canada archaeologists and specialists found walls, platforms and cannon balls and removed 1,500m2 of the terrace’s wooden platform to reinforce the fortification wall. Parks Canada wanted a closed-in storm water retention system in order to enable part of the area under Dufferin Terrace to become a museum. In order to protect this area and make it possible for visitors to actually see and walk around the aged structures, it had to be enclosed with walls and windows. But the storm water that used to come through the Terrace can no longer go into the ground. This created an overflow of rainwater, which had to be detained before it flowed over the cliff and onto the houses and buildings below. The water has to be held and flow at a specified rate into the sewer system of the city.
A novel use of HDPE
The result of extensive work on the site under the Dufferin Terrace is that visitors – an average of 2.5 million/a – now have access to the remains of the basement of Castle St. Louis. The new storm water retention system built under the structure enables the recovery and control of runoff. The installation of the system, along with the creation of an outdoor museum, required JES Construction (Quebec City) to assemble the under-terrace retention system during November and December 2010. JES also rebuilt the steel structure supporting the Terrace.
The storm water retention system is more than 91m long and consists of four rows of 1,200mm diameter watertight and one run of 900mm diameter perforated HDPE pipe. It is capable of containing up to 125m3 of water runoff from the roofs of the buildings and surrounding areas. The pipe used in the watertight sections is Solflo Max double walled (smooth inside, ribbed outside) corrugated HDPE from Soleno, Quebec City, manufactured locally. The corrugated HDPE pipe used is AASHTO M294 certified, meets ASTM standards for F405 and F667, and complies with Canadian Standards Association CAN/CSA B182.8. A 10m long section of its 900mm diameter perforated Solflo Max corrugated HDPE pipe is tied in at the end and will vent any overflow if the system reaches max capacity. Manhole height ranged from 2 to 3.41m. A clean stone cover is used to wick away extraneous water. The system was covered with extruded polystyrene insulation as well as a waterproof membrane and a geotextile added protective layer. Soleno is a member company of the Plastics Pipe Institute (PPI) . Its Executive Director, Tony Radoszewski, observed that this is a novel use of corrugated HDPE pipe, which is normally found in road works, under parking lots and in similar situations.
“To see the attributes of the pipe being used at a very important North American historic site is reassuring,” he said. “The pipe is strong, durable, lightweight and flexible while providing excellent watertight protection for the centuries-old, fragile, delicate site.” The limited space and the age of the site posed logistical challenges for the construction team. Eric Blanchette of JES pointed out that it was impossible to use heavy equipment, which made it like working in a small basement.
“We had to make sure all the pipe could fit between the new support columns,” he said. “One advantage of HDPE pipe is that it is big and strong but still light; so it was easy to take the pipe sections under the terrace. The pipe is 3.08m long – about 10 feet – and easy for our crew to carry and manoeuvre around the support columns.” A second challenge was backfilling. The whole system was surrounded with non-permeable membrane in order to protect and contain any water volume inside. The retention system outlet connects to the city’s system.”
“We needed a product that would provide a very specific solution including being able to handle the water volume, flow rate, as well as having the strength and the projected longevity that a storm water retention system requires,” Bernard Marquis, Eng., Technical Director of Roche Consulting Group (Quebec City), said as he explained that there were, in fact, very few alternatives to the HDPE pipe. “One of the biggest challenges was putting the pipe under the terrace between the columns and support beams. We were able to fit in a mini-excavator that has a cab about as big as a phone booth. The constraint made the installation nearly impossible but HDPE pipe provided a solution – in fact, the only solution.”
Main picture: A new HDPE storm water retention piping system is intended to protect the historic centre of Quebec City, Canada, for the next 100 years.