Engineers who designed the new aboiteau on the LaPlanche River are confident the new structure will last 50 to 75 years.
AMHERST – The new aboiteau on the LaPlanche River should last much longer than the one that was destroyed just months after it was constructed in 2007.
Engineers from CBCL Ltd. held a public information session here Wednesday to discuss the new aboiteau that was announced by the provincial government last week. Estimated at approximately $5 million, the new aboiteau will replace the one that was wrecked in a storm soon after it was completed.
It will also replace the existing wooden aboiteau on the river that has been in place since the 1950s.
“The difference between this one and the last one is the foundation we have designed. The other aboiteau wasn’t built with the same methods as the other one,” Rick Giffin of CBCL said during the meeting. “Our understanding of that project is that settlement occurred. We have about 100 feet of very soft soil and when you put a load on that it will settle.”
This project will include the driving of 93 steel pilings into the bedrock and building the aboiteau on top of those pilings. The box culvert, containing the double gates will sit on top of that cap and an earthen structure will be built on top of that.
He said the new structure will have a design lifetime of 50 to 75 years.
Gordon Smith of CBCL said the company did a lot of study on tides and found the average tide height is approximately 6 metres, with the spring tide maxing out at 7.9 metres.
He also said that in a one-in-100-years storm event the highest tide and taking into account rising sea levels, the highest tide would be 9.5 metres. For that reason, the top of the structure would be 9.8 metres.
In doing their study, he said, CBCL Ltd. balanced the cost of building the aboiteau against what losses would be should the structure be breached by seawater from the Bay of Fundy.
The engineers looked at three structures, including one closer to where the river empties into the bay and one further upstream nearer the town’s sewage treatment facility. He said the most cost-effective location is where the previous aboiteau failed.
During the project, the successful bidder will divert the river and establish a dry working area by building temporary coffer dams.
The aboiteau will also include a fish passage and each of the gates will have a one-foot by one-foot hole to allow fish through.
The new structure will also be twice as big as the existing aboiteau.
The existing aboiteau will remain in place as insurance until the engineers are sure the new one won’t fail.
The tender closes next Thursday and the contract is calling for work to begin in August and take four months to complete.
Several dozen people attended the meeting and for the most part liked what they saw.
Doug Bacon expressed some concern with the construction schedule, saying he’d rather see it build in the spring. He said the fall weather will present a number of challenges and working on the marsh mud is not like working with regular mud.
“I think they’re starting at the wrong time of the year. I think they should have everything lined up to do in the spring,” Bacon said. “Fall weather and marsh mud don’t come together very well. They’re going to find out that it’s going to be a big challenge.”
Bacon said he likes the concept that’s proposed, but is concerned with the potential siltation under the aboiteau and is worried that dyke over the box culvert may not hold over time.
George d’Orsay likes the concept.
“We’re all hoping it works good. Although some don’t think the start time is ideal, I think it’s the right time because they will want some drier soil the start on,” d’Orsay said “We certainly have been wanting this for a long time and finally now it looks like we’re going to get it. I’m pretty happy with what I saw.”
John Atkinson, who owns farmland around the aboiteau, said he liked what he saw but is wondering what happens if it comes in over budget.
“The government is spending a lot of money on this and these guys have studied it a lot and looked at a lot of different variables,” Atkinson said. “My question is what happens if it comes in over budget? Will we have to go back to the drawing board? The engineers said themselves that the existing aboiteau is like a ticking time bomb. Hopefully government will take that into account if the bids are higher than budgeted.”
Atkinson expects there will be challenges, but he’s happy that action is finally being taken after almost 15 years of lobbying government to put a new structure in place.