Slope collapse puts halt to work
A prime example of special challenges in developing steep sloped land in Nanaimo has put a halt to work on a lot in a Departure Bay neighbourhood.
Excavation had begun at 2519 Battersea Rd. when a concrete block retaining wall partially collapsed Feb. 22.
The steep lot fronts Departure Bay Beach and backs onto Seascape Manor condominium property.
John Hessels, site project manager with Lewkowich Engineering Associates, said an unanticipated zone of seepage caused the problem once excavating began.
“There’s no safety issues with neighbouring properties. It’s not an unusual problem for Nanaimo,” he said. “All banks have seepage, but this particular site, for whatever reason, the excavation uncovered a fair heavy seepage zone and that’s what caused the problems.”
Dale Lindsay, city manager building inspections, said there is not going to be any further construction on the property until such time as the bank is stabilized and engineers figure out a plan as to how the house is going to be accommodated on the property.
“There are specific requirements that if you’re building a single-family home on a lot in a steep slope area, you need to have a geotechnical engineer produce a report,” he said. “That was done on this property to ultimately determine that the site is safe and suitable for the use intended.
“Our immediate concern is bank stability, safety of neighbouring property owners and safety of workers so we asked for a revision to that original assessment.”
Hessels said they are in the process of altering the design of the home
‘We are at a stage of redesign. We are suggesting to raise the main floor height of the house,” he said. “We are too far into the cut so we want to bring the house up to create a much safer situation.”
But Charles Thirkill, a fisheries biologist with years of experience with water erosion, said its no surprise the lower half of the slope collapsed and more care than just the city’s steep slope development permit area guidelines are needed in cases like this.
“This area is part of the historic Cilaire bluffs. The substrate is hard pan or sand so it’s inherently unstable and subject to erosion,” he said. “Thirty trees and the vegetation has been removed from the slope which destabilizes it.
“The excavator has been up and down the lower part of the slope which has impacted the ground with weight and vibration, destabilizing the lower half of the slope.
“Having seen the effects of stream erosion, I knew without a shadow of a doubt this was going to happen on this site. I just didn’t think it would be this dramatic.”
An with a good portion of remaining undeveloped land in Nanaimo on hillsides, Thirkill is concerned it could happen again.
“It might appear minor with just one site affected, but it’s a problem we have as a community given all our development is going to be infill and on this type of site,” he said. “Developments are going to be on properties that require certain sensitivity and a great deal of skill and creativity to develop.
“Instead of that, we get this example that could be repeated a hundred times over in the next 10 years unless the city makes sure development is indeed smart. It’s an issue of public safety.”
Lindsay said there are number of areas in the city that are identified as steep slope development permit areas that require a higher level of consideration and planning than what’s required on level ground.
He said the additional challenges are not insurmountable, but certainly require an higher level of awareness.
“The guidelines include an engineer drafting a report and in the case of constructing a house, they then sign on as the registered professional on the project,” said Lindsay. “That engineer now is responsible for the geotechnical design, he’s responsible for a field review which includes slope stability and making sure the work is being conducted properly.”
All reports from engineers go to the city, which, Lindsay said, is equally concerned for public safety and that’s why a review was ordered for the Battersea project.
“There’s no getting around that it is a very steep lot and has challenges when you start constructing on it,” he said. “That’s why we took immediate steps to get an update from the engineer to make sure there isn’t a broader risk.
“The information provided by the engineer says there’s no further undermining of the slope at this time and large failures are not anticipated.”
But Thirkill said just following the guidelines is not enough with these types of lots.
“This is a steep slope, obviously – almost vertical in some places – and they were allowed to go ahead in the worst time of year with rain,” he said. “Ultimately, the city and city council approves and gives the permits go ahead for a development.
“They have to carefully oversee this stuff and if they’re not doing that, they shouldn’t be in business.”