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AGRA Foundations Limited plays a critical role in stabilizing a railway bed near Lillooet, B.C.

By Mark Halsall

Situated in the rugged Fraser River Canyon about 15 km east of Lillooet, B.C., the Fountain Slide is part of an ancient earth ow that’s causing a mountainside to creep slowly but inexorably downwards. A Canadian National (CN) Railway line and as well as B.C.’s Highway 99 run right through this active slide area, and as one might imagine it’s had a substantial impact on both.

The impact on the CN line has been significantly lessoned, thanks to a track mitigation project completed in November 2016 that has improved slope stability around an existing beam and lagging retaining wall that supports the railway bed.

The innovative design-build solution was the work of Nicholson Construction Company and AGRA Foundations Limited, both part of the Soletanche Freyssinet group of companies that performs geotechnical construction work throughout Canada and the U.S.

Jonathan Hazenberg is president of AGRA Foundations, the Edmonton-based firm that served as general contractor for the project. AGRA’s typical scope of work includes pile foundations, retaining structures and shoring systems as well as slope stability projects.

Hazenberg says in the spring of 2016, CN approached a half-dozen or so firms requesting unique proposals for a design-build solution to the slide issue that would mitigate the risks to the railroad’s operations.

“There weren’t a lot of specifications beyond that,” he said. “We felt we were successful on this job at the onset, because we evaluated many different solutions that could be employed. Then we worked with the client to optimize the one that was best suited for this project in consideration of costs, risks, environment, other stakeholders on the project, schedule and safety.”

Hazenberg notes that despite numerous technical and logistical challenges presented by the project, there were no interruptions to train service during the five months it took to complete construction.

“It was a good team success which started immediately with very strong collaboration between the client and AGRA/Nicholson,” he said.

Hazenberg says that prior to the project, the tracks atop the 120-metre long retaining wall had to be realigned regularly to counteract the shifting earth. The slope movement began to pick up in 2015, he says, and eventually reached a rate of three to six millimetres per day, which required CN crews to have to adjust the tracks almost weekly. 

Unusual project
The beam and lagging retaining wall was built in 2008 to protect the railway bed from the loss of ground downhill from it, adding to a shotcrete and tie-back wall that had been installed in 2006. According to Hazenberg, neither of these structures was having any effect on the slide itself, but had acted effectively as a retaining wall supporting the rail bed.

“A challenging thing about this slide is the depth,” he said. “The active slide surface is approximately 16 to 18 metres below ground. These structures...were installed in the upper portion, so they were basically being carried along intact with the slide.”

Hazenberg says the project was very unusual from a geotechnical perspective because they were dealing with what was essentially a failed condition.

“We had a retaining wall that for all intents and purposes was showing signs of distress, and we also had an active slide that was moving at a rate that was considered extremely high,” he said. “Most solutions that you would install would typically be for a static situation where you’re trying to improve a condition or provide an installation that would mitigate future events. In this case here, the event had already happened and we were trying to manage it.”

For the track mitigation project, a two-part solution was employed to first arrest the slide movement and secondly to improve the global slope stability.  at was accomplished by installing structural elements that went all the way through the active slide zone and into the underlying surface below the slip plane.

“We’re satisfied with the result,” said Hazenberg. “The slide has been monitored ever since the completion of the work and our works remain effective.”

Two-part solution
The first part of the solution involved the installation of 250 battered and fully grouted micropiles, which were drilled using a down-hole hammer application.

Phillip Kapronczai, AGRA’s project manager for the site, says the piles extended down below the slip plane of the slide. They acted as ductile shear piles and were made of high tensile strength steel that could withstand some deformation before the slide was halted completely.

The second part of the solution involved the installation of 30 permanent anchors through the existing retaining wall and connected by a cast-in-place concrete waler.

“Those anchors will pick up the load and tension against that waler to provide further resistance,” said Kapronczai.

In addition to the micropiles and the anchoring work, a considerable amount of earthwork was required to establish an access road from Highway 99 and maintain safe working platforms for the drilling rigs and other equipment. These earthworks were completed by a local subcontractor called GNS Contracting Ltd., which was a big help on the project.

“We were not able to bring in additional earth onto the slide area because that would have had a detrimental effect on the actual slide movement,” said Kapronczai. “The challenging aspect of the earthworks was balancing the load with a cut and fill to create these platforms so we’ve minimized any movement.”

Kapronczai says Casagrande C-14 drilling rigs were used for the drilling aspects of the project. Grouting plants, Atlas Copco air compressors and a crane for assisting with the installation of materials were also used on-site, as well as an assortment of bulldozers, excavators and rock trucks that were required for the earthworks portion of the project.

Safety top priority
While the risk of a catastrophic slope failure occurring was considered to be very low, there was always a chance of slope movement accelerating during construction and causing greater distress to the existing retaining structure. For this reason, precautions were taken during every stage to minimize the load on the worksite and maintain the highest safety standards and to protect site personnel.

Worker safety was top of mind at all times and, to this end, there was ongoing slope monitoring throughout the project.

“If there were any red flags, we could react immediately,” said Kapronczai.

This was accomplished using slope inclinometers and real-time surficial monitoring using a CYCLOPS integrated survey network. The monitoring equipment was supplied by SIXENSE.

According to Hazenberg, all of the materials and techniques utilized during construction were prescribed through numerous detailed plans for the project.

“There was a quality management plan, a safety management plan, an environmental management plan and an engineering plan, of course, that were all instrumental to our work execution,” he said.

Kapronczai says because it was a remote project, adequate safety facilities and trained personnel needed to be on site at all times. The location provided additional challenges, including where to house the project’s workers (up to two dozen or so at the peak of construction) but fortunately the Town of Lillooet wasn’t too far away.

“We tried to work with the local community (Lillooet) as much as possible in terms of sourcing a lot of our materials and equipment and for our worker accommodations as well,” said Kapronczai.

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Piling Canada is the premier national voice for the Canadian deep foundation construction industry. Each issue is dedicated to providing readers with current and informative editorial, including project updates, company profiles, technological advancements, safety news, environmental information, HR advice, pertinent legal issues and more.