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VRCA winners tie-down 10-Mile Slide’s moving earth

Jean Sorensen
VRCA winners tie-down 10-Mile Slide’s moving earth
PETER KIEWIT SONS ULC—To slow the movement of a million cubic metres of earth in the 10-Mile Slide area, companies used large diameter blocks with anchors drilled deep into the ground to transfer loads.

The 10-Mile Slide area is considered one of the most complex pieces of geography in B.C. as it is the edge of millions of cubic metres of a post-glacial earth flow on a rugged slope of the coastal mountain range and encompasses both a CN Rail line and Highway 99, an arterial link between Kamloops and Lillooet.

While the bulk of the 13 million cubic metres of the Tunnel Earth flow remains inactive, approximately one million cubic metres of earth, known as the 10-Mile Slide area, has been creeping forward for decades at varying rates causing the road to become unstable. The slide area measured 200 metres in width, 300 metres in length and had an average depth of 18 metres. A 50-per-cent load restriction has been imposed on the highway since a serious destabilization occurred in September 2017 hampering vehicles and impacting the XaXli’p’s Fountain Indian Reserve area, near Lillooet.

The Ministry of Transportation and Infrastructure (MOTI) announced a $60 million fix over two phases which involved stabilizing the bank and road. Two contractors involved in the first phase have received Vancouver Regional Construction Association Silver Awards of Excellence for their work in tying down the mountain of dirt. Being recognized for their work, which started in November 2018, are Peter Kiewit Sons ULC as the general contractor on a project valued at up to $15 million and Con-Tech Systems Ltd., in the manufacturers and suppliers category, for fabricating and supplying huge anchors. Advanced soil anchor work (Phase 1) was successfully completed in February 2019, and as a result of work on the highway and the anchors, load restrictions on the highway were adjusted, allowing commercial vehicles and tour buses that are 27,000 kilograms gross vehicle weight or less to use the highway.

“One of the main challenges of the project was working on an active moving slide. The slide moved an average of 7 mm per day and at times exceeding 12 mm per day,” said Kiewit project manager Tyson Motz. While it may not have seemed much, it amounts to 2.5 metres of movement in a year. “This also meant that the topography at the time of construction had significantly changed since the tender drawings were created.”

It was not practical to remove material because of the slide’s volume. “The bench material cuts were to be utilized for highway improvements as no material could leave the site due to environmental permitting restrictions,” said Motz.

To mitigate these circumstances, Kiewit collected survey data, developed terrain models and worked with MOTI to redesign cuts and fills to find the best design that could provide construction personnel with accessible benches and provide enough material for the highway work. Twice daily Kiewit continued monitoring the site with data analyzed by the MOTI staff to ensure worker safety.

Another challenge that remained was the isolation of the area over the winter months when work progressed. There was no cell phone service. Many delivery companies did not want to travel this stretch of highway during winter. “The project required a detailed procurement plan in order to keep the work progressing without delivery delays,” he said.

(Prior to starting the project, Kiewit dedicated a team to procurement, but also had the construction team work with modellers and schedulers to create detailed work plans that identified schedule risks and ensued the proper plan was in place prior to crew mobilization to expedite construction).

Con-Tech Systems, which supplied the anchors to keep the creeping mound of dirt in place, termed the project one of the most complex emergency projects that the company had been involved with in its 36-year history.

President Horst Aschenbroich said that Phase 1 was really the second part of an emergency response that began when the road was impacted in September, 2017. At that time Kiewit and Con-Tech had worked to stabilize some of the slide and Con-Tech had also supplied anchors to the CN Rail. Approximately 40 anchors had been placed in that early portion of the emergency repair. MOTI then embarked upon a Phase 1 (placing another 44 anchors) with Phase 2 construction (placing a further 200 anchors) to follow.

“We do these projects all the time but this was one of the most challenging projects, mainly because of the logistics and the fact that the project used one of the largest Hot Rolled Threaded Bars. The last job we used this size rebar on was the Wanapum Dam on the Columbia river,” said Aschenbroich.

The project included 1,500 metres of one of the largest double corrosion protected (DCP) anchor bars (65mm Grade 150 Hot Rolled Threaded Bars).

The Phase 1 project utilized 44 precast anchor blocks with the DCP anchors. The anchors were drilled into the bond zone, which was undifferentiated glacial sediment, holding back the slide and transferring the load from the anchor into the ground. The bond length is composed of corrugated high-density polyethylene (HDPE) pipe encapsulating the pre-grout anchor bar, where the corrugation provides bonding between the anchor bar and the soil.

The length of the overburden layer (free length) does not provide anchorage, but serves to facilitate the transfer of the load to the precast anchor blocks to ensure overall stability of the anchor. The free length was larger-diameter, smooth sheathing HDPE to generate the free-stressing zone for the anchor. Once the anchor was installed the hole was grouted. Due to tension cracks in the ground some anchors saw in excess of 2600 litres of grout per hole, far in excess of the neat line quantities.

“Upon completion of a bench of anchors, the slide monitoring program saw significant reduction in slide movement, proof that the design and construction was successful,” said Motz.

The required off-site monitoring, using satellite, caused Con-Tech to propose using RST Instruments Ltd.’s vibrating wire load cells (plus FlexDAQ data logger and GeoViewer software) for real time monitoring. The load cells were seen as a superior alternative system to the project-specified strain gauges, which have to be attached to the bar and grouted into the corrugated sheath before the anchor installation.

“We had used them on a California earthquake site,” said Aschenbroich, after his company had been called to provide anchor material to stabilize an area. The load cells on the bearing plates can be easily reused or replaced and indicate when the anchors are over-stressed and need to be de-tensioned or locked off. (The use of the load cells is now being incorporated into Phase 2, although Con-Tech and Kiewit do not have the Phase 2 contract).

Another project achievement occurred in traffic control on a highway that saw an average of 1,560 vehicles on it daily with 19 per cent heavy vehicles. Traffic control was deemed one of the areas of high risk. Kiewit subcontracted the traffic control to the XaXli’p First Nations, with the contractor’s crew providing training and the integration of the traffic personnel onto the job site. In total, the team completed 4,600 traffic control man-hours without a single incident.

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