C+S April 2020 Vol. 6 Issue 4 (web)

Real-Time Monitoring Advances Landslide Risk Management By Angus W. Stocking The September 30 landslide destroyed everything in its path, including the only access road for Old Fort residents.

the entire town on October 7th, a move that affected 54 homes and about 200 residents. It was a trying and stressful time for everyone involved. Given the size and ongoing downward translation of the landslide, no one at the ministry could say when, or even if, Old Fort Road would be rebuilt (evacuations of people and belongings took place by emergency barge and helicopter from the adjacent Peace River), nor could the ministry be certain that the town itself would be spared. Some residents were worried they might never be able to move back into their houses. Complex Monitoring, Done Just Right Monitoring of the landslide began within days, as ministry personnel quickly implemented daily overflights with lidar equipment. This was an effective and safe first step to provide accurate data that covered the several-acre slide area. Aerial overflights also kept workers off the slide in the earliest and most dangerous days following the initial collapse. But lidar overflights were unsustainable for long-term monitoring because the daily flights were prohibitively expensive. Additionally, the lidar work could only record data every 24 hours and would never be able to provide the real-time information on movement rates that is used to automatically alert workers on the landslide of rapidly increasing movement that might signal another collapse event. “The lidar work was essential in the immediate aftermath of the landslide, and that early data was extremely helpful,” says Ministry Geomatics Survey Supervisor Sean MacIsaac. “But we knew it wasn’t the kind of monitoring we would need to determine when the slide stabilized, and when it would be safe to build a temporary road and restore utilities.” The team reached out to David Rutledge, Leica Geosystems’ director of structural monitoring, as well as measurement solutions provider Spatial Technologies, to design and install a GNSS-based monitoring system within two weeks of the landslide, start to finish. Rutledge, who is based in California and British Columbia, made him- self available immediately for what proved to be one of the most ur- gent and complex of the many monitoring projects he’s been involved with over his career. “Designing a system that would reliably—and cost-effectively—measure the precise displacements in real-time was

On September 30, 2018, the 200 or so residents of British Columbia’s Old Fort (not far from Fort St. John) were shocked by the roar of a landslide occurring a couple of miles outside of town, as 8 million cubic meters (10.5 million cubic yards) of dirt, rock, and big boulders swept down about 1,000 meters (0.6 miles) from a gravel pit to Peace River wetlands, laying waste to everything in their path, including trees, utilities, and—of most concern to residents—about 600 meters (2,000 feet) of the Old Fort Road, the only access for residents. It was a roar heard around the nation, making the evening news and drawing the nearly immediate attention of the B.C. Ministry of Transportation and Infrastructure. The safety of residents trapped in Old Fort was the most pressing con- cern. As the ministry mobilized to investigate, the news wasn’t good. Early site inspections indicated that the landslide was a significant natural hazard that would require a multiyear timeframe to mitigate. Local geologists from Westrek Geotechnical Services were soon able to confirm that the slide was “caused by a failure in the bedrock at the head of the landslide, where a gravel quarry had been operating on the hillside above Old Fort,” and that the ground beneath the slide had been moving for months before the massive collapse. Nor did that massive collapse signal the end of the landslide’s down- ward march. Though it had released a massive amount of stored up energy, the failure was still happening. Three days after the collapse, additional displacements of 20 meters were measured. In the follow- ing days, displacements of 4 to 50 meters were common. These early measurements were so alarming that B.C.’s Public Safety Minister Mike Farnworth took the prudent but controversial step of evacuating


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