The monitoring solution consisted of four major components: • Custom GNSS monuments that were designed by Leica Geosystems to be moved by helicopter onto the active slide area • Leica GM30 monitoring receivers that track the GPS constellation and the GLONASS constellation • A Leica Geosystems designed communication system with three-way failover capability for the highest reliability. • A custom implementation of the Leica GeoMoS Now! online analysis soft- ware for continuous real-time monitoring of landslide movement. Really Stable Monuments… That Move! From a surveyor’s perspective, placing GNSS sensors on an actively moving landslide was an interesting undertaking. The innovative, first- of-its-kind monument design that Rutledge and MacIsaac devised, built, and installed is likely to be influential in landslide monitoring projects for years to come. In order to operate continuously and without electrical grid power, the GM30 receivers required a robust DC power system based around solar energy. This consisted of an appropriately sized solar panel and a battery array capable of providing power in any weather. To keep the entire system “bottom heavy” and resistant to wind toppling, Leica Geosystems designed a minimally expanding concrete cylinder base that weighed well over 1,700 pounds, giving the entire installed as- sembly a weight of over a ton. “The alternative to a system like this would have required getting a drill rig on site to place a deep foundation,” says Rutledge. “But that would have quadrupled costs and likely wouldn’t have been a better solution. We match our monumentation with the expected displacement signal, an often-overlooked aspect of measurement science. The design and special concrete mix meant that we could fabricate everything up near the site in just a couple of days. We felt that we had found a good way to mount the GM30s that worked with the particular conditions in Old Fort, and their performance since then has proved us correct. We now track the slide in the low millimeter range.” One problem remained: How does one install these GNSS stations on terrain that is known to be dangerously unstable? In a word, he- licopters—the initial six concrete bases with mounting brackets, the battery packs and solar arrays, and of course the GM30s were staged at a restricted area near the landslide and then, over the course of one day, lifted and lowered to sites spread out strategically over the landslide (aside from one receiver installed away from the landslide and used as a base station). “In most cases, the sensors were placed on spots that surveyors could walk to safely and then do the necessary assembly on the bases and components as they were lowered in by helicopter,” MacIsaac explains. “But we did place a few in spots where there was no reasonable access by foot, and in those cases we went ahead and lowered in the surveyor too—an unusually exciting day of fieldwork for our team!” With the initial six sensors in place, displacement data were imme- diately available. The lidar flights continued for a couple more days, “Just to provide a check on the GNSS data and help us gain trust in the equipment,” says MacIsaac. “The two sets of data matched very
The size and movement of the landslide put the entire town of Old Fort in danger.
the challenge. We needed a system that could track the slide in three dimensions at the millimeter level around the clock. This was required to understand the mechanics of the slide and to ensure that we were not putting anyone in harm’s way,” Rutledge said. “Sean and his team really understand risk and were open to several innovations that made this solution possible.” For his part. MacIsaac says, “My survey team has been standardized on Leica Geosystems equipment for a long time now. Given the high stakes of the situation, we were confident in Leica’s tried and tested approach.”
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csengineermag.com april 2020
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