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Home arrow Authors   The American Surveyor     

Guardians of the Rock Print E-mail
Written by Gavin Schrock, PLS   
Friday, 13 April 2012

A pioneering system for monitoring rockslides on Norway's fjords.

A 1.825Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

A Young Boy's Discovery
More than 50 years ago, a Norwegian farm boy left his family home near the shore of a remote fjord to climb the 40 percent slope towering above. A stocky, healthy lad, Per Åknes managed the climb with the same ease as the family goats. But something seemed out of place: nearly 880 m (2,917 ft) up the rocky face, a crack the size of his small fist had formed. As the years passed, Åknes noted that the crack was widening--until today that same crack is more than 15 m (48 ft) wide: the entire slope is slowly accelerating towards a potential rockslide.

Åknes' discovery set off a chain of events that resulted in an ambitious plan utilizing state-of-the-art research and technologies to detect and provide an early warning of the potential failure of the steep rocky slope.

The Åknes /Tafjord region of western Norway is a UNESCO (United Nations Educational, Scientific and Cultural Organization) World Heritage site and a popular tourist destination. Numerous cruise ships enter through narrow openings along the rugged coastlines into steep, fingerlike fjord complexes--miles and miles of pristine beauty. Located in the Stranda municipality along the Åknes reach of a much larger fjord complex, the region is also home to many small villages and farms.

Evidence of rockslides in ancient and modern history is readily visible along the fjords. The most notable example happened in 1934 near the far end of the Tafjord arm of the same fjord complex; a rock slide much the same size as the potential Åknes slide created a "tsunami" wave resulting in the deaths of 40 local residents and property damage representing tens of millions in adjusted dollars.

Evaluating Potential Rock Slides
The people of the fjords have come to terms with the risk of potential rockslides, planning well for such events. Local communities have disaster preparedness plans and systems to warn of potential rock-slide-created "tsunamis;" such warning systems and shelters are commonplace along the fjords. With enough warning, the people can seek high ground or hardened shelters that can withstand brief but powerful inundations. Scientific research has also identified the telltale warning signs leading up to such events.

Studies of past slides and other active slide areas have provided detailed models of the behavior of such slopes in the weeks and days prior to such failures. According to the studies, a slope like the one at Åknes would accelerate from the current few centimeters per year to several centimeters per day beginning about two weeks prior to the failure.

A comprehensive ongoing survey of the hundreds of miles of scenic Norwegian fjords has identified numerous potential rockslides, the Åknes site being the largest. This is a predictable rock slide, nearly 600 m (1,969 ft) wide, more than 1 km (3,280 ft) long and with some parts close to 200 m (656 ft) deep. It is estimated that this large of a rockslide plunging into the fjord could generate run-up waves as high as 90 m (295 ft) and endanger villages in the narrow waterways.

The sheer size of the potential slide and its proximity to local towns and villages gave rise to calls within the Norwegian government and local communities to find ways to provide as much warning time as possible. In short, the existing early warning precautions are sufficient, but "earlier" warning is desired. With legacy methods, small movements would be noticeable by visual inspection alone only in the final days before a slide; and only in the final hours would the cracking rock face issue telltale audible cues. The challenge was how to detect the more subtle acceleration that would herald the impending slide as early as two weeks prior to the event. What was needed was a comprehensive, high-precision monitoring system that could interface with early warning systems.

A Bold Plan
The Norwegian government and the local Åknes/Tafjord Early-Warning Center enlisted the geo-monitoring company Cautus Geo AS to provide onsite instrumentation, data management and the early warning system. Their plan was to employ multiple monitoring components to interface with a new early warning system being constructed for the region. While each individual monitoring technology, system and methodology could possibly stand alone in detecting the progression of downslope movements, in this safety-of-life situation it was decided not to rely on a single component or monitoring system.

In the design phase, Cautus Geo key project engineer Lars Krangnes considered the inherent challenges of constructing these multiple systems in such a remote and steep environment and harsh climatic conditions. "Every person and each piece of equipment had to be transported by helicopter," noted Krangnes. "Instruments, fuel, and supplies--even the bags of concrete needed to construct a control building had to be flown in." The instrumentation ranged from simple extensometers (dubbed "crackmeters") to measure the expansion of the upper crack to the Trimble® S8 Robotic Total Station monitoring dozens of targets over the entire slope to the sophisticated Trimble Net R9 GNSS network tracking the movement of the slope.

Sophisticated Monitoring Tools
"Our S8 total station stands behind a bay window in the primary observation and utility building outside the unstable slope," says Krangnes. "The building is climatically controlled and also houses generators, key data processing and communications equipment."

This total station is the 21st-century descendant of a surveyor's instrument: that little "telescope on a tripod" you might see along the highways and at construction sites. The similarity ends at the tripod. The high-tech gadgetry inside the Trimble S8 Total Station can automatically perform in minutes what might have previously taken an entire surveying crew days to do. Instead of the mechanical "protractor-like" plate used to measure angles on the old instruments, the total station now rotates precisely on a magnetic drive; instead of measuring distances with chains and tapes, the total station has a built-in precision laser.

Together with the Trimble S8 Total Stations, Trimble 4D ControlTM Software is used to control the total station and analyze its data. "The robotic total station continuously cycles through a series of observations of 30 prism targets over the entire slope," Krangnes adds. "It can detect subtle movements ranging from the current several millimeters per month rates to the expected centimetersper-day movement that would happen prior to a collapse."

The next major system for tracking the movement of the slope is a network of Global Navigation Satellite System (GNSS) units. There are 10 Trimble NetR9 GNSS units--consisting of high-precision antennas and dual-constellation, dual-frequency receivers--onsite that have their positions compared every 15 minutes, 4 hours and 12 hours to two GNSS units placed on stable ground offsite. A monitoring GNSS unit can utilize not only the satellites from the U.S. GPS constellation but also those of the Russian Federation's GLONASS system. In the future, such NetR9 units may also be used to track the planned European Galileo constellation, China's planned Compass constellation, and other regional augmentations constellations such as the Japanese QZSS--and more systems are also being proposed by other countries. This GNSS-monitoring system can resolve positions at a high frequency (up to 50 HZ) to high accuracies by utilizing geodetic-grade GNSS antennas and receivers as well as Trimble 4D Control Software, a suite of "motion engines" that apply multiple advanced mathematical algorithms in real-time to the GNSS observations.

The GNSS network is currently one of the most important and trusted monitoring systems at Åknes and provides high-accuracy 3D data. "The frequency and accuracy of the 3D results, along with the robustness and stability of the network makes the system very well suited for a challenging site like Åkneset," says Krangnes.

Other surface-movement detection instruments employed at the site include a laser ranging system aimed at larger, heated targets. There is also groundbased real-time radar (operating from another structure across the fjord) with companion reflectors on the slope and post-processed InSar (synthetic aperture radar) measurements from satellites. These systems are augmented by more tightly constrained periodic observations that Krangnes calls "campaign-style" surface scans with a portable scanner. A portable scanner creates a kind of 3D "photograph" of the rock surface, with each "pixel" representing an actual laser-measured point. To support these other instruments, there are also climatic stations and numerous live Web cameras.

With the surface rigorously monitored, the next challenge is monitoring subsurface conditions to gauge the depth and movement of the moving rock layer. Krangnes describes the first solution as "geophones that listen to the moans and groans of the subsurface rock that is under the tremendous pressures of the downhill slide." He adds, "We also drilled deep boreholes extending below the rock layer to stable bedrock below." These boreholes house the long cables of extensometers and tilt meters; additional instrumentation in the boreholes gauges water pressure and temperature changes deep within the rock face, providing more data to correlate with readings from the other instruments. Krangnes summarizes the surface and sub-surface instrumentation as being "like the monitors in a hospital's Intensive Care Unit (ICU); each reports individual readings that collectively give a total picture of the health of the slope."

Managing The Warning System
Perhaps less exciting but just as important as the high-tech monitoring gear are the system's vital communications and data management components. The various monitoring tools are connected via redundant wireless communications systems that are operated, like the monitoring instruments, in a preventivemaintenance mode: key parts are replaced on a predetermined schedule before they have problems. Processing is handled, both onsite and offsite, via robust broadband connections to a series of external communications nodes in local communities and beyond. The data management is multi-tier; each element provides a data import tier, temporary storage, a backup and failure tier, and then a data transfer system feeding a database server. Krangnes explains that, "The technical monitoring instruments, geotechnical instrumentation, and surveying systems all feed a single database." The sophisticated management system called Cautus Web was developed by Cautus Geo based on many years of monitoring experience. This management system is subsequently tapped by monitoring systems such as motion engines and generates results as web-based presentations in real-time.

These vital communications links interface with a sophisticated early warning system set up by Cautus Geo for the Norwegian government and the Åknes/ Tafjord Early Warning Center. The automated reports are sent directly to the operators of the Early Warning Center. The Cautus Web Monitoring system also accommodates functionality for viewing and analyzing data in different ways. This includes complex alarm functionality, maps with GIS functionality as well as ad-hoc reporting and data feeds for the ongoing and evolving research of this type of rockslide. "This project is not a pilot or an experiment," says Krangnes. Instead, he adds, "this project is designed as an active monitoring and early warning system using multiple and redundant instrumentation to assure success." However, any additional research done or lessons learned at the site will surely benefit future rockslide monitoring projects.

"Based on experience from other rockslides and measured yearly movement for this site, a theoretical acceleration diagram has been defined," Krangnes says. "From these figures, alarm levels have been defined to evaluate emergency plans for the region." Under certain scenarios, and as early as two weeks prior to a predicted failure, decisions might be made to suspend shipping in the fjord and issue warnings to local residents. In the final days before a predicted collapse, evacuation orders may be issued for some communities. Even in the worst-case scenario, as little as five minutes warning would enable residents to seek high ground or shelter.

New Tools For Monitoring Hazards Worldwide
There are similar sites in other parts of the world where these types of technologies are being applied to geophysical and structural hazards. Examples include plate tectonics (earthquake) monitoring in Sichuan China and in the Pacific Northwest of the U.S.; volcanoes in Alaska and Papua; a tsunami detection system on India's offshore islands; mudslides in New Zealand and Italy; dams and bridges in Washington State; and open pit mines in South Africa. These new tools contrast with legacy systems by detecting actual movement with much higher precision, with less reliance on derived or modeled results. Example: traditional seismic monitoring with strong motion sensors is outstanding in reporting the magnitude (essentially how much the ground "shook"). While great strides have been made in extrapolation of trends with these legacy systems, it has only been the recent availability of multi-tiered high-precision networks that comprehensive trending and true warning systems for localized geophysical events may be achieved. A multi-tiered approach can employ a system like the one at Åknes which includes Trimble 4D Control Software to monitor purely localized geophysical events like rockslides or subsidence, and then connect into a larger system like Trimble VRS and Trimble VRS3NetTM App Networks to monitor regional geophysical effects like plate tectonics, volcanoes, and earthquakes. Many countries, states, and localities have these Trimble VRS or Trimble VRS3Net App regional networks in place to serve real-time corrections for surveying, construction, agriculture and/or purely for plate tectonic science. Localized monitoring solutions like Trimble 4D Control Software are designed to be able to tap the power of these regional networks as the backbone of the localized monitoring network.

This project on the Norwegian Fjords represents a great step forward in the evolution of monitoring and warning systems for catastrophic slope failures and may be applied to other geophysical phenomenon. In summary of this groundbreaking initiative, Norway's Minister of Local Government and Regional Development Magnhild Meltveit Kleppa stated, "Through the comprehensive monitoring and safety systems in the Åknes-Tafjord project, the threat to people's life and health is substantially reduced."

Note: A gorgeous video about what Cautus Geo is doing can be found at http://www.youtube.com/watch?v=a9IOzzykb5Q

Unless noted, all images courtesy Lars Krangnes, Cautus Geo AS.

Gavin Schrock is a licensed land surveyor and tech writer in the Pacific Northwest.

 
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