When Mountains Move
It’s a scene from a Hollywood movie: A mountainside in Austria begins slipping down into the picturesque lake below. Emergency teams alert nearby residents and instruct them to flee from their homes. The landslide threatens to bury neighborhoods and carry dozens of buildings into the lake. As the disaster unfolds, they learn that it will be months before they can safely return to their homes and businesses. For the residents and authorities, it’s a battle against nature. And, for a while, nature has the upper hand.
It’s not fiction. The landslide took place near the town of Gmunden, Austria, in an area known as the Gschliefgraben, or “sliding ditch.” Located on the east side of Lake Traun, the Gschliefgraben had long been known as a potential hazard (see sidebar on page 22). But for more than 100 years, there had been little reason for alarm. That changed in November 2007 when a forester making routine checks discovered that a road in the Gschliefgraben had moved.
The Moving PyramidThe trouble had been building for months. In April 2006, a rockfall dumped approximately 92,000 cubic yards (70,000 m3) of debris into the Gschliefgraben valley. Heavy rainfall in November 2007 set the earth in motion. The accumulated material began to move, sliding 1,600 feet down the mountain across an area 330 feet wide. In some areas, the material was up to 65 feet deep. At first, the slide moved about 10 feet per day. By mid-December, a mass of earth larger than the Great Pyramid of Giza was moving as much as 15 feet per day. The homes and businesses along Lake Traun lay directly in its path.
The task of managing the emergency went to the Austrian Wildbach und Lawinenverbauung (Austrian Service for Avalanche and Torrent Control) together with a crisis team that was led by the mayor of Gmunden. To support the crisis team, the Wildbach assembled a team of experts, including geologists, geophysicists, engineers, surveyors and technical specialists. Because of the size and location of the landslide, the crisis team declared the entire area to be a disaster zone. On December 3, 55 homes were evacuated and the roads and businesses on the eastern shore of Lake Traun were closed.
The Wildbach team of experts initiated immediate countermeasures. To divert the water from the slope, teams dug ditches in the upper part of the Gschliefgraben and drilled wells to drain lower layers. They installed bores and pilings to slow the debris movement. And they examined the lake bed and debris cone for any notable distortion or fissures. In mid-December, the slope stabilized and a few homes were reoccupied, but the danger had not passed. Earth movements were still occurring in the upper part of the Gschliefgraben, and the fire and safety teams patrolled hourly. Construction of ditches and wells continued, and the wells reached depths of 560 feet.
In January, warm weather caused more meltwater to flow into the Gschliefgraben, and the trouble soon increased. The water leached through the upper layer of earth and accumulated underground producing high water pressure in the deeper layers. The underlying marlstone threatened to turn into a lubricating slush that could start a major slide. Underground movement pushed the surface layers up into peaks as high as 26 feet. By the end of January, there were 80 new dewatering wells with 20 more yet to be drilled. As much as 53,000 gallons of water were pumped out of the wells each day.
The effects of the slide had moved out of the valley down to the shoreline of Lake Traun. Simple level measurements revealed that three houses at the outflow had moved downhill by as much as 0.06 feet in a single day. The shore road developed cracks that were more than 1.5 feet long and nearly 1 inch wide. And the experts believed that the earth was moving even faster below the ground level.
Throughout the winter and spring, the Wildbach team worked at a feverish pace. Their objectives were to reduce the amount of water lubricating the slide, remove debris and control the flow’s direction. They called on technical teams, drillers and heavy equipment to transport the scree away. The slide area now looked like a construction site. To reduce truck traffic, a conveyor transported scree to a disposal barge that dumped it into the middle of the lake.
Their efforts paid off. By mid-May the slope was moving only a few centimeters per day. The worst was over.
Mounds of Data from a Mountain of MudFrom the very beginning, the Wildbach experts had needed current and accurate information about the size and behavior of the landslide. They considered using total stations to monitor the slide. But setting stable points for total stations within the slide area was impossible, and problems with visibility to monitoring points in the rapidly moving valley ruled out instrument locations outside the active area. Because the ground was moving so fast, even day-to-day traverse work for monitoring wouldn’t work. It was obvious to the Wildbach team that GPS was a faster and more flexible solution.
In early December, they borrowed a GPS receiver from the Ministry of Agriculture and took daily real-time kinematic (RTK) measurements of the motion in the Gschliefgraben. After a few weeks, it became clear that the Wildbach needed their own GPS system, and the team purchased a Trimble 5800 GPS Receiver and Trimble TSC2 Controller running Trimble Survey Controller Software. “We needed a GPS sytem that was fast, highly portable and simple to operate,” says Harald Gruber, an engineer from the avalanche-control authority. “The 5800 proved to be an excellent solution for this project.”
Because of the thick forests and mountainous terrain, the line-of-sight radio communications common to RTK were not practical. Fortunately, the Gschliefgraben lies within the NetFocus Realtime Network (RTN) operated by Energie AG, Austria’s electric utility. The NetFocus RTN uses Trimble VRS technology to provide centimeter-level positioning services throughout central Austria. Comprising 10 Trimble NetRS Reference Receivers and Trimble RTKNet Software, the NetFocus RTN delivered a steady flow of RTK corrections to the Gschliefgraben. The use of Energie AG’s RTN proved to be an important timesaver. Because the Wildbach surveyors did not need to worry about setting up local base stations, they could go wherever they were needed on short notice.
For data communications, the surveyors carried General Packet Radio Service (GPRS) cellular phones to receive RTK corrections from the NetFocus RTN. The phones connected via Bluetooth to the Trimble TSC2 Controller, which in turn had a separate Bluetooth connection to the Trimble 5800 GPS Receiver. The cable-free setup made life easier for the field surveyors.
At the start of the work, the Wildbach experts established approximately 150 monitoring points in and around the slide area. Anticipating that the moving earth would destroy some of the points, the surveyors purposely set more than they needed. They were right--by late summer, fewer than 70 points were available for GPS measurements. However, even with the loss of more than half of the monitoring points, there was sufficient information to characterize the motion. Using the Trimble system and collecting 30 seconds of data at each point, the surveyors could measure all of the points in less than three hours. “We decided to use Trimble instruments for measuring because they are user friendly and also very precise,” says Hofrat Diplom-Ingenieur Wolfgang Gasperl, an engineer with the Wildbach’s forestry service.
Each day’s data went to Gruber. After downloading the data collector into Trimble Geomatics Office Software, Gruber analyzed the results in an Excel spreadsheet and then used GIS software to develop graphics and reports. The measurements with the Trimble 5800 documented the surface movements. According to Gruber, the RTK accuracy was good for when the slide was moving quickly; higher accuracy is desirable during periods of slow or subtle motion. The ability of the Trimble 5800 to collect data for detailed computations and analysis made the instrument well-suited for the project.
Most of the GPS measurements were taken in the upper part of the Gschliefgraben and in the area of the debris outflow fan. Since much of the slide area was covered with trees, the GPS work was more challenging. But as work progressed and trees were cleared, GPS measurements became the primary source of information on the slide’s behavior. GPS points also provided control for five flights of airborne laser scans. The Wildbach team used Gruber’s data to determine the necessary countermeasures. Based on the GPS information, they planned additional wells, pipes and ditches, construction of protective walls, removal of earth mounds and tree clearing.
Taking StockBy June 2008, the Wildbach experts could reflect on the experience and lay new plans. It had been a huge effort. More than 5 million cubic yards of moving earth was stopped. Seventeen thousand truckloads had carried 327,000 cubic yards of soil and rock to be hauled away or dumped in the lake. To capture and carry water out of the valley, workers had installed 220 drainage wells, 6 miles of ditches and 3,600 feet of pipe. Trees had been cleared from 54 acres, and 1.2 miles of emergency and auxiliary roads were built.
Numerous measures are under way to help avoid a disaster in the future. Over the next 10 years, the Austrian government will invest upward of ï€¿11 million ($15 million) on drainage, flood prevention, reforestation and monitoring. The monitoring will utilize remote-sensing technologies including airborne laser scanning and echo sounding. Subsurface data will be taken through core drilling, inclinometers, piezometers, and seismic, geophysical and soil mechanics surveys. Surface monitoring will include crack maps and observations, terrestrial surveys and Webcam observation.
The GPS receiver is still at work making weekly measurements on 66 fixed points. “Thanks to this GPS monitoring, we were able to preserve the homes of the residents of Gmunden,” Gasperl says. “We hope that the danger will be avoided permanently through our preventative measures.”
Sidebar: The Makings of a LandslideThe trouble in the Gschliefgraben came as no surprise to geologists. Landslides had been occurring there since the Ice Age. The narrow Gschliefgraben valley lies between two mountains and drops from a height of 2,790 feet down to the shore of Lake Traun at 1,390 feet with an average slope of more than 15 percent. Every year, heavy rain or rockfalls detach thousands of cubic meters of rock and soil from the rugged slopes of the Traunstein mountain. This scree collects in the upper part of the Gschliefgraben and weighs down on the slope below. The scree accumulates under a top layer of marl (or marlstone), a loose mixture of materials that erodes easily. When rainwater or snowmelt from the Traunstein collects in the valley, it mixes with the marl, which then acts as a lubricant. When there is enough material and water, gravity takes over and the valley begins to slide.
Landslides are common in the Alps; in past centuries, the Gschliefgraben has pushed many buildings into the lake. Major slides were documented in 1470, 1660 and 1734. On calm days, divers in Lake Traun are said to be able to make out the historic farms that now rest on the lake bed. Although the danger of the Gschliefgraben has been thoroughly researched, there remains no way to bring the problem under complete control. Work to update local regulations began in 1974, and a danger-zone plan to prohibit construction in the slide area went into effect in 1987. The measures implemented in 2008 will provide further safeguards.