Measuring Mudslides: Tracking Extreme Forces
Tons of debris, mud and water rush precipitously down mountainsides into valleys during mudslides. To gain a better understanding of the forces at work, the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) is using a special scale equipped with measurement technology from HBM.
"If you've ever seen a mudslide rush down a mountain, you won't soon forget the thundering and roaring," said Dr. Yolanda Deubelbeiss, a scientist with WSL.
Mudslides occur frequently in the mountains during or after heavy precipitation or when the snow melts. When loose material becomes saturated with water, it starts to slide, taking more debris down with it along the way. Trees and boulders can also be caught up in the flow and tumble down to the valley, often following the course of creeks or gullies at speeds up to several meters per second, with devastating results for everything in the way. "It is especially dangerous if the mudslide takes a new route outside a creek bed. Then it can cause serious damage to houses, bridges or roads," explained the geologist.
Hazard maps indicate the extent to which an area is at risk of mudslides and how much damage could potentially occur. They are based partly on the results of computer simulations using defined scenarios but also, and more importantly, on field observations and damage data from earlier mudslides. This knowledge plays a role in regional planning or may lead to the construction of barriers and dams or widening of creek beds. "Computer models only allow us to draw theoretical conclusions about the flow behavior of a mudslide. To provide better protection for people against these enormous forces, we need a better understanding of what happens inside a mudslide. That will advance our work on the simulation models and make them more realistically reflect natural processes," explained Deubelbeiss. That might make it easier to predict, for example, how far some pieces could stray from the main flow.
With this purpose in mind, WSL set up a mudslide observation station in 2000 at the Illgraben catchment, a debris flow torrent in the Canton of Wallis in the Swiss Alps. "The location is ideal, as the Illbach creek is one of the most active mountain torrents in the Swiss Alps and several mudslides occur there every year, so we can measure the natural processes," said Deubelbeiss. Since 2004, a video camera, ultrasonic sensors and radar measuring devices have been added to the mudslide scale. "The scale helps us better understand the physical processes inside a mudslide instead of just looking at them from the outside," the expert continued. According to the researchers at the institute, the project is the first and largest of its kind in the world.
Decoupled Force Measurements
The scientists used the concrete foundation of a bridge for the scale. Its flat U shape hugs the Illbach creek bed. The actual scale is embedded in this foundation. A steel plate eight meters square, twelve millimeters thick and weighing 300 kilograms is supported by a steel frame consisting of HEB360 sections (2800 kilograms), which in turn rest on the measuring cells.
"It was no simple matter to build a scale capable of measuring such enormous dynamic forces. The mudslide is constantly in motion, after all. It doesn't stop so we can weigh it," said Bruno Fritschi, measurement technology expert at WSL. To represent the forces in a mudslide, the scale records vertical normal forces. These are the forces that exert pressure on the ground from above. It also determines horizontal shear forces simultaneously. This is the loading produced as the material moves continuously forward. The flow depth, flow speed, and pore water pressure are also measured. "First, we obtain a realistic overview of the forces at work in a mudslide through the combination of these data points. At the same time, this interaction is the greatest challenge for the measurement technology because the values have to be represented separately from each other," explained Fritschi. "That's why we decided to use HBM force transducers. Their products not only offer precise sensor systems; they allow us to measure vertical force without horizontal forces affecting the result."
A critical aspect of this measurement process is the transfer of force from the steel plate via an elastomer bearing onto the load cell (type C2, 50 metric tons). Type ZEL elastomer bearings consist of steel plates and layers of rubber arranged on top of each other and joined by vulcanization. When they transfer a force, the elastic portion practically eliminates the horizontal weight: The layers move so that lateral force effects are not transferred to the load cell. Two type U2A load cells also absorb the horizontal force of the mudslide (20 metric tons), minimizing force shunts with joint sleeves.
Withstanding Heavy Impacts
The fact that the scientists measure the results in metric tons is an indication of the enormous forces involved. "So the measurement technology not only needs to return precise results, it has to withstand a lot," Fritschi continued. "The large boulders in a mudslide roll over the scale at high speeds, producing heavy impacts. The force transducers must be able to withstand this massive loading. We can't build in any overload protection – the mudslide simply weighs what it actually does."
Based on measurements made so far in Illgraben, that could be up to 40 metric tons of compressive load moving along at up to six meters per second. The elastomer bearings in HBM sensor systems absorb these forces. However, the forces are not the only challenge for the technology: it also needs to work reliably in an extreme environment. "The scale is surrounded by mud and water. It's cold in the winter and hot in the summer. The system is kept dry, but the conditions for the measurement are extremely harsh," added Fritschi.
First General Overall Renovation - 2012
After eight years of operation without having any significant maintenance done, the scale needed renovations in April 2012. The steel beams had become bent and the plate had worn away due to friction and boulder impacts. "We could no longer exclude the possibility of a breakthrough, which could have destroyed the force measurement cells under the plate," continued Fritschi.
The renovation was extensive: first, the team had to divert the Illbach creek and clear access routes for the excavator. Next came the actual job of replacing the steel plate and beams. Fortunately, the scientists were able to continue using the existing measurement technology by mounting the elastomer bearings and measuring cells on a new steel structure.
Before the team lowered the new steel plate on the framework, they first checked the measuring cells with no load. To do this, they weighed the steel frame and cover plate individually and compared the data with the measured values recorded at the beginning of the project.
"We had suspected that the zero point of the two measuring cells had changed over the years," said Fritschi, explaining the process. "And the values did have a deviation of 10 tons, but the results showed that the deviation was constant and we could therefore factor it into the results very easily. This means we can continue using the old measuring cells." A replacement was initially not possible because the anchor bolts fastening the measuring cells in place could no longer be released.
Half a year later, after the summer mudslide season, the scientists finally removed the measuring cells with considerable effort to test them in the lab and correct any deviations or replace them if they were faulty. The measuring cells will be reinstalled in the scale in Spring 2013. Fritschi concluded: "We would like to continue research here at Illgraben for a few more years – we don't understand all the physical processes in a mudslide yet."