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 HBK 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 HBK 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.
