Load cells and landslides: understanding debris-flow flexible barriers
As we write this, the news is again dominated by a storm story, as Tropical Storm Freddy causes devastation in the African country of Malawi with flooding and mudslides. Last month heavy rain in Brazil caused landslides in San Paulo State, and widespread flooding in the US state of California and New Zealand have triggered landslides too. Sadly, all these events have claimed lives, alongside widespread destruction of homes and infrastructure.
Even in areas where landslides are normally quite frequent, the problem is growing, as a paper in “Nature” magazine explains:
“The frequency of landslides or debris flows is high in the mountainous areas of Southwest China due to steep passages, abundant rainfalls and solid-fragment sources1. As affected by the surge of extreme rainstorm events worldwide in recent years, some low frequency-landslides areas have turned into high ones, and the scale of hazards increases, which is difficult to accurately estimate and is causing considerable threats to residents and infrastructures nearby, as well as difficulties in the design of prevention and control measures.”
Testing the netting
A team of researchers wanted to check how efficient the flexible barrier usually used to retain small-scale debris flow in terms of its stiffness. They noticed that:
“The wire net component of the structure may be breached prior to the failures of supporting cables or anchorages, indicating a gap between structural design theory and actual engineering function.”
This net is held in place by three cables at the top, middle and bottom, with the cables interlaced with the net. Load cells were used to measure the force on the cables. To test this, a flume was constructed and the middle and bottom cables fitted with a load cell at each end of the cable cable and attached to the steel posts either side of the test flume. To properly assess the impact during each test in the flume:
“The load cells and the high-speed camera are triggered simultaneously to ensure the tensile loads and structural deflections to be synchronized.”
The University of Pennsylvania in the US also used a similar flume construction in their investigation of landslide mobility. Their testing device included six load cells to assess the impact force of sliding granular masses.
Holding back the mountain
In a previous blog, we explored how railway engineers took measure for “tying down the mountain of dirt” at the aptly named 10 Mile Slide region in British Columbia, Canada. You can read the story here.
Avalanches and snow stability
Load cells have also be used to investigate the conditions and snow structure that can place skiers at risk of self-triggered avalanches. See our blog on snow loads.
When the snow melts, there can also be considerable issues with mudslides. Since the year 2000, there has been a mudslide observation station at the Illgraben catchment, a debris flow torrent in the Swiss Alps. In order to measure vertical force without the influence of horizontal forces, a team created a unique scale.
“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 ZEL elastomer bearings consist of steel plates and layers of rubber (are) 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 (other) load cells also absorb the horizontal force of the mudslide (20 metric tons), minimizing force shunts with joint sleeves.”
Slip slidin’ away **
If you have a project where you need to assess forces on, in or around shifting surfaces, contact us. We’re always happy yo talk through your requirements and help design an effective testing set-up using load cells.
Know the phrase but just can’t place it? Here’s the Paul Simon song!
