Debris flows are a particularly destructive class of landslide, in that large volumes of wet soil and rock can move as liquefied masses at very high velocity, and with little warning. Debris flows may be triggered by earthquakes, volcanic eruptions, or rising groundwater. Because of their rapid motion, they can result in a large number of fatalities; an extreme example is the destruction of San Salvador?s suburb of Santa Tecla by an earthquake-triggered debris flow in 2001, resulting in over 700 deaths. The US has not experienced such a devastating event, but flow slides triggered by rainfall or earthquakes have caused significant material damage, and a number of fatalities, particularly in densely-populated areas of California. Both human and material losses are expected to rise with climate change, and as urbanization of landslide-prone areas continues.
Debris flows have the particular feature of being able to run out for long distances over very flat slopes. In order to define high-hazard zones and plan possible defensive measures, it is necessary to calculate potential slide velocities and run-out lengths. Modeling these slides is difficult; when the slide is triggered, the sliding material is observed to liquefy, i.e. high internal water pressures force the soil grains apart, and the mass behaves as a heavy viscous fluid. However, as the slide moves down-slope, it segregates into a drained ?snout? of coarser material which could be expected to brake the flow, and a still-liquefied interior, where the finer portion of the soil mass pushes the slide forward. How these two zones interact, so that the overall mass maintains a high ?efficiency? which allow it to travel long distances even on flat slopes, is not well understood, although it is obviously related to the slides? capacity to maintain high internal fluid pressures over a long runout. In many cases this seems surprising, since the grain size distribution of even the interior, liquefied, sliding material would suggest a relatively rapid dissipation of the water pressures required to maintain the soil in a liquefied condition. Sophisticated numerical models for debris flow motion have been developed, but a fundamental problem is that they require, as input, a prediction of the internal water pressures during sliding.
In order to clarify this basic problem, this project will develop two sizes of instrumented ?smart rocks? using recently-developed MEMS instrumentation: a ?smart pebble? of golf-ball size to measure how the interior particles vibrate, and how water pressure develop and dissipate in the liquefied interior of the sliding mass; and a ?smart cobble? of soft-ball size, more heavily instrumented, so as to also be capable of tracking how coarser particles move towards the landslide snout during sliding. Both will be ovoid in shape, to simulate natural rocks, and have metal casings to make them easier to find at the end of the slide, using a metal detector. The first two years of the project will be spent in design and extensive laboratory testing of the smart rocks, by a team which will include graduate students from mechanical and civil engineering as well as civil, mechanical and electrical engineering undergraduates.
In the third year, the smart rocks will be deployed in large-scale artificial flow slides at the USGS experimental landslide flume in the Willamette National Forest, Oregon; the research team which operates the flume is highly supportive of this project. Results from these tests will be used by the project team to calibrate and refine existing models of debris flows in a way which has not been possible to date. Further, a well-established partnership program with local middle and high schools will seek to involve students and teachers as observers and participants in designing the smart rocks and interpreting the results.
Debris flows are a particularly destructive class of landslide, in that large volumes of wet soil and rock can move as liquified masses at very high velocity, and with little warning. Debris flows may be triggered by earthquakes, volcanic eruptions, or rising groundwater. Because of their rapid motion, they can cause large property destruction and loss of life. In order to define high-hazard zones and plan possible defensive measures, it is necessary to calulate slide velocities and run-out lengths. Modeling these slides is difficult; the sliding mass is observed to segregate into a drained 'snout' of coarser material which tends to impede the flow, and a finer liquified interior which impels the slide forward. How these two zones interact, so that the overall mass maintains a high 'efficiency' is not well understood. To help provide useful information on debris flow behavior, two unique instrumented 'smart rocks' were created to measure water pressure and rock movement during debris flow tests, one softball size and the other a small cylinder. The larger 'smart rock' was placed in debris flow tests at the USGS experimental landslide flume in the Willamette National Forest, Oregon. Acceleration and pressure data captured by the instrumentation was downloaded to a computer and compared favorably to data from stationary sensors located on the flume. While some 'smart rock' flume testing data is available, much more is needed to significantly improve the basic understanding of a highly destructive class of landslides, and to help calibrate and refine numerical models of debris flows. Given the results of our research, it is now possible to build and deploy multiple sensors to further expand and refine the information gathered from experimental test slides. The broader impact of our work is the evential improvement of defense measures in debris-flow prone areas, whether in the form of better zoning or design of possible physical defenses. From the educational viewpoint, development and deployment of the 'smart rocks' involved four graduate students in civil and mechanical engineering, as well as a team of undergraduate students in civil engineering.
