Episode 33 Soil testing in the world's biggest blender - with Scott Brandenberg

Today, DesignSafe radio host Dan Zehner starts a conversation with geotechnical research engineer Scott Brandenberg, engineering professor at UCLA. In his investigations, Brandenberg employs the very large geotechnical centrifuge at the UC Davis Center for Geotechnical Modeling, a NHERI experimental facility.   Brandenberg was raised on a cattle ranch, where he helped his father fix machinery. What hooked him on engineering as a kid, he says, was entering a toothpick bridge competition. He majored in geotechnical engineering at Cal Poly in San Louis Obispo, and in graduate school at UC Davis, he did research with professors Ross Boulanger and Bruce Kutter. Brandenberg enjoyed grad school at UC Davis so much that he ended up completing his PhD there. Although he has been on faculty at UCLA for about 12 years, he spends much of his research time at the UC Davis centrifuge — a world-class facility that’s available to researchers everywhere.   Geotechnical centrifuge. Brandenberg describes the nine-meter radius centrifuge, which was originally used by NASA to test components in high-G fields. The machine can reach up to about 80 Gs. When the centrifuge spins at 60 Gs, the nine-foot arm is spinning about one-and-a-half times per second. Brandenberg jokes: “It’s like the world’s biggest blender.”   Brandenberg explains how soil modeling via centrifuge works, including the scaling effect, and why understanding soil behavior is so important in seismic engineering. Centrifuge testing mimics real, field-level stress conditions — the behavior of soil under stress.   Spinning — and shaking. Not only does the contraption spin, Brandenberg explains that the soil models are built in containers that rest on top of a shake table. Then, while the soil models are spinning around, researchers impose earthquake motions on them. He explains the scaling effect that high-G force has for simulating earthquakes. Time gets compressed, he says; it takes mere seconds to impose a shaking-motion equivalent to a one-minute-long earthquake.   In each soil model, hundreds of sensors monitor and record acceleration, displacement, and even water pressure inside the soil. Researchers also embed structures with strain gauges mounted to them to measure the bending or the axial load demands on a structure.   Brandenberg emphasized that researchers make models to capture fundamental mechanisms of loading, not to mimic the world perfectly. By measuring simplified models that let them capture fundamental load mechanisms — researchers ultimately understand how engineers should be doing design calculations for real infrastructure, on real sites that are more complicated and difficult.   In the second half of the podcast, Brandenberg provides a fascinating and detailed overview of his first major research project, which was to study propagation of earthquake ground motions through soft soil layer —  from painstakingly building the models, to testing them and then analyzing the results.   Among other things, Brandenberg explains why it’s important to measure the sheer strength properly over a wide range of shaking intensities, not just for the really strong ground motions, a finding he says is in parallel with other fundamental profiling studies.