The effect of bulk density on debris flow mobilization, Marin County, CA

Abstract

I tested the hypothesis that soils with low bulk density are more likely to mobilize into a debris flow during a shallow landslide event. Laboratory tests have demonstrated that loose, low bulk density soils contract during deformation causing elevated pore pressures and liquefaction. This mechanism for debris flow mobilization is not observed in the densest soils which fail as slump blocks. Previous methods of measuring bulk density are time consuming and imprecise, making them inadequate for testing this idea in the field. I measured bulk density using an instrument called the Mold Impression Laser Tool (MILT), a portable 3D scanner developed by NASA and modified by USGS for field measurement of soil density. Using the MILT, I performed bulk density measurements in soils adjacent to 15 shallow landslide scars in Marin County and compared the density of soil at debris flows sites to soil at slump sites. The average bulk density of debris flows (1.341 +/- 0.046 g/cm3; mean +/- S.E.) was significantly lower than the density of slumps (1.604 +/- 0.016 g/cm3; mean+/- S.E.) and the critical bulk density that separated mobilized failures from slumps was between 1.48 g/cm3 and 1.57 g/cm3. These findings suggest that slopes susceptible to debris flows can be identified by measuring soil bulk density.