HR: 14:55h
AN: H43G-06    [Abstracts]
TI: Using Controlled Landslide Initiation Experiments to Test Limit-Equilibrium Analyses of Slope Stability
AU: * Reid, M E
EM: mreid@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 910, Menlo Park, CA 94025 United States
AU: Iverson, R M
EM: riverson@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Ct. Bldg. 10, Vancouver, WA 98683 United States
AU: Brien, D L
EM: dbrien@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd. MS 910, Menlo Park, CA 94025 United States
AU: Iverson, N R
EM: niverson@iastate.edu
AF: Iowa State University, Department of Geological and Atmospheric Sciences, Ames, IA 50011 United States
AU: LaHusen, R G
EM: rlahusen@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Ct. Bldg. 10, Vancouver, WA 98683 United States
AU: Logan, M
EM: mlogan@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Ct. Bldg. 10, Vancouver, WA 98683 United States
AB: Most studies of landslide initiation employ limit equilibrium analyses of slope stability. Owing to a lack of detailed data, however, few studies have tested limit-equilibrium predictions against physical measurements of slope failure. We have conducted a series of field-scale, highly controlled landslide initiation experiments at the USGS debris-flow flume in Oregon; these experiments provide exceptional data to test limit equilibrium methods. In each of seven experiments, we attempted to induce failure in a 0.65m thick, 2m wide, 6m$^{3}$ prism of loamy sand placed behind a retaining wall in the $31\deg$ sloping flume. We systematically investigated triggering of sliding by groundwater injection, by prolonged moderate-intensity sprinkling, and by bursts of high intensity sprinkling. We also used vibratory compaction to control soil porosity and thereby investigate differences in failure behavior of dense and loose soils. About 50 sensors were monitored at 20 Hz during the experiments, including nests of tiltmeters buried at 7 cm spacing to define subsurface failure geometry, and nests of tensiometers and pore-pressure sensors to define evolving pore-pressure fields. In addition, we performed ancillary laboratory tests to measure soil porosity, shear strength, hydraulic conductivity, and compressibility. In loose soils (porosity of 0.52 to 0.55), abrupt failure typically occurred along the flume bed after substantial soil deformation. In denser soils (porosity of 0.41 to 0.44), gradual failure occurred within the soil prism. All failure surfaces had a maximum length to depth ratio of about 7. In even denser soil (porosity of 0.39), we could not induce failure by sprinkling. The internal friction angle of the soils varied from $28\deg$ to $40\deg$ with decreasing porosity. We analyzed stability at failure, given the observed pore-pressure conditions just prior to large movement, using a 1-D infinite-slope method and a more complete 2-D Janbu method. Each method provides a static Factor of Safety (FS), and in theory failure occurs when FS $\leq$ 1. Using the 1-D analysis, all experiments having failure had FS well below 1 (typically 0.5-0.8). Using the 2-D analysis for these same conditions, FS was less than but closer to 1 (typically 0.8-0.9). For the experiment with no failure, the 2-D FS was, reassuringly, $>$ 1. These results indicate that the 2-D Janbu analysis is more accurate than the 1-D infinite-slope method for computing limit-equilibrium slope stability in shallow slides with limited areal extent.
DE: 1824 Geomorphology (1625)
DE: 1829 Groundwater hydrology
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting