HR: 1340h
AN: V43B-1429    [Abstracts]
TI: Granular Friction in Laboratory Shear-Zone Tests on Volcanic Sediments
AU: * Samuelson, J
EM: jsamuels@geosc.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, Deike Building, University Park, PA 16802 United States
AU: Marone, C
EM: cjm38@psu.edu
AF: Department of Geosciences, The Pennsylvania State University, Deike Building, University Park, PA 16802 United States
AU: Voight, B
EM: voight@ems.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, Deike Building, University Park, PA 16802 United States
AB: We report on detailed laboratory experiments designed to elucidate the frictional behavior of volcanic materials, including pyroclastic flow debris from Soufriere hills volcano, Montserrat, and lahar deposits from Mt. St. Helens. Experiments were conducted in a servo-controlled, double-direct shear apparatus by shearing two 5-mm thick layers of loosely packed pyroclastic material between three roughened forcing blocks under conditions of monitored temperature and humidity. The central block is driven between the stationary side blocks at a precisely controlled displacement rate (typically 10 $\mu$m/s). We studied the effects of loading velocity, normal stress, grain size distribution, and water content. Normal stress was maintained constant during shear. A range of grain sizes and grain size distributions were examined, using material up to 1.0 mm in diameter. Median diameter (Md$\phi$) and the phi deviation measure ($\sigma$$\phi$) were varied, as well as several other distribution parameters. Experiments were conducted on pyroclastic material of fine (0.063-0.125 mm) and coarse (0.5-1.0 mm) grain sizes, as well as two broader grain size distributions (0.125-1.0 mm, and 0-1 mm). Using a normal stress range of 0.75 to 8 MPa we created a Coulomb-Mohr envelope and found that the coefficient of internal friction varies from 0.56 to 0.64 over this grain size range. The residual coefficient of sliding friction increases slightly, from 0.63 to 0.64, with increasing grain size and decreases from 0.61 to 0.56 with a widening of the grain size distribution. Smaller grain sizes and wider size distributions also exhibited higher apparent cohesion. For experiments on the natural grain size distribution (0-1 mm), we varied the shear velocity in the range 10 to 900 $\mu$m/s. These data indicate that the value of residual sliding friction increases with slip velocity, and thus exhibits velocity strengthening frictional behavior. A series of variable normal stress experiments were run at 100 $\mu$m/s so as to create a Coulomb-Mohr envelope that indicates the coefficient of internal friction increases from 0.56 at a shear velocity of 10 $\mu$m/s to 0.65 at 100 $\mu$m/s. Also the apparent cohesion is seen to fall from 78.6 kPa in the low velocity case to 23.6 kPa in the high velocity case. Ongoing experiments include slide-hold-slide tests to identify time-dependent frictional healing effects and saturated tests to investigate the effects of water. Our results indicate that the frictional properties of granular, volcanic sedminent are sensitive to shear velocity, normal stress, and variations in grain size distribution.
DE: 8404 Ash deposits
DE: 8499 General or miscellaneous
DE: 5199 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting