HR: 0800h
AN: C31B-0329    [Abstracts]
TI: Laboratory Study of Frictional and Rheological Properties of Till From Matanuska Glacier, Alaska
AU: * Rathbun, A P
EM: arathbun@geosc.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, 503 Dieke Building, University Park, PA 16802 United States
AU: Marone, C J
EM: cjm@geosc.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, 503 Dieke Building, University Park, PA 16802 United States
AU: Anandakrishnan, S
EM: sak@essc.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, 503 Dieke Building, University Park, PA 16802 United States
AU: Alley, R B
EM: ralley@essc.psu.edu
AF: Department of Geosciences, The Pennsylvania State University, 503 Dieke Building, University Park, PA 16802 United States
AB: Recent studies show that small-amplitude stress changes can trigger ice sheet motion and subglacial seismicity. Deformation in the subglacial region plays a key role in determining slip behavior, including creep, transient slip, and stick-slip (seismic) motion. However, progress in understanding these phenomena is limited by uncertainty in the rheology and frictional properties of glacial till. We report on detailed laboratory experiments to measure the frictional constitutive properties of till from the Matanuska Glacier. Experiments were conducted in a servo-controlled, double direct shear apparatus with dried till samples sheared at displacement rates of 1 to 300 $\mu$m/s and normal stresses of 0.5 to 5.0 MPa. Till was sheared in a three-block arrangement in which two layers are sandwiched between a central forcing block and two stationary blocks. The nominal frictional contact area is constant during shear. We studied the effect of layer thickness, shear-zone surface roughness, sliding history, normal stress, and slip velocity. In one set of experiments, the frictional contact area was 100 cm$^{2}$ and the forcing blocks were grooved perpendicular to the shear direction with a depth of 0.8 mm and wavelength of 1 mm. A second set of forcing blocks had contact dimensions of 15 cm x 15 cm, and these surfaces had two wavelengths of roughness: grooves of wavelength 1 mm and depth 0.7 mm were superimposed on grooves of wavelength of 10 mm and depth 0.7 mm. All experiments were done at room temperature and humidity. The till had grains ranging from 6.3 mm to finer than .063 mm with a mean of 2.67 mm. Sample thicknesses were 0.7 and 1 cm for the smaller set of blocks and 3 and 6 cm for the larger blocks. In all cases the grain size is less that 1/10 the layer thickness to minimize boundary effects. In the thicker layers, we investigated the role of shear localization by intentionally constraining the shear zone width. Preliminary results at normal stress of 0.5 MPa show stable sliding and give residual values of sliding friction of 0.65 at 1 $\mu$m/s increasing linearly with the log of velocity to a maximum of 0.77 at 300 $\mu$m/s. This trend indicates velocity-strengthening behavior, in which friction increases with sliding velocity. Increasing normal stresses to 1.0 and 5.0 MPa yields values for sliding friction of 0.61 to 0.64 and 0.56 to 0.60, respectively, showing that the degree of velocity-strengthening varies with normal stress. Coulomb-Mohr failure envelopes give an internal friction angle of 31.5 degrees and cohesion of 4.5 kPa. Our results indicate velocity-strengthening and thus imply that shear of the Matanuska glacial till will be dominantly stable under the conditions studied.
DE: 9310 Antarctica
DE: 9315 Arctic region
DE: 1827 Glaciology (1863)
SC: Cryosphere [C]
MN: 2004 AGU Fall Meeting