HR: 0800h
AN: C41A-0054    [Abstracts]
TI: A laboratory apparatus to measure clast-bed contact forces
AU: * Cohen, D
EM: dcohen@iastate.edu
AF: Department of Geological and Atmospheric Sciences, Iowa State University, 253 Science I, Ames, IA 50011, United States
AB: Glacier dynamics, sediment transport, and erosion are controlled in part by processes occurring at the interface between basal ice and bedrock. One critical parameter is the contact force between a clast and the bedrock. This force affects many processes such as basal friction which regulates sliding speed, slip resistance which influences basal shear stress and may cause micro-seismic events associated with slip instabilities, abrasion which controls rates of erosion, landscape evolution, and production of sediments. Despite field and laboratory evidences indicating that contact forces may be up to one order of magnitude higher than estimated from leading theories, no studies have yet measured with precision the magnitude of contact forces and how contact forces vary as a function of key glaciological variables such as basal melt rate and effective pressure. An apparatus was designed to make two independent measurements: (1) the contact force between a clast and a hard bed as a function of melt rate and effective pressure; (2) the drag force on an identical clast away from the bed as a function of the ice speed. The contact force differs from the drag force because of the presence of the bed which modifies the ice flow field. Measurement (2) is necessary to estimate the rheological properties of the ice and to quantify wall- (bed) effects on the drag force. The apparatus consists of a hydraulic press that pressurizes an ice cylinder, 24~cm high and 20~cm in diameter, to 1.0 - 1.5~MPa. The ice cylinder is contained inside a polycarbonate vessel. Above and below the ice cylinder are three disks: an aluminum disk sandwiched between two Delrin disks. The aluminum disks are hollow and used to circulate a fluid at a controlled temperature. The Delrin disks are used to isolate the ice from the cold room and to control the flow of heat to the ice block. The ice is kept at the melting temperature by circulating a fluid in channels inside the polycarbonate vessel and in the top aluminum disk. The fluid is pumped from an external bath at a controlled rate and temperature. The melt rate at the ice-bed (bottom Delrin disk) interface is controlled by a fluid circulating in the bottom aluminum disk at a temperature slightly above the melting temperature. The fluid is pumped to the disk by a second external bath. To measure the drag force ice exerts on a clast, a Delrin sphere is suspended by Kevlar filaments to a load cell above the ice cylinder. The sphere remains immobile as ice moves downward by melting at the ice-bed interface. To measure the contact force, a Delrin sphere sits above a ceramic rod running through the center of the three bottom disks. The rod transmits the contact force to a load cell beneath the apparatus. Temperature is monitored with thermistors at the ice-bed interface and in the ice. Ice motion is measured with a LVDT. Water pressure at the ice-bed interface is measured with two piezometers. Three drains on the outside perimeter of the Delrin bottom disk remove melt water. The effective pressure underneath the sphere is controlled by a valve that opens or closes a drain near the center of the bottom Delrin disk.
DE: 0738 Ice (1863)
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 0794 Instruments and techniques
DE: 1815 Erosion
DE: 1862 Sediment transport (4558)
SC: Cryosphere [C]
MN: 2007 Fall Meeting