HR: 0800h
AN: T41F-1306    [Abstracts]
TI: Interpreting the Frictional Behavior of the Smectite Clay Montmorillonite
AU: * Moore, D E
EM: dmoore@usgs.gov
AF: U. S. Geological Survey, Mail Stop 977 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Lockner, D A
EM: dlockner@usgs.gov
AF: U. S. Geological Survey, Mail Stop 977 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: Montmorillonite has been the most widely tested phyllosilicate mineral in soil and rock mechanics friction investigations, but many of the published data are contradictory, with reported values of the coefficient of friction (= shear stress/effective normal stress) ranging from 0.06 to 0.78. We report some new laboratory and petrographic data which illustrate that this wide variation is in part a function of the large difference in coefficient of friction between thoroughly dried (0.7 or greater) and water-saturated (0.3 or lower) montmorillonite. Dry montmorillonite gouge is subject to standard frictional processes such as abrasion, wear, and fracture during shear. In contrast, shear of water-saturated montmorillonite gouge is concentrated in thin films of water that are adsorbed onto the (001) surfaces of the platy grains. Our recent studies suggest that the water-saturated shear strength of sheet-structure minerals increases with the strength of the bonding of the polar water molecules to the (001) surfaces, and the relative weakness of water-saturated montmorillonite may be largely owing to its small layer charge. Values of the coefficient of friction for montmorillonite that are considered to represent water-saturated, equilibrated conditions increase from 0.06 at effective normal stresses below 1 MPa to 0.30 at 300 MPa. This correlation is attributed to decreasing thickness of the surface water films with increasing effective normal stress. Similar stress dependence of frictional strength can be demonstrated for the serpentine minerals, muscovite, biotite, phlogopite, chlorite, kaolinite, and talc, and it is considered to be characteristic of sheet silicates. This behavior contrasts with that of most other silicate minerals, for which the coefficient of friction exhibits little pressure sensitivity below 200 MPa effective normal stress and then decreases at higher stresses (Byerlee's law). Most of the published strength data for montmorillonite fall outside the range of values for water-saturated, equilibrated samples. Of these, the samples that are overly strong for a given set of experimental conditions may have been only partially saturated. Those samples that are weaker than the water-saturated, equilibrated samples at a given effective normal stress appear to result from inadequate drainage and consequent build-up of internal pore pressure. The velocity dependence of montmorillonite strength has not been extensively investigated, but water-saturated montmorillonite gouge is velocity-strengthening over the range of conditions tested to date whereas dry and partially saturated montmorillonite gouge may be velocity weakening at some velocities. These results highlight the hazards of interpreting fault-zone behavior based on experiments that do not approximate natural conditions.
DE: 8199 General or miscellaneous
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting