HR: 17:30h
AN: S14C-07    [Abstracts]
TI: Compaction Can Limit Peak Vertical Velocity at Yucca Mountain
AU: * Andrews, D J
EM: jandrews@usgs.gov
AF: U. S. Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Compaction is irreversible decrease of volume of a porous material subject to compressive stress, and it is an effective mechanism to attenuate strong P waves. Compaction of porous unsaturated tuffs at Yucca Mountain can occur if compressive stress reaches a level never reached before since the deposition of the tuff. Compaction can place a physical limit on peak vertical ground velocity at the site of the proposed radioactive waste repository. A constitutive model for compaction is formulated that fits laboratory data on porous sandstone from Wong et al. (JGR 102, 3009-3025, 1997). In stress space, yielding occurs on a Mohr-Coulomb surface at low effective pressure and on an elliptical end cap at higher effective pressure. Stress in the model scales with P*, the effective pressure at crushing under hydrostatic loading, which depends on porosity, grain size, cementation, and other grain-scale properties of the sandstones. The data are fit with a non-associative flow rule, in which plastic strain has a compactive volumetric component at effective pressure greater than 0.1P* and a dilatant component at effective pressure less than 0.1P*. The model is used in dynamic calculations of normal-faulting earthquakes on the Solitario Canyon fault with complete stress drop through the seismogenic depth of the crust and with strong directivity of P waves toward the repository. Compaction occurs in the very porous Calico Hills Tuff unit below the repository. If the yield surface is chosen to be only slightly above the initial stress state in the Calico Hills Tuff, then peak vertical velocity at the repository is limited to 3.6 m/s. In that case, the value of P* used for tuff is 0.3 times P* for sandstone with equal porosity. More laboratory data are needed on compaction of tuffs at Yucca Mountain to establish the velocity limit.
DE: 5120 Plasticity, diffusion, and creep
DE: 7212 Earthquake ground motions and engineering seismology
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Seismology [S]
MN: 2007 Fall Meeting