HR: 0830h
AN: NG41C-0082    [PDF]
TI: Critical Threshold for Spontaneous Failure: Macro- and Micro- Behavior of Granite Loaded to Failure
AU: * Katz, O
EM: odedk@mail.gsi.gov.il
AF: Geological Survey of Israel, 30 Malkhe Yisrael St., Jerusalem, 95501 Israel
AU: Reches, Z
EM: Reches@earth.es.huji.ac.il
AF: School of Geology & Geophysics, Univ. of Oklahoma, 100 E. Boyd st., Norman, OK 73019 United States
AB: The ultimate strength, time-dependence creep and associated microstructure of granite samples are examined as an attempt to characterize the critical parameters of brittle rock failure. We loaded triaxially 27 cylinders of the medium grain-size Mount Scott granite (western Oklahoma) under dry, room temperature conditions. Thirteen of the samples were loaded under confining pressure ranging from 0 to 50 MPa, and the group of 14 samples was loaded under confining pressure of 41MPa, for which the ultimate strength is Us = 586+-16 MPa. The 14 samples were loaded up to pre-selected differential stress (NDS) that ranges from 0.54 Us to 1.05 Us, and were then held under constant stroke for periods as long as six hours. The failure could be reasonably well predicted by two macroscopic parameters. One is the maximum differential stress: the eleven samples loaded under NSD $\leq$ 0.95 did not fail during the six hours of hold period, whereas the three samples loaded by NDS>0.95 failed spontaneously after a few seconds to an hour of hold time. The high Weibull parameter (m=13-22) of strength distribution of a heterogeneous rock is in agreement with this observation. The second parameter is the ``crack volumetric strain'' (CVS) that increases monotonously for NDS$\leq$0.95, but at NDS>0.95 it reaches a critical value of ~0.001 beyond which it is poorly constrained (with CVS approaching 0.005). We mapped the microfractures in thin-sections prepared from 5 deformed samples that cover the full loading range: 0.00, 0.57, 0.88, 0.96 of the rock strength and failure. The microstructural thin-section maps provided quantitative damage intensity (approaching 0.2) and fractal dimensions of the microfractures length distribution (1.5 for unloaded sample and 2.2-2.4 for loaded samples); these maps however, provide no critical failure indicator. Which of the examined parameters could be used to determine a critical failure state in an active fault-zone? We believe that the "crack volumetric strain" is the most promising one as in the experiments it displays critical behavior and in the field it could be detected by velocity decrease of guided waves.
DE: 7223 Seismic hazard assessment and prediction
DE: 8020 Mechanics
DE: 8030 Microstructures
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting