Micromechanics of failure in brittle geomaterials. Final technical report (for 7/1/1994 - 8/31/2000) Page: 4 of 6
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Mechanics of compressive failure in sandstone
We conducted a comprehensive study of the inelastic and failure behavior of 6 sandstones
(Adamswiller, Berea, Boise, Darley Dale, Kayenta and Rothbach) with porosities ranging from
15% to 35%. Most experiments were conducted in triaxial compression on samples saturated
with distilled water. A broad range of effective pressures were selected so that the transition in
failure mode from brittle faulting to cataclastic flow could be observed. To investigate the effect
of loading path, some triaxial extension tests were also conducted.
The mechanical data for saturated sandstone deformed in triaxial compression were presented
and analyzed by Wong, David and Zhu . Under relatively low effective pressures, samples
fail by brittle faulting with dilatancy and strain softening. Under relatively high effective
pressures, samples fail by cataclastic flow with shear-enhanced compaction and strain hardening.
The initial yielding is identified with the onset of shear-enhanced compaction. We have obtained
one of the most complete data sets for the compactive yield stress in sandstone. Our new data
validate several key concepts currently assumed in continuum models of compaction. First, the
compactive yield stresses map out an approximately elliptical envelope in stress space, in
agreement with the critical state and cap models. Second, the normality condition of plasticity
theory can be adopted to associate a flow-rule with the compactive yield envelope. The data
provide additional insight into microstructural controls on compaction behavior. The compactive
yield envelopes expand with decreasing porosity and grain size R. The critical stress for the
onset of grain crushing and the brittle-ductile transition both scale as ( R)'32, and geologic data
on tectonic faulting in siliciclastic formations (of different porosity and grain size) are consistent
with this model. While our data are in reasonable agreement with current continuum models for
compactive failure, they also underscore the inadequacy of certain theoretical models for the
development of dilatancy and shear localization.
Effect of water on compressive failure of sandstone
It has generally been recognized that the presence of interstitial fluid can lower the
compressive strength through chemical processes such as stress corrosion and the Rehbinder
effect. Published data indicate that the extent by which strength is reduced in the presence of
water is highly variable in sandstone. To investigate this question, hydrostatic and triaxial
compression tests were performed on sandstone under nominally dry and saturated conditions at
room temperature. Significant differences were observed in the strain hardening behavior of
nominally dry and water-saturated samples deformed at the same effective pressure and strain
rate. Zhu and Wong  observed that water-weakening effects in both the brittle faulting and
cataclastic flow regimes are particularly pronounced in sandstones with about >10% feldspar.
Unlike previous studies which focused on comparison of the compressive strength, our
measurements include volumetric strain, porosity change and acoustic emission and such an
integrated approach provides additional insight into the coupling between water-rock interaction
and mechanical deformation. The data allow us to formulate a theoretical model to consistently
interpret the dilatant and compactive failure behaviors due to presence of water and initial
damage in a fracture mechanics framework [Baud, Zhu and Wong, 2000]. A review of our
research on mechanical compaction was also presented [Wong and Baud, 2000].
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Wong, Teng-fong. Micromechanics of failure in brittle geomaterials. Final technical report (for 7/1/1994 - 8/31/2000), report, December 1, 2000; United States. (https://digital.library.unt.edu/ark:/67531/metadc733923/m1/4/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.