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| Titel |
Failure-plane angle in Bentheim sandstone subjected to true triaxial stresses: experimental results and theoretical prediction |
| VerfasserIn |
Xiaodong Ma, John Rudnicki, Bezalel Haimson |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250087742
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| Publikation (Nr.) |
 EGU/EGU2014-1800.pdf |
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| Zusammenfassung |
| We conducted true triaxial tests in the high-porosity (n = 24%), quartz-rich (95%), Bentheim
sandstone. An important objective was to investigate the dependence of failure-plane angle θ
(angle between the normal to the plane and Ïă1 direction) on the prevailing stress conditions.
We employed two distinct loading paths, and seven Ïă3 magnitudes (between 0 and 150 MPa).
In tests using the common loading path, Ïă2 and Ïă3 were fixed, while Ïă1 was raised
monotonically to failure. In tests using the novel loading path (which facilitate
comparison with theoretical predictions), Ïă3 was fixed, and the Lode angle, Î (=
tan-1 [(Ïă1 - 2Ïă2 + Ïă3) / 30.5(Ïă1 – Ïă3)]) was kept constant by raising Ïă1 and Ïă2
simultaneously at a set ratio b [= (Ïă2 -Ïă3)/(Ïă1 -Ïă3)] until failure occurred. Six
stress ratios b (= 0, 1/6, 1/3, 1/2, 3/4, 1), i.e. six Î (= tan-1 [(1-2b) / 30.5]) values
from +Ï/6 (axisymmetric compression) to –Ï/6 (axisymmetric extension) were
used.
In axisymmetric common loading path tests, failure-plane angle θ generally declined as
the applied Ïă3 = Ïă2 increased from about 80° at Ïă3 = Ïă2 = 0 MPa to 0° at Ïă3 = Ïă2 = 150
MPa (forming compaction bands). In tests where Ïă3 - Ïă2, the resulting failure-plane strike
was consistently parallel to Ïă2 direction. For low Ïă3, θ typically rose by up to 12° as Ïă2 rose
from Ïă2 = Ïă3 to Ïă2 = Ïă1. However, the rise in θ with Ïă2 tended to diminish at
higher Ïă3. A limiting case occurred at Ïă3 = 150 MPa, where failure plane remained
at 0°, regardless of the rise in Ïă2. In the novel loading path tests, failure-plane
angle θ declined monotonically for any given Lode angle Î, from roughly 80° to
0°, as the mean stress at failure (Ïăoct,f) rose from about 20 MPa to around 220
MPa; for a constant Ïăoct,f, θ typically increased from 10° (at Ïăoct,f = 20 MPa)
to 30° (at Ïăoct,f = 220 MPa) as Î dropped from +Ï/6 (Ïă2 = Ïă3) to -Ï/6 (Ïă2 =
Ïă1).
We compared the measured θ with that predicted using equation 28 in Rudnicki (2013), an
extension of the Rudnicki and Rice (1975) prediction to include the third stress
invariant Î. (Space does not permit detailing the equation in this abstract.) The theory
treats octahedral shear stress at failure (Ïoct,f) and the resulting θ as dependent on
Ïăoct,f and Î. We used two series of the novel loading path tests: axisymmetric
compression (Î = +Ï/6) and pure shear (Î = 0) to constrain that dependence. The failure
conditions in the novel loading path tests were then simulated to compare the predicted
failure-plane angles with the experimental results. The predictions were in general
agreement with the experimental data, except when Î = -Ï/6 (Ïă2 = Ïă1). In the
common loading path tests, failure prediction replicated the general rise of the
experimentally observed θ with Ïă2 for a given Ïă3,as well as the diminished rise at high Ïă3
magnitudes.
The reasonable agreement between the predicted and the observed failure-plane
angle demonstrated the applicability and the limitations of Rudnicki’s (2013) theory. |
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