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| Titel |
From KTB amphibolite to Bentheim sandstone: the diminishing effect of the intermediate principal stress on faulting and fault angle |
| VerfasserIn |
B. Haimson, X. Ma |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063885
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| Zusammenfassung |
| The introduction of the “true triaxial” testing machine by Mogi (JGR, 1971) and later by
Haimson and Chang (JGR, 2000; IJRMMS, 2000) was instrumental in discovering the
hitherto largely unrecognized effect of the intermediate principal stress (Ïă2) on rock brittle
failure (or faulting) and fault angle (angle between fault-normal and direction of the major
principal stress Ïă1). It was observed that generally for a given Ïă3, the threshold of strain
localization and the level of Ïă1 at which rock fails (Ïă1,peak) rise monotonically with Ïă2
beyond the base magnitudes under axisymmetric compression (AC) when Ïă2 = Ïă3. An
inflection point is reached at some Ïă2that defines its maximum Ïă1,peak. Further
rise in Ïă2 leads to a gradually lower Ïă1,peak. However, even when Ïă2 approaches
Ïă1, strength is still somewhat higher than under AC. Similarly, fault angle for a
given Ïă3increases with the rise in Ïă2, at least until the maximum Ïă1,peak is reached.
These important roles of Ïă2 in fault formation and angle are totally neglected by the
commonly accepted Mohr-Coulomb theory, which assumes that faulting is a function of
only the two extreme principal stresses, and considers fault angle a unique material
property.
Our first true triaxial experiments, conducted on two crystalline rocks (under 1%
porosity), Westerly granite (Haimson and Chang, IJRMMS, 2000) and KTB amphibolite
(Chang and Haimson, JGR, 2000), exhibited a remarkable Ïă2 effect. At low Ïă3 (for example:
30 MPa), raising Ïă2 increased Ïă1,peak in the amphibolite by up to 59% (at Ïă2 = 200 MPa)
over its AC magnitude. Similarly, peak Ïă1,peak in the granite at Ïă3 = 20 MPa increased by a
maximum of 49% (at Ïă2 = 200 MPa) over its Ïă2 = Ïă3 level. As Ïă3was raised, the
increase in strength dropped steadily, but even at Ïă3 = 100 MPa maximum Ïă1,peak
in both rocks increased by 18% to 39% over the base level. The increase in fault
angle with the rise in Ïă2 reached a maximum of 20Ë for all levels of Ïă3 in the
KTB rock. In the granite, fault angle steepened by 10Ë to 15Ë as Ïă2 rose above
Ïă3.
A milder effect of Ïă2 on strength and fault angle was observed in a low porosity (Ï = 7%)
siltstone, extracted from the TCDP, Taiwan test hole (Oku, et al, GRL, 2007). At
low Ïă3 (25 MPa), the maximum Ïă1,peak was 28% larger than the AC level, when
Ïă2 reached 200 MPa. At higher Ïă3 (100 MPa), the maximum Ïă1,peak, reached at
Ïă2 = 300 MPa, was only 12.5% higher than the strength at Ïă2 = Ïă3. Fault angle
increase with Ïă2 for the same Ïă3 was limited to less than 10Ë , irrespective of Ïă3
level.
Currently, we are studying the true triaxial mechanical behavior of two quartz-rich
sandstones. In the Coconino sandstone (Ï = 17%), an even smaller Ïă2 effect was observed.
Ïă1,peak as a function of Ïă2 at Ïă3 = 20 MPa reached a maximum of just 10.5% higher than
under AC. At Ïă3 = 150 MPa, Ïă1,peak maximum increase was also about 10%. Fault angle rise
with Ïă2 for the same Ïă3, was less than 10Ë at any Ïă3 level. By far the smallest Ïă2 effect on
rock strength and fault angle was found in the high porosity (Ï = 25%) Bentheim
sandstone. In this rock at low Ïă3 = 30 MPa, Ïă1,peak reached a maximum of just 4% over
its AC magnitude. At higher Ïă3, such as at Ïă3 = 150 MPa, it showed a maximum
increase of 8% as Ïă2 rose. Average fault angle decreased from about 80Ë at Ïă3 = 0
MPa, to 48Ë at Ïă3 = 80 MPa, to near 0Ë at Ïă3 = 150 MPa (compaction band).
For any given Ïă3, however, fault angle rise with Ïă2 was limited to less than 10Ë
.
In conclusion, our research shows that the intermediate principal stress affects the
resistance to faulting and fault angle in both crystalline and clastic rocks. However, the effect
in clastic rocks is less pronounced, and appears to decrease with the rise in porosity. |
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