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| Titel |
Running dilatancy banding as a mechanism of jointing: Experimental and numerical models |
| VerfasserIn |
Alexandre Chemenda, Si-Hung Nguyen, Jean-Pierre Petit, Julien Ambre |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049678
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| Zusammenfassung |
| There are basically two approaches to the problem of quasi-brittle fracture/rupture of
geomaterials. One is the fracture mechanics dealing with stability conditions of cracks
characterized by a strong stress concentration at the tips causing crack propagation. The other
approach is the formation of deformation localization bands as constitutive instabilities,
whose onset in quasi-brittle rocks can be considered as corresponding to the inception of
rupture. We investigate the conditions of applicability of these end-member approaches and
show a continuous transition from one to another with an increase in the confining pressure P
in the experimental extension tests on a synthetic physical rock analogue (granular, frictional,
cohesive and dilatant) material GRAM1 . Discontinuities/fractures perpendicular to the
least (axial) stress Ïă3 were generated in GRAM1 samples. These fractures form
dynamically and are of two types defined by the mean stress Ïăor P . When Ïă is very small,
the fractures form through mode I cracking with Ïă3 equal to the material tensile
strength. The fracture walls have smooth surfaces in this case. Increase in Ïă causes
increase in Ïă3 at fracturing, which becomes less negative and reaches small positive
(compression) values, while the failure still occurs along a discontinuity perpendicular
toÏă3. Thus, the discontinuities generated starting from a certain Ïă value cannot be
mode I fractures. Increase in Ïă also results in changes in the relief of the surfaces of
discontinuities after their postmortem opening (separation of the walls): the surfaces become
rougher, with the topography features forming faint/delicate plumose patterns very
similar to those on the geological joint walls. SEM observations of the unopened
discontinuities show that they represent several grain sizes-thick bands of a material
which underwent a heterogeneous decohesion and volume/porosity increase. This
suggests a dilatancy within bands. After opening they become fractures with plumose
fractography. As indicated, these fractures could not be formed through the mode I
mechanism. It is suggested that the formation mechanism represents a running constitutive
instability in the form of dilation banding. This instability is reproduced in the
numerical models showing fracturing pattern very similar to that observed in the
experiments. The morphological similarity between the experimentally generated
plumose-surface fractures and natural joints surfaces is shown. On the other hand, MEB
observations evidence a textural similarity between the experimental bands and natural
unopened incipient joints found in fine grained rocks. It is suggested that propagating
dilation bands could be an important mechanism for the generation of natural joints. |
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