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Analysis of faults terminaisons


at the metric scale, shear faults display horse-tail terminaisons.





Figure. Orientation distribution of faults and associated farctures in a granite pluton coming from a Variscan shear zone domain (Borne granite, Cévennes, SE-France). Note that in such rocks ( non soluble ) shear fracture terminations have horse-tail configurations. Exercice : place the orien-tations of of s1 and s 3.




Horse tail fault terminaisons are also observed at the regional scale (1) Stephanian (coal) and Mesozoic rocks ; (2) granite ; (3) Cevennes Palaeozoic Schists ; (4) migmatites ; (5) shows the quantity of displacement along the fault.


VI. Mohr envelop and Coulomb critèrion


1. The Mohr envelop


Back to the cylindrical specimen subjected to a confining pressure slitho = 1/2 (s1 + s3). Let us determine experimentally the value of s1 (vertical pressure) leading to rupture, for a set of slitho values.

s1 - slitho [= s1 - [1/2 (s1 + s3)] = 1/2 (s1 - s3)] gives the radius of the Mohr circle at rupture under slitho as the lithostatic pressure. The envelop of all the Mohr circles (at rupture) is the curve tangent to the circles : it is called the Mohr envelop. It provides a characterization of the material (Figure). Each point of this curve gives the maximum, or critical (tc, snc) values that the material can sustain without breaking (or just before breaking).



Figure. The envelop of the Mohr circles at rupture (or Mohr envelop) is characteristic of a material.

Date: 2014-12-28; view: 958

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II. FRACTURE MODES | Coulomb criterion
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