Analysis of faults terminaisons
… at the metric scale, shear faults display horsetail terminaisons.
Figure. Orientation distribution of faults and associated farctures in a granite pluton coming from a Variscan shear zone domain (Borne granite, Cévennes, SEFrance). Note that in such rocks (« non soluble ») shear fracture terminations have horsetail configurations. Exercice : place the orientations of of s_{1} 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 s_{litho} = 1/2 (s_{1} + s_{3}). Let us determine experimentally the value of s_{1} (vertical pressure) leading to rupture, for a set of s_{litho} values.
s_{1}  s_{litho} [= s_{1 } [1/2 (s_{1} + s_{3})] = 1/2 (s_{1}  s_{3})] gives the radius of the Mohr circle at rupture under s_{litho} 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 (t_{c}, s_{nc}) 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: 20141228; view: 1900
