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 s_litho = 1/2 (s₁ + s₃). Let us determine experimentally the value of s₁ (vertical pressure) leading to rupture, for a set of s_litho values.

s₁ - s_litho [= s₁ - [1/2 (s₁ + s₃)] = 1/2 (s₁ - s₃)] 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: 2014-12-28; view: 2251