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For rock slopes, different from those in soil, the MohrCoulomb failure criterion can not be used to define the resistance of the material; however with this method is described a procedure that allows the application of the classical methods of Limit Equilibrium even in rocky slopes.
In this purpose are defined the angle of shearing and the cohesion along the sliding surface according to the following expressions:
where:
σc is the uniaxial compressive strength of the rock;
A, B, T constants function of the lithotype and the quality of the rock (table below);
N normal stress at the base of the slice.
The constants A, B and T are determined as a function of the classification of the rocks according to Bieniawski (RMR index) or according to Barton (Q index).
Between the two classifications, on the base of 111 examples analyzed, was found the following correlation:
Lithotype Rock quality 
Limestones Dolomites Marl 
Shales Siltstones

Arenites Quartzites 
Andesites Basalts Rhyolites 
Amphibolites Gneiss Granites Gabbros 

RMR =100 Q = 500 
A = 0.816 B = 0.658 T = 0.140 
A = 0.918 B = 0.677 T = 0.099 
A = 1.044 B = 0.692 T = 0.067 
A = 1.086 B = 0.696 T = 0.059 
A = 1.220 B = 0.705 T = 0.040 
RMR = 85 Q = 100 
A = 0.651 B = 0.679 T = 0.028 
A = 0.739 B = 0.692 T = 0.020 
A = 0.848 B = 0.702 T = 0.013 
A = 0.883 B = 0.705 T = 0.012 
A = 0.998 B = 0.712 T = 0.008 
RMR = 65 Q = 10 
A = 0.369 B = 0.669 T = 0.006 
A = 0.427 B = 0.683 T = 0.004 
A = 0.501 B = 0.695 T = 0.003 
A = 0.525 B = 0.698 T = 0.002 
A = 0.603 B = 0.707 T = 0.002 
RMR = 44 Q = 1 
A = 0.198 B = 0.662 T = 0.0007 
A = 0.234 B = 0.675 T = 0.0005 
A = 0.280 B = 0.688 T = 0.0003 
A = 0.295 B = 0.691 T = 0.003 
A = 0.346 B = 0.700 T = 0.0002 
RMR = 3 Q = 0.1 
A = 0.115 B = 0.646 T = 0.0002 
A = 0.129 B = 0.655 T = 0.0002 
A = 0.162 B = 0.672 T = 0.0001 
A = 0.172 B = 0.676 T = 0.0001 
A = 0.203 B = 0.686 T = 0.0001 
RMR = 3 Q = 0.01 
A = 0.042 B = 0.534 T = 0 
A = 0.050 B = 0.539 T = 0 
A = 0.061 B = 0.546 T = 0 
A = 0.065 B = 0.548 T = 0 
A = 0.078 B = 0.556 T = 0 
Relation between the classification of the rocks and the parameters A, B and T
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