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GEO5

Geotechnical software

# Parameters of rocks

Rock parameters of orientation with respect to strength of rock in pure compression

 Compressive strength of rock σ ci [MPa] Strength parameter of rock after Hoek m i [-] GSI [-] Cohesion of rock c [kPa] Angle of internal friction of rock φ [°] 150 25 75 7000 - 13000 46 - 68 80 12 50 3000 - 4000 30 - 65 50 16 75 2000 - 4000 40 - 60 30 15 65 1000 - 2000 40 - 60 20 8 30 400 - 600 20 - 44 15 10 24 300 - 500 24 - 38 5 10 20 90 - 100 23 - 28

Unlike soils (both cohesive and cohesionless) the magnitude of the angle of internal friction (sometimes refer to as the angle of shear strength) varies and depends on the current state of stress in the rock body. Graphically it is represented by the angle of the tangent to the envelope of Mohr circles constructed for the ultimate stress state. The value of φgradually decreases with the increasing value of stress σ. If the elastic regime is exceeded (onset of plastic deformation) we set φ = 0. As a representative value of the angle of internal friction φwe denote the value φ0 associated with the stress σ = 0. In practical applications the part of the Mohr envelope between tensile Rt and compressive Rd circles is usually replaced by the tangent to both circles (see Fig.) The magnitude of the angle of internal friction then follows from:

Determination φ0 from Mohr circle

The angle of internal friction can be estimated by measuring angles of slip planes on remaining parts of tested specimens together with the following formula:

Some values of orientation:

 weathered sand conglomerate, lowly cracked 35 – 44° unweathered clay slate, medium cracked 30 - 40° unweathered tuff, medium cracked 33 – 42° unweathered diabase 39 – 50° unweathered phantanite, lowly cracked 45 – 52°