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GEO5

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Parameters of Rocks

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

Compressive strength of rock

Strength parameter of rock after Hoek

GSI

[-]

Cohesion of rock

Angle of internal friction of rock

σci [MPa]

mi [-]

[-]

c [kPa]

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 φ gradually 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 φ we 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°