Rock properties - Geotechnical Investigation and Design Tables - page 106

9.9 Rock strength due to failure angle
•

The confining stress affects the rock strength but is not as significant a factor as
with the soil strength.

•

The table is for zero confining stress.

Table 9.9 Relative strength change due to discontinuity inclination (after Brown
et al. 1977).

Angle between failure plane and Major principal stress at failure Comments

major principal stress direction (relative change)

0 ◦ 100% Horizontal

15 ◦ 70% Sub-horizontal

30 ◦ 30%

45 ◦ 15%

60 ◦ 20%

75 ◦ 40% Sub-vertical

90 ◦ 70% Vertical

9.10 Rock defects and rock quality designation
•

The RQD is an indicator of the rock fracturing.

•

RQDmeasurement methods do vary. Measure according to the methods described
in Chapter 3.

Table 9.10 Correlation between Rock Quality Designation (RQD) and discontinuity
spacing.

RQD (%) Description Fracture frequency per metre Typical mean discontinuity

spacing (mm)

0-25 Very poor 15 60 > <

25-50 Poor 15-8 60-120

50-75 Fair 8-5 120-200

75-90 Good 5-1 200-500

90-100 Excellent 1 500

≤ >

9.11 Rock laboratory to field strength
•

The RQD does not take into account the joint opening and condition.

Table 9.11 Design values of strength parameters (Bowles, 1996).

RQD (%) Rock description Field/laboratory

compressive strength

0-25 Very poor 0.15

25-50 Poor 0.20

50-75 Fair 0.25

75-90 Good 0.3-0.7

90 Excellent 0.7-1.0 >