Page 370 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 26 Geological Strength Index 325

TABLE 26.4—Cont’d

Appearance of rock mass Description of rock mass Suggested
value of D
Very poor quality blasting in a hard rock
tunnel results in severe local damage, D ¼ 0.8
extending 2 or 3 m, in the surrounding
rock mass.

Small-scale blasting in civil engineering D ¼ 0.7
slopes results in modest rock mass damage, Good
particularly if controlled blasting is used blasting
as shown on the left-hand side of the D ¼ 1.0
photograph. However, stress relief Poor blasting
results in some disturbance.

Very large open pit mine slopes suffer D ¼ 1.0
significant disturbance due to heavy Production
production blasting and also due to stress blasting
relief from overburden removal. D ¼ 0.7
In some softer rocks, excavation can be Mechanical
carried out by ripping and dozing and excavation
the degree of damage to the slopes is less.

Sources: Hoek et al., 2002; Hoek, 2007.

as stress relief due to removal of the overburden of the rock mass results in disturbance of
the rock mass. It is considered that the “disturbed” rock mass parameters with D ¼ 1 in
Eqs. (26.7) and (26.8) are more appropriate for these rock masses (Hoek et al., 2002).

Thus, UCS of a rock mass obtained from Eq. (26.6) is

qcmass ¼ qc Á sn ð26:10Þ
and uniaxial tensile strength (UTS) of a good rock mass is

qtmass ¼ À s qc ð26:11Þ
mb