Page 169 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 10 Rock Mass Index 137

FIGURE 10.1 The graphical combination of block volume (Vb), joint condition factor (jC), and jointing
parameter (JP).

FIGURE 10.2 The combination of the parameters used in RMi. (From Palmstrom, 1996)

SCALE EFFECT

Significant scale effects are generally involved when a sample size is enlarged from
laboratory size to field size (Figure 3.1). From the calibration described earlier, RMi
is related to large samples where the scale effect has been included in JP. The joint size
factor (jL) is also a scale variable. However, for massive rock masses where the jointing
parameter JP % 1, the scale effect for the UCS (qc) must be accounted for as qc is related
to the 50 mm sample size. Barton (1990) suggested from data presented by Hoek and
Brown (1980) and Wagner (1987) that the actual compressive strength for large field
samples with diameter (d, measured in millimeters) may be determined using the
following equation (Figure 10.3):

qc ¼ qcoð50=dÞ0:2 ¼ qcoð0:05=DbÞ0:2 ¼ qco Á f ð10:4Þ

where qco is the UCS for a 50 mm sample size.
Equation (10.4) is valid for a sample diameter up to several meters, and may,

therefore, be applied for massive rock masses. Thus, f ¼ (0.05/Db)0.2 is the scale factor

for compressive strength. The approximate block diameter in Eq. (10.4) may be found