Page 167 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 10 Rock Mass Index 135

TABLE 10.2 The Joint Length and Continuity Factor (jL)

jL

Joint Continuous Discontinuous
length (m) Term
Type joints joints**

<0.5 Very short Bedding/foliation parting 3 6

0.1–1.0 Short/small Joint 24

1–10 Medium Joint 12

10–30 Long/large Joint 0.75 1.5

>30 Very long/ Filled joint, seam or shear* 0.5 1
large

*Often a singularity (special feature), and should in these cases be treated separately.
**Discontinuous joints end in massive rock mass.

Source: Palmstrom, 1996, 2000.

of the block volume, Vb, which can be found from field measurements, and the joint
condition factor, jC, which is the result of three independent joint parameters (roughness,

alteration, and size, Eq. 4.5a).

Results from large-scale tests and field measurements of rock mass strength

have been used to determine how Vb and jC can be combined to express the jointing
parameter, JP. Calibration has been performed using known test results of the UCS
and the inherent parameters of the rock mass. The values for Vb and JP are plotted in
Figure 10.1, and the lines representing jC have been drawn. These lines are expressed as

JP ¼ 0:2 ðjCÞ0:5 Á ðVbÞD ð10:2Þ

where Vb is given in m3 and D ¼ 0.37 Á jCÀ0.2.
Joint condition factor (jC) is correlated with jR, jA, and jL as follows:

jC ¼ jLðjR=jAÞ ð10:3Þ

Various parameters of RMi and their combination in the RMi are shown in Figure 10.2,
whereas the ratings of joint roughness (jR), joint size and termination (jL), and joint
alteration (jA) are listed in Tables 10.1, 10.2, and 10.3, respectively. Joint roughness
(jR) together with joint alteration (jA) define the friction angle as in the Q-system of
Barton, Lien, and Linde (1974) in Chapter 8. The classification of RMi is presented
in Table 10.4.

For example, jC and JP are most commonly given as

jC ¼ 0:2 Vb0:37 and JP ¼ 0:28 Vb0:32

For jC ¼ 1.75 the jointing parameter can simply be expressed as

JP ¼ 0:25 ðVbÞ0:33
and for jC ¼ 1 the jointing parameter from Eq. (10.2) is expressed as

JP ¼ 0:2 Vb0:37