Page 271 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 18

Slope Mass Rating

The Mother Nature is Motherly!

Veda, Gita, and Durgasaptashati

THE SLOPE MASS RATING

For evaluating the stability of rock slopes, Romana (1985) proposed a classification
system called the “slope mass rating” (SMR) system. SMR is obtained from Bieniawski’s
rock mass rating (RMR) by subtracting adjustment factors of the joint–slope relationship
and adding a factor depending on method of excavation

SMR ¼ RMRbasic þ ðF1 Á F2 Á F3Þ þ F4 ð18:1Þ

where RMRbasic is evaluated according to Bieniawski (1979, 1989) by adding the ratings
of five parameters (see Chapter 6). F1, F2, and F3 are adjustment factors related to joint
orientation with respect to slope orientation, and F4 is the correction factor for method of
excavation.

F1 depends upon parallelism between joints and slope face strikes. It ranges from
0.15 to 1.0. It is 0.15 when the angle between the critical joint plane and the slope face
is more than 30 and the failure probability is very low; it is 1.0 when both are near

parallel.

The value of F1 was initially established empirically. Subsequently, it was found to
approximately match the following relationship:

F1 ¼ ð1 À sin AÞ2 ð18:2Þ

where A denotes the angle between the strikes of the slope face (as) and that of the joints
(aj), that is, (as À aj).

F2 refers to joint dip angle (bj) in the planar failure mode. Its values also vary from
0.15 to 1.0. It is 0.15 when the dip of the critical joint is less than 20 degrees and 1.0 for

joints with dips greater than 45 degrees. For the toppling mode of failure, F2 remains
equal to 1. So

F2 ¼ tan bj ð18:3Þ

F3 refers to the relationship between the slope face and joint dips.
In planar failure (Figure 18.1), F3 refers to a probability of joints “day-lighting” in

the slope face. Conditions are called “fair” when the slope face and the joints are parallel.

If the slope dips 10 degrees more than the joints, the condition is termed “very

Engineering Rock Mass Classification 231
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