Slope Mass Rating (SMR)

Slope mass rating is a system of classification developed by Romana (……)as an extension of Bieniawski's rock mass rating approach for application to rock slopes. RMR is widely used in tunnels and it is not suitable for slopes due to the fact that joints are a more governing parameter for stability in slopes. Some parameters are introduced to include the attitude of discontinuities to assess slope stability

The proposed ‘Slope Mass Rating’ (SMR) is obtained from RMR by subtracting a factorial adjustment factor depending on the joint-slope relationship and adding a factor depending on the method of excavation.

SMR = RMR + (F1 . F2 . F3) + F4

The RMR is computed according to Bieniawski’s 1979 proposal, adding rating values for five parameters:

(i)            Strength of intact rock;

(ii)          RQD (measured or estimated);

(iii)         Spacing of discontinuities;

(iv)         Condition of discontinuities; and

(v)          Water inflow through discontinuities

RMR has a total range of 0 - 100. The adjustment rating for joints is the product of three factors (F1*F2*F3) as follows:

F1 depends on the parallelism between joints and slope face strike. It ranges from 1 (when both are near parallel) to 0.15 (when the angle between them is more than 30 degrees and the failure probability is very low). These values are found to match approximately the relationship:

where, A is the angle between strike of the slope face and the joint.

F2 is related to joint dip angle in the planar mode of failure. It is a measure of the joint shear strength. Its value varies from 1 (for joints dipping more than 45 degrees) to 0.15 (for joints dipping less than 20 degrees). It has been found to match approximately the relationship:

(B)

where, B denotes joint dip angle. For toppling mode of failure, F2 remains 1.

F3 reflects the relationship between the slope face and the joint dip.

F4 is a factor for the method of excavation and its adjustment factor has been fixed empirically as follows: (Table 2.1).

Table 2.1: Adjustment ratings for method of excavation (after Romana, 1995)

 Method of excavation Natural slope Pre-splitting Smooth blasting Blasting or mechanical Deficient Blasting F4 +15 +10 +8 0 -8