In applying this classification system, the rock mass is divided into a number of structural
regions and each region is classified separately. The boundaries of the structural regions
usually coincide with a major structural feature such as a fault or with a change in rock
type. In some cases, significant changes in discontinuity spacing or characteristics, within
the same rock type, may necessitate the division of the rock mass into a number of small
structural regions.
The Rock Mass Rating system is presented in Table 4, giving the ratings for each of the
six parameters listed above. These ratings are summed to give a value of RMR. The
following example illustrates the use of these tables to arrive at an RMR value.
A tunnel is to be driven through slightly weathered granite with a dominant joint set dipping
at 60o against the direction of the drive. Index testing and logging of diamond drilled core
give typical Point-load strength index values of 8 MPa and average RQD values of 70%.
The slightly rough and slightly weathered joints with a separation of < 1 mm, are spaced
at 300 mm. Tunnelling conditions are anticipated to be wet.