The working Party are grateful to Messrs. Franklin, Broch & Walton for their comments. We would disagree with the use of a system of strength classification based on equal divisions because it would tend to create unusually large units in the strength scale within areas where the natural properties of rock would appear to make subdivision desirable. For example, the VH group of rocks covers a range of unconfined strength from about 50 to 170 MN/m2 and within this range one would find granites in differing states of alteration and sandstones in varying states of cementation. It would also appear to us difficult to describe rocks with a strength less than about 50 MN/m2 as being of ‘high strength’. We fully accept that any such scale must be arbitrary but we prefer a scale that is ‘natural’ and related to the properties of rocks and equivalent engineering materials, rather than a scale which is forced and not related to the occurrence of rock materials of the greatest engineering significance.
We feel that the comments in the second paragraph might be a little pedantic. The Report does not attempt to provide a universal system of description of rock masses but one that can fairly be applied to boreholes. We feel that the modified terms suggested by the writers are somewhat clumsy and could lead to difficulties in practice. The question of rock quality classification is clearly a matter of opinion and we are of the view that such scales should be