Dear Sir,

We have some suggestions regarding the report of the Working Party on Rock Core Logging. The logarithmic scales for both strength and bedding plane spacing are welcome. Do, however, the arguments in favour of the proposed strength classification outweigh the practical advantages of a system based on equal divisions, when only slight modifications would be required for sub-divisions evenly spaced on a logarithmic scale?

Use of adjectives such as weak, strong, close or wide can also lead to difficulties. The relationship between terms such as ‘moderately weak’ and ‘moderately strong’ may not be immediately obvious. When setting up a system for rock quality classification based on both strength and fracture spacing (Franklin, Broch and Walton, 1971) some of these difficulties are overcome. A common nomenclature (low, high etc.) was used for both axes and also for rock quality (Figure 1), terms such as ‘low strength’ and ‘low fracture’ spacing replacing terms such as ‘weak’, ‘thin’ and ‘close’. There is little to be gained by differently classifying the spacing of joints, bedding planes or other types of structural discontinuity.

Attention is drawn to the ‘point load test’ as an alternative to the ‘uniaxial test’ for strength classification. Results for the two types of test are closely correlated. The point load test, however, uses portable field apparatus and requires no specimen preparation.

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