DESCRIPTION AND CLASSIFICATION OF WEATHERED ROCKS FOR ENGINEERING PURPOSES - THE BACKGROUND TO THE B55930-1981 PROPOSALS

With the impending publication of the first revision of BS5930:1981 it is pertinent to set down the background to the simple statements made for the descriptive terms proposed for the weathering of rock material and the scale of weathering grades of the rock mass. The seminal accounts published in the 1950s related to the weathering of granite at engineering sites in the Snowy Mountains, Australia, by Moye, and a scientific account of the weathering of granite in Hong gong by Ruxton and Berry. Fifteen years later, these early accounts provided the basis for two Geological Society Engineering Group Working Party Reports in 1970 and 1972 as guides to good site investigation practice. A statement on the weathered state of a rock, or engineering soil, comprised only one element of a scheme of description in which the rock name was prefixed by selected descriptive terms of the rock in the hand specimen as a material and in the mass, with suffixes used to indicate the main engineering properties, one of which was the estimated mechanical strength of the rock material. Because a similar descriptive scheme was adopted for BS5930:1981, it was felt appropriate at that stage to omit any reference to strength in the proposed scale of weathering grades of the rock mass and the descriptive terms for the weathering of rock material. However, the strength criterion was dealt with separately in the full engineering description. This change was part of a growing dissatisfaction with some aspects of the earlier descriptive schemes for rocks. After publication of BS5930:1981, the descriptive scheme for weathering was later criticized as being too restricted in scope and not easily applicable to a wide range of rock types and structural situations, although this was the original intention. As a basis for discussion, a reversion to Australian experience was proposed using a much simplified classification. The proposal envisaged four so-called condition-terms combined with descriptive terms for rock material strength, closely parallel to the original Moye classification of weathered granite material. This scheme holds out the prospect of application to a wide range of rock types in varying structural conditions.