Terms which precisely describe the spacing of joints and bedding planes are needed in engineering investigations of rock masses. Several schemes have been used by geologists for the description of bedding units and these were summarized by Ingram (1954). Because of their variations and irregular scales he proposed a classification based on a logarithmic scale starting at 10 mm. His system has been used by many geologists but it is considered inadequate at the widely spaced end for much engineering work. More recently schemes have been defined by workers in engineering geology and rock mechanics with subdivisions for the wider spacings. Some of the classifications are shown in Table 1.

Different descriptive terms for the spacings of bedding planes and joints, viz. thick-thin, close-wide, are preferred in order to emphasize their generally different characteristics, e.g. the abrupt change in lithology, permeability and strength which can occur at bedding planes. This follows Deere (1952) whose scheme has been reproduced in text books by Obert & Duvall (1967) and Stagg & Zienkiewicz (1968). His classification is not, however, completely regular, it does not conform with common geological usage and more subdivision is considered necessary below 300 mm (12 in) spacing.

Although there is reluctance at proposing an additional scheme, it is considered that a classification combining features of some previous scheme is required. A system which defined the size of the unit blocks of intact rock as an extension of the scale used for size distribution (c.p. 2001) was considered. Such a

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