the word ‘mass’ is actually defined in as ‘the volume of ground which will be affected by or will affect the engineering work’. Much research has been undertaken into thecharacteristics and behavior of discontinuities. Whole conferences, such as that on‘Rock Joints’ held in 1990 (Barton and Stephenson 1990) have examined the subject ingreat detail. However, there is a major gap between acquiring such knowledge by researchand the application of that knowledge to practice. The problem is that the investigatormust forecast the character of the discontinuities in the mass, and thence theirlikely behavior, from the evidence available from outcrops or boreholes. Adequate datais seldom available.