An investigation has been made into the reproducibility of measurement of joint spacing in rock. The rocks studied were sandstone, mudstone and limestone exposed in the Kielder Aqueduct Tunnels; the same limestone exposed in a nearby quarry was also studied. Joint spacing measurements made on site were also compared with measurements made from stereophotographs. It was found that for six observers measuring a 10m long scanline, the number of joints recorded can vary by up to a factor of nearly four without there being any real difference in the joint spacing of the rock. Measurements made from stereophotographs showed more joints than measurements made on site. When the joint spacing results were used to assign the rocks to recognised joint spacing classification categories, the results never varied by more than two adjacent categories, suggesting that the effects of the variation may not be so serious from the engineering classification standpoint. This was confirmed when it was found that the variation does not affect the overall rating of the rock mass using the Bienwiawski rock mass classification. (A)

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