POTENTIAL ERRORS IN CHARACTERIZING JOINT PROPERTIES

Rock mass classification is an important part of many engineering projects, particularly where underground structures are involved, but also with respect to slope stability, construction, and excavation. Hudson and Harrison describe rock mass classification as a compromise between the use of a complete theory and completely ignoring rock properties. They also note, however, that rock mass classifications provide short-cuts to obtaining information on rock mass properties. These properties are difficult to assess, but provide direct guidance for engineering design. It is thus important that potential sources of error be recognized and reduced as much as possible. Jointing is considered by many to be the most important rock mass property, and the one that most strongly affects any engineering activity. Two types of error or bias with respect to joint spacing and joint trace length will be described: 1) those resulting from the method of field data collection and 2) those due to sampling errors. These errors are applicable to joints in all types of rocks, although the examples used are based on data from joints in granite. The implications of these errors may be significant and could result in inaccurate rock mass classification which could in turn effect engineering design and cost estimates.