Difficult rock comminution and associated geological conditions

Conditions encountered by the authors on several projects (tunnel boring, blast hole drill-ing, and large diameter pile socket drilling) were unanticipated, undetectable, and even difficult to identify even after they had been encountered. These conditions have caused failure of mechanical components, low penetra-tion rates, and remarkably high cutter wear. Coincidentally, all projects had one or more geotechnical conditions in common. The diffi-culties encountered, however, could not be attributed to geotechnical factors measurable by classical laboratory or field methods. The difficulties were not discernible or predictable by conventional exploration, rock mechanics testing, or analysis, prior to tender A study of common elements and an analysis of these conditions have shed some light on how to avoid similar difficulties. These prob-lems may be avoided using exploration and testing specifically tailored to mechanical excavation, and by investigation for conditions which have been associated with such difficul-ties in the past. What can be learned from these widely undoc-umented and unpublished experiences? How were these conditions missed? What are the underlying causes and motive mechanisms? How can such problems be avoided in the future? In all case histories presented here, problems or difficulties were encountered. It is the intent of this paper to use these negative experiences to produce new possibilities for avoiding them in the future