Development of a preliminary rock mass classification scheme for near-surface excavation

In rock excavation work, the resistance of a rock mass depends partially on the mode of loading, shape of the loading instrument, rock moisture conditions and the orientations of the significant flaws relative to that of the applied force. Flaws or discontinuities in rocks may be cracks, bedding planes, weak grain boundaries and faults. Laboratory tests on rocks are often conducted on small cores of reasonably intact material. Generally, rocks decrease in strength with increase in sample size because the probability of the presence of large strength-controlling flaws increases with sample size. This is a plausible explanation for the disparity between laboratory and field specific energies often observed in rock excavation. Since the synergistic effects of discontinuity properties, scale of excavation and moisture condition can not be effectively evaluated using laboratory test results solely, it is necessary to use judgement along with such results in rating rock masses. Such a rating may involve the formulation of a single numerical index which must account for the significant properties, weighted to incorporate their relative importance. In this report, a preliminary classification system is formulated. It is called the Rock Trenchabiiity Classification (RTC). Consideration is given to rock material strength; rock brittleness; discontinuity spacing; orientations and linkage; and the scale of the excavation. Results of decades of investigations by many workers on various aspects of rock mass strength are taken into consideration.