ABSTRACT
Strength of a rock mass largely depends on the density, nature and extent of the fractures within it. Rock mass strength also relates to rock strength, weathering and water conditions.
FRACTURE DENSITIES Rock fractures include microfissures (spacing mostly 1mm-1cm), joints (1cm-1m) and faults (>1m). Also bedding, cleavage, schistosity. Fractures allow inelastic deformation and reduce rock mass strength to 1/5 to 1/10 of the intact rock strength. This fraction may be known as the Rock Mass Factor. Assessing fracture density is subjective, except by RQD. Rock Quality Designation (RQD) is a fracture quantification on borehole core > 50 mm diameter; lengths of core pieces are measured as they come from the drill barrel, and:
• RQD = X(core lengths >10 cm ) x 100/borehole length Values of RQD > 70 generally indicate sound rock. FRACTURE ORIENTATION Influence of orientation is only assessed subjectively in terms of favourability with respect to potential failure by sliding or rotation at a particular site or part of a site. Importance of orientation is shown by UCS variation in blocks of slate with well-defined cleavage.
