Intact rock versus rock mass

A hand specimen of granite may be stronger than the best concrete. A cliff of the same granite can still collapse. The difference is between intact rock, the solid material between fractures, and the rock mass, the intact blocks together with the joints, bedding planes, faults and weathered zones that cut through them. Engineering works deal with the rock mass.

Intact rock strength is measured in the laboratory: uniaxial compressive strength on cylindrical cores, and the quicker point load index, which crushes a core or lump between two cones and correlates roughly with compressive strength. Intact rock is also characterised by its density, porosity, modulus and durability, for example its resistance to slaking in water.

The discontinuities control the mass. Their orientation relative to a slope or tunnel decides whether blocks can slide or fall; their spacing decides the block size; their roughness, aperture and infilling decide their shear strength; and water in them adds pressure and can soften infilling clays. A rock mass is therefore weaker, more deformable and more permeable than its intact pieces, and its properties depend on scale: a small sample may contain no joints, a large excavation many.

Engineers bridge the gap with rock mass classification systems and with strength criteria that downgrade intact properties according to the quality of the mass. Field mapping of joints, core logging, and in-situ tests such as plate loading and pressuremeter tests give the information needed.