Geology Reference
In-Depth Information
(a)
(b)
Figure 1.8.
(a) Orthogonal fractures exposed in quarry near Blackingstone Rock, eastern Dartmoor, south-
western England (b) Orthogonal fractures cut by faults exposed in quarry near Lugo, Galicia.
granite at the surface is flexible in thin sheets. Like most rocks fresh granite has a considerable
compressive strength, but it also possesses a high tensile strength (Dale, 1923).
Fresh granite is of low porosity and permeability, but is pervious because near the surface it is
characteristically fissured and fractured. Porosity, also known as mass permeability, refers to the ratio
of the volume of voids to the total volume of rock expressed as a percentage. Porosity depends on the
shape of constituent grains as well as their sorting, packing and cementation. A mass of closely packed
uniform spheres consists of 26% by volume of voids or pore space, but in a crystalline medium such as
fresh granite the value is commonly less than 1%. Permeability, also known as primary permeability,
