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Porosity :
Porosity is the property of the rocks and soil to contain interstitial pore spaces.
Some rocks are dense and devoid of interstices while some are quite porous
and can absorb water readily. Such water holding capacities of the rocks and
soils are dependent on the geologic formations, shape, size, arrangement,
interconnection and extensiveness of the voids in which water can accumulate
and move. For example sandy formations in alluvium can absorb water freely
and can move, while in solid granite it can neither absorb water or there is any
free movement of water. Porosity is expressed as a percentage of the void
volume in a given volume of rock/soil. If ? is the porosity, then porosity or
= 100 W
V
Where W = volume of water required to fill or saturate all the pore space, and V
is the total volume of rock or soil.
Porosity can be of primary or secondary depending upon the formation of void
spaces. In unconsolidated sedimentary formations like alluvium deposits,
porosity is primary. Secondary porosities develop in consolidated sedimentary,
igneous and metamorphic formations due to availability of joints, fissures, faults
etc. which were the results of earlier geological processes of stress and strain
in the rock formations.
Table 6.1 : Some representative values of porosity (after Morrison and
Johnson as in Todd 1995)
