Porosity and permiyability

Introduction: Porosity is a measure of the voids in sediment and rock, and the masses they form. Permeability is a measure of the extent to which these voids are connected.

Porosity: Porosity is a measure of the volume of voids in a material or mass. In materials porositydepends upon the space between grains; in masses it would also include any spaceprovided by open fissures and joints.

Inter granular porosity is often determined in the laboratory by comparison between dry and saturated weights of the sample. To become fully saturated, voids must be in contact, and both interconnections and voids must be large enough to allow the flow of water under reasonable pressures. One or both of these conditions is often not satisfied, particularly for fine grained materials, and the results of such tests are properly described as apparent porosity. Note that the execution of the porosity test may damage the sample. There are some porous rocks, such as vesicular basalts, that are porous but whose porosity cannot be measured by conventional means because the voids are not in contact i.e. the material is impermeable.

Permeability: Permeability is a general term which describes the ability of a porous medium to allowthe flow of fluid through it. For flow to take place through a saturated materialthere must be a difference in total head. In engineering geology the usual concern iswith the flow of water through a granular soil or rock mass. The head of water h is theheight of the water level above the point at which the head is measured. Thus, the pressureof water at that point p = hγw, where γw= unit weight of water.In 1856, Darcy conducted laboratory experiments to analyze the flow of waterthrough sand, and showed that, if Q is the quantity of water passing through a sandtube of cross-sectional area A in time t, then the specific discharge q of water flow isgiven by:

In this experiment the flow was inter granular and non-turbulent; these are conditions most likely to be encountered in engineering geology except where flow is fast as in gravel, open rough fractures in rock or through some clays where the clay minerals may interfere with free flow.  Darcy also showed that the specific discharge was directly proportional to the hydraulic gradient (i), which is the loss of total head per unit length of flow (Fig. 2.4). Thus q∝i and q = Ki, where the constant of proportionality K (the coefficient of permeability) is dependent upon both the properties of the porous medium and the fluid. Since the fluid is water, K is also called the hydraulic conductivity. The properties of the fluid which effect K are fluid density and viscosity, which change with temperature. K is therefore temperature dependent.

 Thus:

Where v = a theoretical velocity of the flow across the area A. The coefficient K has units of velocity.