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Geology and Soil Mechanics, UW-Stout
Permeability and Dewatering
Permeability refers to the propensity of a material to allow fluid to move through its pores or interstices. Permeability is an important soil parameter for any project where flow of water through soil or rock is a matter of concern. For example, seepage through or under a dam, drainage from subgrades or backfills, knowing the rate at which a well can "recharge", and dewatering for construction projects under or near the water table.
There are several factors that influence the permeability of a soil (or rock material): the viscosity of its water (which is slightly influenced by temperature), size and shape of the soil particles, degree of saturation, and void ratio. The void ratio is slightly different than the density of a material. The void ratio is the ratio of volume of voids to volume of solids. However, for a given soil, permeability is inversely proportional to soil density. The more tightly a materials particles are packed, the tendency for the material to allow water to flow through it is reduced.
In construction, if an excavation is to be done below the water table, a dewatering plan needs to be followed. A highly permeable soil will require a pumping system with a comparatively large capacity. A low permeable soil may not require any pumping. In addition, fill material that is highly permeable can usually be placed and compacted immediately, but low permeable fill my require days of drying before being placed and compacted.
The fundamental description of permeability is based on the equation q=vA which takes the familiar form similar to river discharge. The variable q is the discharge (Vol/Time), v is the apparent velocity, and A is the area that is related to the geometry of the situation. Now, Darcy's Law describes the factors important in determining the value of v, which is
where k is a constant for the material and is called the coefficient of permeability, and i is the hydraulic gradient which is related to the water pressure. The following table lists some soil permeabilities.
Permeability Coefficient, k
Some methods used to determine the permeability of a material include: (1) Falling-Head Laboratory test, (2) Constant Head laboratory test, and (3) an in-field pumping test.
Example Laboratory Test (Constant-Head Method)
Figure 1 is a sketch of the apparatus used to determine the coefficient of permeability in the laboratory. The soil sample should be as close to un-disturbed as possible. While performing the test, one has to maintain a constant water level at the top by adding water at a rate of q for some time interval of t. Then
and Q is the volume of water collected in time t from the outlet. The cross-sectional area of the specimen is indicated by A.
Example Field Test
Figure 2 is the sketch of a soil aquifer profile. To perform this test, a pumped well and two observation wells need to be dug at the site. Thus, by measuring the pumping rate q, and knowing the water table depth in two observation wells, we can establish a value of k for field conditions. The equation in these conditions is
This measurement also requires the pumping rate to be a value that keeps the water level the same over time in the pumping well.