The results from the field and laboratory test program described in the previous chapter may vary with respect to both depth and areal extent, even if the same basic soil type is known to exist over much of the site. The soil layer with the most restrictive permeability is taken as the design basis for those systems that depend on infiltration and percolation of water as a process requirement. In other cases, where there is considerable scatter to the data, it is necessary to determine a "mean" permeability for design.

If the soil is uniform, then the vertical permeability (Kv) should be constant with depth and area, and any differences in test results should be due to variations in the test procedure. In this case, Kv can be considered to be the arithmetic mean as defined by Equation 3.1:

where Kam i the arithmetic mean vertical permeability, and K1 through Kn are individual test results.

Where the soil profile consists of a layered series of uniform soils, each with a distinct Kv generally decreasing with depth, the average value can be represented as the harmonic mean:

( j A

( j A d2

f d \ dn



V Ki

V K2


Khm = Harmonic mean permeability.

D = Soil profile depth.

dn = Depth of nth layer.

If no pattern or preference is indicated by a statistical analysis, then a random distribution of the K values for a layer must be assumed, and the geometric mean provides the most conservative estimate of the true Kv:

where Kgm is the geometric mean permeability (other terms are as defined previously).

Equation 3.1 or 3.3 can also be used with appropriate data to determine the lateral permeability, Kh. Table 2.17 presents typical values for the ratio Kh/Kv.

Kam n

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