Predicting the saturated permeability of soils or aggregates based on theoretical considerations has turned out to be difficult as permeability is dependent on a number of factors such as grading, void ratio, soil texture and structure, density and water temperature (Cedergren, 1977). Therefore, several empirical equations for estimating the permeability have been proposed in the past. These equations frequently include parameters linked to the grading curve of the material or their void ratio.
Hazen (1911) proposed an equation of the permeability of loose clean filter sand as:
K = cD2l0 (2.22)
where c is a constant that varies from 1.0 to 1.5 when the permeability K is in cm/s and the effective size D10 is in mm. Hazen’s equation is only valid for limited grain size distributions of sandy soils. A small quantity of silt or clay particles may change the permeability substantially. It is seldom a good means of estimating K as illustrated by Fig. 2.7.
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Another form of equation that has been frequently used in estimating the permeability of sandy soils is based on the Kozeny-Carman equation (Das, 1997; Carrier, 2003):
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10-
C
0)
и
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« 10-4
Fig. 2.7 Illustration of mis-match between Hazen’s estimation and measured values from road aggregates (adapted from Jones and Jones, 1989) where c is a constant. Samarasinghe et al. (1982) proposed a similar equation for normally consolidated clays:
en
K = c (2.24)
1 + e
where n and c are experimentally determined parameters. These equations are almost certainly improvements overEq. 2.22.
