Environmental Engineering Reference
In-Depth Information
where
w
unit weight of water; c
v
coefficient of consolidation; m
v
coefficient of
volume change.
In any oedometer tests c
v
and m
v
commonly depend on the pressure applied to the soil,
so the permeability k also varies depending on the applied pressure. If a Rowe consolida-
tion cell is used with radial drainage (rather than vertical), the horizontal coefficient of
consolidation and hence horizontal permeability will be calculated. Details are given in
Head (1985).
Similarly, the consolidation phase of a triaxial test can be used to estimate m
v
and c
v
and hence the permeability. If radial drainage is promoted by surrounding the sample with
filter paper, the horizontal coefficients of consolidation, volume change and permeability
will be determined.
An advantage of using triaxial testing is that saturation of the sample can be assured by
back pressure saturation. However the permeability of the filter paper drains may control
the flow from the sample (Germaine and Ladd, 1988), so care must be taken in using them.
6.3.3.2
Estimation of permeability of sands from particle size distribution
An approximate estimate of the permeability of sands and sandy gravels can be obtained
from the particle size distribution of the soil. The most commonly used of the available
formulae are:
(i)
Hazen's formula
. The most commonly quoted form of the equation is
kC(D)
10
2
(6.47)
where C
factor, usually taken as
0.01; D
10
effective grain size in mm; k
per-
meability in m/sec.
It should be noted that Hazen's formula was developed for clean sands (less than
5% passing 0.075 mm) with D
10
sizes between 0.1 mm and 3.0 mm. Even within these
constraints C is quoted to vary from 0.004 to 0.015 (Holtz and Kovacs, 1981 and
Head, 1985). Lambe and Whitman (1979) show values of C varying from 0.0001 and
0.0042 with an average of 0.0016 for a range of soils from silt to coarse sand. Sherard
et al. (1984) suggest C
0.0035 for graded filter materials.
It is recommended that if the formula is used, an appropriate degree of conser-
vatism be applied to the value of C selected. Depending on the particular circum-
stances, conservatism may involve selection of a low or high value of C. Hazen's
formula is NOT applicable to clays, or to coarse gravels.
(ii)
Kozeny-Carman formula.
Head (1985) discusses the application of the Kozeny-
Carman formula and suggests the form
3
2
fS
e
k
m/sec
(6.48)
2
1
e
6
d/
12
where S
specific surface
(6.49)
and d
1
, d
2
maximum and minimum sized particles in mm; e
void ratio; f
angu-
larity factor
1.1 for rounded grains; for 1.25 for subrounded grains; 1.4 for angu-
lar grains.
