Environmental Engineering Reference
In-Depth Information
where
permittivity
kn
cross plane permeability coefficient
t
geotextile thickness at the normal pressure on the geotextile.
In Darcy's Law:
q i A
(9.5)
h
t
kn
A
(9.6)
So
kn
t
q
(h)A
(9.7)
where
q
flow rate
h
head loss across the geotextile
A
area of flow (i.e. of geotextile).
As pointed out by Bertacchi and Cazzuffi (1985) and others, the permittivity is depend-
ent on the normal stress applied to the geotextile i.e. the ability to transmit water is
reduced as the geotextile is placed under stress which compresses the geotextile and the
values adopted for design should account for this. The permittivity is also affected by
clogging of the geotextile by fine soil. This is discussed further below.
In some applications the geotextile may be used to transmit water along its plane i.e. it
is in itself performing a drainage function. This is measured by transmissivity
and is
given by:
qL
hW
kt
(9.8)
p
where
transmissivity
k
p
permeability in the plane of the fabric
t
thickness of the fabric
q
flow rate in the plane of the fabric
L
length of the fabric
W
width of fabric
h
head lost.
As shown in
Figure 9.34
,
transmissivity is also affected by the applied normal stress.
9.7.2
Geotextile filter design criteria
9.7.2.1
General requirements
The requirements for the design of geotextile filters are as for conventional filters:
-Prevent erosion of soil particles from the base soil they are protecting;
- Allow drainage of seepage water;
- Be sufficiently durable to resist damage during construction and in service.
The first is achieved in a manner similar to that for conventional filters, but using the
equivalent opening size (EOS) of the geotextile, instead of D
15F
.
