Geoscience Reference
In-Depth Information
•
A geogrid over a loose sand subgrade reduces the displacement.
Table 16.11
Geotextile function in roadways (Koerner, 1995).
Geotextile function
Unsoaked CBR value
Soaked CBR value
Separation
8
3
≥
≥
Separation with some nominal reinforcement
3-8
1-3
Reinforcement and separation
3
1
≤
≤
16.12 Geotextiles as a soil filter
•
The geotextile filter pore sizes should be small enough to prevent excessive loss of
fines.
•
The geotextile filter pore size should be large enough to allow water to filter
through.
•
The geotextile should be strong enough to resist the stresses induced during
construction and from the overlying materials.
•
Geotextile permeability is approximately equivalent to a clean coarse gravel or
uniformly graded coarse aggregate (
>
10
−
2
m/s).
Table 16.12
Criteria for selection of geotextile as a filter below revetments
(McConnell, 1998).
Soil type
Pore size of geotextile O
90
Cohesive
O
90
≤
10 D
50
Uniform (U
5), uniform
O
90
2.5 D
50
O
90
D
90
<
≤
≤
Uniform (U
<
5),Well graded
O
90
≤
10 D
50
Non cohesive
Little or no cohesion and 50%
O
90
200
m
≤
µ
by weight of silt
•
Uniformity Coefficient, U
=
D
60
/D
10
.
•
Geotextiles should have a permeability of 10 times the underlying material to
allow for in service clogging.
•
Geotextile filters can be woven or non-woven that meet the above specifications.
•
Woven geotextiles are less likely to clog, however have a much narrower range of
applicability (medium sand and above). However, non-woven geotextiles predom-
inate as filters due to its greater robustness and range of application. Non-woven
geotextiles are therefore usually specified for filters.
16.13 Geotextile strength for silt fences
•
The geotextile strength required depends on the posts spacing and the height of
impoundment (H).
-
The ultimate strength of a typical non reinforced silt fence geotextile is
8-15 kN/m.
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