Geoscience Reference
In-Depth Information
15.18 Drainage measures factors of safety
•
Large factors of safety are applied in drainage situations due to the greater
uncertainties with ground water associated issues.
Table 15.18
Factors of safety for drainage measures.
Drainage element
Factor of
Comments
safety
Pipes
2
To avoid internal piezometric pressures.
Granular material
10
To avoid permeability reduction due to fines or turbulent
flows.
Geotextiles
10
To account for distortion and clogging.
Blanket drain on flat slope
10
To avoid permeability reduction due to fines or turbulent
flows.
Blanket drain on steep slope
5
eg chimney drains, which uses graded filter or geotextile.
Geocomposite
4
To account for crushing.
15.19 Aggregate drains
•
Aggregate drains are often used for internal drainage of the soil.
Table 15.19
Aggregate drains.
Aggregate type
Advantages
Disadvantages
Open graded gravels - french drain
Good flow capacity
Clogging by piping from
surrounding soils
Well graded sands - filter sands
Resists piping. Useful in reduction
Low flow capacity
in pore water pressures
Open graded gravels wrapped in
Resists piping. Reasonable flow
Depth limitation
geotextile
capacity
15.20 Aggregate drainage
•
Aggregate drains are sometimes used with or in place of agricultural perforated
pipes. The pipes channel the already collected water while the aggregate drains
the surrounding soils.
•
The equivalent permeability for various size aggregate is provided in the table.
•
There is a significant advantage of using large size aggregate in terms of increased
permeability (flows) and reduced size.
•
No factors of safety apply.
•
I
=
1% to minimise turbulent effects in the aggregate.
Search WWH ::
Custom Search