Page 439 - Planning and Design of Airports

Airport Drainage 379

1. To prevent clogging of a perforated pipe with filter material,
the following requirement must be satisfied:

85% size of filter material ∗
> 1
Diameter of perforatioon

2. To prevent the movement of particles from the protected
soil into the filter material, the following conditions must be
satisfied:

15% size of filter material ≤ 5
85% size of protected sooil

and

50% size of filter material
≤ 25
50% size of protected sooil

3. To permit free water to reach the pipe, the following condi-
tion must be fulfilled:

15% size of filter material
≥ 5
15% size of protected sooil

A typical example of design is shown in Fig. 9-16. Concrete sand
has proved to be a satisfactory filter material for the majority of fine
soils which are drainable. A single gradation of filter material is pre-
ferred for simplicity of construction.
Filter materials tend to segregate as they are placed in trenches. To
minimize this tendency, the material should not have a coefficient of
uniformity greater than 20. For the same reason, filter materials should
not be skip-graded. Filter materials should always be placed in a moist
state. The presence of moisture tends to reduce segregation.
Drainability of Soils
Certain types of soils, such as gravelly sands, sand, and sandy loams,
are usually self-draining and require very little, if any, subsurface
drainage. Subsurface drainage can be effective for draining clay
loams, sandy clay loams, and certain silty loams. The amount of sand
in these soils largely determines how drainable they are. For soils
containing a high percentage of silt and clay, subsurface drainage
becomes very problematic.

∗ This means that 85 percent (by weight) is finer than the specified size.