Page 438 - Planning and Design of Airports

378 Airp o r t D e sign

3. Porous concrete pipe collects water by seepage through the
concrete wall of the pipe. This type of pipe is laid with the
joints sealed.
4. Skip pipe manufactured of both vitrified clay and cast iron
is a special type of bell-and-spigot pipe with slots at the
bells.
5. Farm tile is made of clay or concrete with the ends separated
slightly to permit the entrance of water. This type of pipe is
rarely used on airport projects.

Pipe Sizes and Slopes
Experience has shown that a 6-in-diameter drain is adequate, unless
extreme groundwater conditions are encountered. If desired, the
flow may be estimated by means of the available theories for soil
drainage [7]. These theories require knowledge of the effective
porosity and coefficient of permeability of the soil which is being
drained, as well as the head on the pipe and the distance which the
water must flow to reach the drain. Rarely is theory relied on to com-
pute pipe sizes.
The recommended minimum slope for subdrains is 0.15 ft in
100 ft. A minimum thickness of 6 in of filter material should surround
the drain. The gradation of the filter material is discussed in succeed-
ing paragraphs.

Utility Holes and Risers
For cleaning and inspection, utility holes and risers are often installed
along the drains. The Corps of Engineers recommends that utility
holes be placed at intervals of not more than 1000 ft, with one riser
approximately midway between the holes [7]. The function of the
riser is to be able to insert a hose for flushing the system. The function
of a utility hole is to permit inspection of the pipes.

Gradation of Filter Material
The term filter material applies to the granular material which is used
as backfill in the trenches where subdrains are placed. To permit free
water to reach the drain, the filter material must be many times
more pervious than the protected soil. Yet if the filter is too pervious,
the particles of soil to be drained will move into the filter material
and clog it.
On the basis of some general studies conducted by K. Terzaghi,
the Corps of Engineers has developed an empirical design for filter
material which has been substantiated by tests [10]. The criteria for
selecting the gradation of the filter material are as follows: