TM 5-818-8/AFJMAN 32-1030
for the placement of the geotextile and protects the
placed in the trenches to facilitate movement of
geotextile from punctures and abrasion due to the
the water from the track.
large aggregate pieces that are on the subgrade.
b. Highway Grade Crossings.
(1) Drainage in a grade crossing is generally
b. While the use of protective clean sand (less
parallel to the rails until the pavement and road
than 5 percent passing the No. 200 sieve) extends
shoulder have been cleared. Once clear of the
service life of a geotextile, there are also several
crossing itself, the drainage should be turned
disadvantages. These disadvantages include the
perpendicular to the track and discharged away
extra cost of the sand, the increase in rail height
from the track structure. A perforated drain pipe,
(which results from the extra thickness in the
either wrapped with a geotextile during installa-
track structure), and the difficulty and cost of
tion or prewrapped, may be placed in the trench to
placing the sand layer during construction or
assist the flow of water from within the crossing to
rehabilitation.
the ditches outside of the crossing area. Such
5-6. Drainage
drainpipes should be placed in the trench with the
line of perforations facing downward. The ends of
Adequate drainage is the key to a stable railroad
the perforated drainpipes and the geotextile under
track structure. During the design of a new track
the crossing should be laid with sufficient fall
or a track rehabilitation project, provisions for
toward the side ditches to prevent water from
improving both internal and external track drain-
ponding in the crossing area. Whether perforated
age should be included. Drainage provisions that
pipes are used or not, the shoulders at the corner
should be considered include adequate (deep) side
of the crossing should be removed, and the ends of
ditches to handle surface runoff, sufficient crown
the geotextile turned down so that the geotextile
in both the subgrade and subballast layers to
facilitates drainage under gravity toward the side
prevent water from ponding on the top of the
ditches.
subballast or subgrade, installation of perpendicu-
(2) In cold climates it is common to salt and
lar drains to prevent water accumulation in the
sand highways, including grade crossings, which
track, and French drains where required to assist
can lead to ballast fouling in the grade crossing.
in the removal of water from the track structure.
One method of preventing or minimizing this
During track rehabilitation, the creation of bath-
ballast fouling is to encapsulate the ballast in a
tub or canal effects should be avoided by having
geotextile. The provision for drainage in this type
the shoulders of the track below the level of the
of installation would be the same as discussed
ballast/geotextile/subgrade interface. Geotextiles
above.
should not be placed in a railroad track structure
until existing drainage problems are corrected.
c. Turnout Applications.
(1) The installation of a geotextile under a
Proper maintenance of railroad drainage facilities
turnout is basically the same as installation in
is described in TM 5-627.
any other segment of track. In the vicinity of a
5-7. Typical Sections
is more difficult to achieve because horizontal
Figure 5-1 presents typical cross sections of the
distances for subsurface flow are about doubled
railroad track structure showing the recommended
and gradients are about halved. Thus, there are
use of a geotextile in the track.
reasons for using geotextiles to promote lateral
5-8. Special Applications
drainage under a turnout where none is used in
a . I n s t a l l a t i o n of G e o t e x t i l e s B e l o w N a t u r a l
adjacent straight sections. If this is done, it should
Ground Level. In some locations, the elevation of
extend at least 25 feet away from the turnout
the track structure may be such that the geotex-
itself to provide a transition section. As with road
tile is placed below the level of the natural
crossings, particular attention should be given to
ground. Where the natural ground surface is ele-
the removal of surface water from the turnout
vated above the geotextile, steps should be taken
area.
to prevent the inflow of water. A French drain
(2) Many geotextile manufacturers produce
installed along the edge of the track and lined or
specially packaged units ready-made for quick
completely encapsulated in a geotextile to filter
application under turnouts varying from No. 8 to
the inflow of surface water may be used to direct
No. 20.
water away from the track structure. In extremely
d . R a i l C r o s s i n g s ( D i a m o n d s ) . The use of a
flat areas it may be necessary to construct perpen-
geotextile in the track under a rail crossing is very
dicular side ditches and soak-away pits from the
similar to the road crossing application. The de-
track structure to allow the water to drain out of
sign and installation process must provide ade-
the French drains. Slotted drain pipes can be
quate drainage.
5-3
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