TM 5-818-8/AFJMAN 32-1030
CHAPTER 5
R A I L R O A D TRACK CONSTRUCTION AND REHABILITATION
the track will be worsened. In any track construc-
5-1. General
tion or rehabilitation project, adequate drainage
The use of geotextiles in a railroad track structure
must be incorporated in the project design.
is dependent upon many factors including the
traffic, track structure, subgrade conditions, drain-
5-2. Material Selection
age conditions, and maintenance requirements. In
a. Based on current knowledge, woven geotex-
railroad applications, geotextiles are primarily
tiles are not recommended for use in railroad track
used to perform the functions of separation, filtra-
applications. Test installations have shown that
tion, and lateral drainage. Based on current
woven geotextiles tend to clog with time and act
knowledge, little is known of any reinforcement
almost as a plastic sheet preventing water from
effect geotextiles have on soft subgrades under
draining out of the subgrade.
railroad track. Therefore, geotextiles should not be
used to reduce the ballast or subballast design
ture of military railroads should be nonwoven,
thickness. Geotextiles have found their greatest
needle-punched materials that meet the require-
railroad use in those areas where a large amount
ments listed in table 5-l.
of track maintenance has been required on an
c. ASTM D 4886 is used to measure the abra-
existing right-of-way as a result of poor drainage
sion resistance of a geotextile for use in a railroad
conditions, soft conditions, and/or high-impact
application. Indications are that abrasion is
loadings. Geotextiles are normally placed between
greater for geotextiles placed during track rehabil-
the subgrade and ballast layer or between the
itations where the rail remains in-place than for
subgrade and subballast layers if one is present. A
geotextiles placed during new construction or reha-
common geotextile application is found in what is
bilitations where the existing rail, ties, and ballast
commonly known as "pumping track" and "ballast
are removed and the subgrade reworked. This may
pocket areas." Both are associated with fine-
be due to the differences in the surface upon which
grained subgrade soil and difficult drainage condi-
the geotextile is placed. In new construction the
tions. Under traffic, transient vertical stresses are
subgrade surface is normally graded, compacted
sufficient to cause the subgrade and ballast or
and free from large stone. During in-place rehabil-
subballast materials to intermix if the subgrade is
itations the old ballast may be removed by under-
weak (i.e. wet). As the intermixing continues, the
cutting or ploughing which leave ballast particles
ballast becomes fouled by excessive fines contami-
loose on, or protruding from, the surface, creating
nation, and a loss of free drainage through the
a rough surface for placement of the geotextile.
ballast occurs as well as a loss of shear strength.
5-3.
Application
The ballast is pulled down into the subgrade. As
this process continues, ballast is forced deeper and
Geotextiles should be used to separate the ballast
deeper into the subgrade, forming a pocket of
or subballast from the subgrade (or ballast from
fouled and ineffective ballast and loss of track
subballast) in a railroad track in cut sections
grade control. Ballast pockets tend to collect wa-
where the subgrade soil contains more than 25
ter, further reducing the strength of the roadbed
percent by weight of particles passing the No. 200
around them and result in continual track mainte-
sieve. Geotextiles are also used in embankment
nance problems. Installation of geotextiles during
sections consisting of such material where there is
rehabilitation of these areas provides separation,
less than 4 feet from the bottom of the tie to the
filtration, and drainage functions and can prevent
ditch invert or original ground surface.
the reoccurrence of pumping track. Common loca-
5-4. Depth of Placement
tions for the installation of a geotextile in railroad
track are locations of excessive track maintenance
Technical Manual TM 5-850-2/AFM 88-7, chap.
resulting from poor subgrade/drainage conditions,
2 specifies a minimum ballast thickness of 12
highway-railroad grade crossing, diamonds (rail-
inches. An additional minimum of 6 inches of
road crossings), turnouts, and bridge approaches. If
subballast may be used in areas where drainage is
a geotextile is installed in track without provisions
difficult. The actual total ballast/ subballast thick-
made for adequate drainage, water will be re-
ness required is a function of the maximum wheel
tained in the track structure and the instability of
load, rail weight, size, tie spacing, and allowable
5-l
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