BRO RAIL - Flipbook - Page 6
IMAGE C: To stabilize the sub-ballast under a new rail line, geogrids are simply
rolled out directly on top of the prepared subgrade.
IMAGE D: Existing track can be jacked up so that new geogrid and ballast
can be added.
Tensar® TriAx® Geogrid is quick and easy to install. Smooth,
graded surfaces are ideal, but if this is not possible, any
protruding objects including tree stumps, branches, boulders,
etc. should be removed. Then, any hollows resulting from
removal operations should be backfilled. Whenever possible,
the subgrade surface should be crowned slightly to promote
positive drainage away from the construction area.
There are distinct methods for placing geogrids for (1) the
construction of new rail lines and (2) the rehabilitation of
existing rail lines. To stabilize the sub-ballast under a new
rail line, geogrids are simply rolled out directly on top of the
prepared subgrade (Image C). A 1 ft to 3 ft (300 to 900 mm)
shingle-style overlap, placed in the direction of the fill
advancement, is sufficient to ensure stability across the
installation. The actual overlap length and any anchoring
pattern required are dependent on the subgrade strength
(the weaker the subgrade, the greater the overlap).
Multiple approaches may be used to rehabilitate an existing
rail line. The installed track and underlying ballast may be
completely removed, after which the geogrid is placed in a way
similar to new rail construction. Alternatively, the existing
track may be jacked up, and the underlying ballast removed
prior to geogrid installation (Image D). The track is then
lowered, and a track-mounted ballast machine places and
compacts new ballast on top of the geogrid.
Special track-mounted maintenance equipment has also been
developed recently for ballasting operations. This ballasting
machine lifts and cleans existing ballast as it advances down
the track. For projects requiring the stabilization of recycled
ballast, the machine can be modified to roll out the geogrid
during routine maintenance operations without a significant
addition of time.
Compaction of loosely placed ballast is typically completed
with a track-mounted tamping machine. To prevent damage
to the geogrid, a minimum clearance of 4 in. (100 mm)
between the bottom of the compaction tines and the surface
of the geogrid is required.