1 MARCH 2000
and extruded polystyrene foam boards are the two most commonly used synthetic insulations. Extruded
polystyrene is generally the most popular. In some instances, organic material such as peat, bark, wood
chips, or timbers and mineral materials such as slag or gravel have also been used. Placement of the
insulation can be between the ballast and sub-ballast, sub-ballast and the subgrade, or anywhere in
between. However, the insulation should be placed deep enough in the cross section to prevent damage
by maintenance equipment. In general, a minimum depth of 24 in. is recommended. Results of field and
laboratory tests have shown that extruded polystyrene retains its thermal properties and does not absorb
a significant amount of moisture if the board is protected from deformation. Therefore, the thickness of
the cover above the insulation is usually determined by the vertical stresses caused by the dead and live
loads and is limited to 1/5 of the compressive strength of the insulating material. Extruded polystyrene is
commercially available in compressive strengths of 40, 60 and 115 psi. Physical properties, available
thicknesses and sheet sizes are available from manufacturer's specifications. TM 5-852-6/AFM 88-19,
chapter 6 provides calculations of frost depth where insulation is used.
h. Transition Zones. A gradual transition is required between areas with significantly different frost
heave behavior. The transition distributes differential heave over a distance and thus reduces its
detrimental affects. Transitions between cuts and fills and at culvert crossings and bridge approaches are
typically 75 to 100 ft long.
i. Construction Procedures. Subgrade will be excavated and scarified to a predetermined depth based
on field conditions, and then wind-rowed and bladed to achieve adequate blending. This helps to ensure
a high degree of uniformity of soil conditions and to eliminate any isolated pockets of soil with higher or
lower frost susceptibility. It may be necessary to remove isolated pockets of either low or high frost-
susceptible material. In these cases, the soil should be excavated to the full frost depth and replaced with
the surrounding soil. Stones (6-in. in diameter or larger) or large roots must be removed from any fill in
the full depth of the frost penetration. This includes any stones encountered during subgrade preparation.
Failure to remove these items can result in track roughness as the stones and roots are gradually heaved
upward. In rock excavations, positive drainage should be supplied so that no pockets of water are left in
the zone of freezing. The irregularity of the isolated pockets may lead to non-uniform heaving. At the
transition between cut and fill sections, transition sections should be used as previously discussed.
Frequently, rock joints and fractures are full of frost susceptible soil. Rock joints or fractures encountered
in the subgrade should be cleaned out to the full depth of frost penetration and these joints replaced with
nonfrost susceptible material.
a. Importance. Although not a component of the track or roadbed, drainage (or lack of it) can have a
major impact on track strength and longevity. Without proper drainage, track will fail to perform as
designed or intended.
b. Design Guidance. TM 5-820-4/AFP 88-5, chapter 2 will be used for the design and construction of
drainage structures, except as modified by the following paragraphs.
c. Side Ditches.
(1) Most commonly, drainage is provided by open ditches running parallel to the track. In terminals
and in other level areas, subdrainage or other alternative drainage designs will be required.