1 MARCH 2000
(4) Determine ballast depth from equation 6-11:
h = Ballast depth in inches.
pm = Ballast surface stress (psi).
pc = Design subgrade bearing capacity (psi).
a. Subgrade Stability and Track Performance. The subgrade is the prepared earth on which the
railroad ballast section and track structure are built. If the subgrade does not have sufficient stability, it
will be impossible to maintain proper track alignment, profile (surface), and cross level.
b. Soil Investigation. Prior to initial design of the track structure, a geotechnical investigation should be
performed along the proposed alignment to determine soil type, strength, bearing capacity, location of
groundwater tables, natural water content, and compaction characteristics. Additional geotechnical
borings, laboratory testing, and engineering analysis will be required in areas where bridges or other
special structures are to be constructed. Chapter 1, part 1 of the AREMA Manual provides
recommendations for geotechnical investigations.
c. Design of Cuts and Fills and Subgrade Preparation. Chapter 1, part 2 of the AREMA Manual
provides recommendations for design of cuts and fills and subgrade preparation. Table 1.2.5 of the
AREMA Manual indicates soils types and their suitability as railroad subgrades.
d. Track Structure Design Bearing Capacity - Cohesive Soils. For cohesive soils, and for track
expected to carry the usual military traffic level of less than 5 million gross tons per year, design bearing
capacity for the track structure may be set at the soil's unconfined compressive strength at its natural
water content. For unusual cases, where annual traffic volumes are projected at higher than 5 million
gross tons, 80 percent of the unconfined compressive strength may be used as the design bearing
e. Soil Stabilization. Information on the use of soil stabilization and the design and construction of soil
stabilized subgrades is presented in TM 5-822-14/AFMAN 1019. Especially in frost areas, soil stabilizers
should be used with caution and only after intensive laboratory testing, including a frost susceptibility test
and a freeze-thaw durability test.
9. FROST DESIGN MODIFICATIONS.
a. Frost Heave Conditions. Frost heaving is the rising of the soil due to the growth of ice lenses or ice
segregation during freezing. There are three basic conditions that must be present for ice segregation to
occur: a frost susceptible soil, a source of water, and freezing temperatures. A change in any of the three
conditions will affect the amount of heave.