UFC 3-260-02
30 June 2001
(2) Material requirement. A complete investigation will be made to determine the source,
quantity, and characteristics of available materials. A study should be made to determine the most
economical thickness of material for a base course that will meet requirements. The base course may
consist of natural materials or processed materials, well-graded and high-stability, as referred to in
Chapter 8. All base courses to be placed beneath airfield rigid pavements will conform to the following
requirements:
(a) Well-graded course to fine.
(b) Not more than 85 percent passing the 2-millimeter (No. 10) sieve.
(c) Not more than 15 percent passing the 0.075-millimeter (No. 200) sieve.
(d) PI not more than 8 percent.
However, when it is necessary that the base course provide drainage, the requirements set forth in
TM 5-820-2/AFM 88-5, Chapter 2, will be followed. When frost penetration is a factor, the requirements
set forth in Chapter 20, herein, will be followed.
(3) Compaction requirements. High densities are essential to keep future consolidation to a
minimum; however, thin base courses placed on yielding subgrades are difficult to compact to high
densities. Therefore, the design density in the base course materials should be as required in
Chapters 7 and 8.
(4) Modulus and Poisson's ratio. The modulus values of unbound granular bases (subbases)
will be determined from cyclic triaxial tests on prepared samples. The recommended test procedure is
outlined in Appendix O. The outputs from the test procedure are measures of modulus of elasticity and
Poisson's ratio. Triaxial compression tests should be conducted at confining pressures of 13.8, 34.5,
41.4, and 68.9 KPa (2, 5, 6, and 10 psi). Axial stresses should be applied that result in ratios with
confining stresses (F1/F3) of 13.8, 20.7, 27.6, and 34.5 KPa (2, 3, 4, and 5 psi). Plots of resilient modulus
versus first stress invariant (F1 + F2 + F3 or Fx + Fy + Fz) should be prepared and an average relationship
established. From this relationship, a value of resilient modulus at a first-stress invariant of 68.9 KPa
(10 psi) should be selected. No well-defined relationships exist for Poisson's ratio. However, plots of
Poisson's ratio versus ratio of axial to confining stress (F1/F3) may be made and representative values
selected. The modulus value of granular material may also be estimated from the relationship in a chart
in which the modulus is a function of the underlying layer and the layer thickness. The chart and the
procedure for use of the chart are given in Appendix I. However, it is recommended that the chart be
used in conjunction with test results to determine a representative modulus rather than as the sole
method. A Poisson's ratio of 0.3 will be used unless there is a reason to believe that it is significantly
different for the material in question.
d. Subgrade Soils.
(1) General. Subgrades may be divided into the general classes of cohesive and cohesionless
soils. Repeated loadings affect both cohesive and cohesionless soils. Cohesionless sands, gravels, or
sand-gravel combinations will respond much like granular bases or subbase. Cohesive soils are more
sensitive to repeated loadings. The resilient modulus of cohesive subgrades generally increases with
load repetitions provided the level of stress is lower than that required to initiate shear failure. However,
the number of stress repetitions required before a stable condition is reached may be greater than for
bound bases, granular bases, or cohesionless subgrades.
19-5
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