UFC 3-260-02
30 June 2001
a. Navy Minimum Subbase CBR. On Navy airfield pavements, material with a minimum CBR of
30 should be used in the upper 152 millimeters (6 inches) of the subbase.
b. Exceptions to Gradation Requirements. Cases may occur in which certain natural materials
that do not meet the gradation requirements may develop satisfactory CBR values in the field.
Exceptions to the gradation requirements are permissible when supported by adequate in-place CBR
tests on construction that has been in service for several years.
c. Example. As an example of the selection of a design CBR for subbases or select materials,
consider the following material.
Soaked laboratory CBR = 40
Maximum size, millimeters (inches) = 50 (2.0)
Percent passing 2.0 millimeters (No. 10) = 85
Percent passing 0.075 millimeters (No. 200) = 14
Liquid limit = 12
Plasticity index = 3
The design CBR for this material would be 30 rather than the measured value of 40 because 80 percent
passing the 2.0 millimeters (No. 10) sieve is the maximum permitted for higher CBR values and this
material has 85 percent passing.
5. SEPARATION LAYERS. The gradation requirements shown in paragraph 4 are the maximum
allowable limits. The designers can and should include additional gradation requirements to ensure that
this material will meet the requirements for a separation layer as described in EI02C202/AFJMAN 32-
1016. These additional gradations are dependent on the base course or drainage layer gradations and
the gradations of the existing subgrade material; therefore, the designer should tailor these changes for
each project.
6. STABILIZED SELECT MATERIALS AND SUBBASES. The design of pavements using stabilized
soils is discussed in Chapter 9 of this document and in TM 5-822-14/AFJMAN 32-1019.
7. DESIGN FOR SEASONAL FROST CONDITIONS. In areas where the pavement will be subject to
cycles of freezing and thawing, Army and Air Force pavements will be designed in accordance with the
requirements in Chapter 9.
8. DRAINAGE LAYERS. The requirements for drainage layers used for subbase are presented in
EI 02C202/AFJMAN 32-1016 and NAVFAC DM 21.06. For pavements in nonfrost areas and having a
subgrade with a permeability greater than 20 feet/day, one can assume that the vertical drainage will be
sufficient such that no drainage layer is required. Also, flexible pavements in nonfrost areas with a total
thickness of 8 inches or less are not required to have a drainage layer. For pavements requiring
drainage layers, the design of the drainage layer shall be based on the premise that the capacity of the
drainage layer should be greater than the volume of water entering the pavement and that the drainage
layer, if saturated, should reach a degree of drainage of 0.85 within 1 day after the inflow of water stops.
The degree of drainage for the drainage layer is defined as the volume of water that has drained from
the layer over a specified time period divided by the total volume of water in the layer that can be drained
by gravity.
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