UFC 3-260-02
30 June 2001
Number Equal to or Greater
Percent Equal to or Greater
CBR
than Each Different Value
than Each Different Value
4
4
4
4
20
(20/20)100 = 100
5
5
5
5
5
16
(16/20)100 = 80
6
6
6
6
6
11
(11/20)100 = 55
7
7
6
(6/20)100 = 30
8
8
4
(4/20)100 = 20
10
2
(2/20)100 = 10
11
1
(1/20)100 = 5
(3) Plot CBR versus percent equal to or greater as shown in Figure 6-2.
(4) Enter Figure 6-2 at 85 percent. Continue to plotted curve then down to design CBR value
of 4.7. If a sample from a test location has a value so low (indicating a weak area) that it is not
representative of the other tests in the area, obtain additional samples to determine the extent of the
area and whether special consideration is required. Where soil conditions vary substantially, a separate
set of CBR determinations will be required for each distinct soil type.
4. SUBGRADE MODULUS OF SOIL REACTION. The strength of the subgrade is expressed in terms
of the modulus of soil reaction (k) for rigid pavement design. The k value will be determined by the field
plate bearing test as described in CRD-C 655.
a. Strength Test. The field plate bearing test will be performed on representative areas of the
subgrade, taking into consideration such things as changes in material classification, fill or cut areas,
and varying moisture (drainage) conditions which would affect the support value of the subgrade. While
it is not practical to perform a sufficient number of field plate bearing tests to make a statistical analysis
of the k value, a sufficient number must be performed to give confidence that the selected value will be
representative of the in-place conditions. This means that at least two tests for each significantly
different subgrade condition should be conducted. Considering the limited number of measured k values
that can be obtained, maximum use of other pertinent soil data must be made to aid in the selection of
the design k value. The pavement thickness is not affected appreciably by small changes in k values.
6-3
Previous Page
