UFC 3-260-03
15 Apr 01
constructed so that the voids fall at about the lower limit of the specified allowable range, it is quite prob-
able that aircraft with relatively high-pressure tires will produce sufficient densification to reduce appre-
ciably the voids in the total mix. When the voids fall below the specified minimum (TI 825-01/AFM 32-
1124 (I)/NAVFAC DM 21.10), the pavement must be considered to be in a critical condition. These
conditions cannot be translated into numerical evaluations, but they should be discussed in the
evaluation report and summarized so that responsible engineers will have the information available.
b. Base Course and Subgrade.
(1) In the construction of airfield pavements, definite degrees of compaction are specified for
the subgrade and base course to prevent excessive densification under traffic and the consequent
development of surface roughness "birdbaths" and loss of grade. The specification of definite degrees
of compaction is also necessary because the design CBR values are based on assumed degrees of
compaction.
(2) To evaluate the base, subbase, and subgrade from the standpoint of future compaction, it
is necessary to compare the in-place densities, in percentage of ASTM D 1557 maximum density, with
the design requirements for the various loads and gear configurations that the pavement is expected to
support. If it is found that the in-place density of a layer is appreciably lower than that required, it must
be assumed that traffic will densify the layer in time. Density requirements at various depths are
discussed in TI 825-01/AFM 32-1124 (I)/NAVFAC DM 21.10.
(3) The effect of further compaction on strength of base and subgrade should also be con-
sidered. Some cohesive soils, when highly saturated, may develop pore pressures under traffic of
heavy wheel loads and show serious loss of strength. A clue to the possibility of this happening can be
obtained by comparing the in-place density and moisture contents with those of the laboratory com-
paction tests made at three compaction efforts to determine the line of optimums. This is illustrated in
figure 5-47 by a line drawn through the three optimum moisture contents. Pore pressure seldom devel-
ops unless the moisture and density are such that, when plotted on a diagram similar to that of fig-
ure 5-47, the point falls to the right of the line of optimums. Therefore, the moisture and density of the
soil being tested can be plotted on the laboratory chart and studied to determine if future compaction will
produce pore pressures. For example, consider point A plotted in figure 5-47 at a moisture content of
16 percent and a density of 1,649 kilograms/cubic meter (103 pounds per cubic foot). Assume this
represents a subgrade that should have 95 percent of ASTM D 1557 maximum density. If further com-
paction occurs, the density will increase to about 1,681 kilograms/cubic meter (105 pounds per cubic
foot) (point A' on the curve for 26-blow effort). Since this is to the left of the line of optimums, no pore
pressures will develop. If the example had been a subgrade with a moisture content of 18 percent
(point B), the increased compaction would cause the density to be plotted to the right of the line of opti-
mums (B'), and pore pressures would result. The CBR that would develop under this condition could be
estimated from laboratory CBR tests in which the material was compacted to the same density and
moisture content.
(4) In an evaluation, lack of specified compaction will not make it necessary to lower the load-
carrying capacity of the facility below that derived on the basis of thickness and CBR. However, if the
measured densities are considerably less than those specified, this should be discussed in the
evaluation report. It should be noted that materials of low density combined with low moisture content
may not densify under traffic, but subsequent increases in moisture content will permit densification.
Statements of possible amount of settlement due to densification should be included in the evaluation of
pavements being subjected to channelized and heavy wheel-load traffic. In the case of cohesive mate-
rials that may develop pore pressures, a study of the possibility of loss in strength should be made and
the lowest probable CBR estimated. This estimated value should be considered in selecting the evalu-
ation CBR for the material.
5-6
Previous Page
