E. M. PART XV
for design of rigid pavements should be avoided if at all possible over group F3 soils and particularly over group F4 soils.
If special needs or conditions dictate the use of rigid pavements over group F3 and F4 soils, a curve is included on figure
10, so that design can be made.
Use of the reduction in subgrade strength method for design of pavements for lightweight highspeed aircraft will give
thicknesses adequate to carry traffic, but possibly may result in objectionable surface roughness due to heaving or
subsidence. In such cases, design studies should include the compilation of frost heaving and settlement experience
records from existing airfield or highway pavements in the vicinity where conditions are comparable. The amount, type,
and distribution of ice formation in that portion of the existing frozen soil that will be thawed after pavement construction
should be determined and an estimate made of the magnitude and probable unevenness that will result from future
subsidence. Based on these design studies, the base-course thicknesses should be increased as necessary, over the
thickness obtained by the reduction in strength method, to hold the differential surface heave or subsidence to a tolerable
amount, or provisions made for resurfacing pavement periodically for a period of years to maintain a level surface. In
order to accomplish this, base courses up to 6 feet in thickness may be required to reduce prohibitive differential
heaving. Where the subgrade contains large, concentrated, buried ice masses, ice-filled polygon fissures and intense ice
lens formations which will be subject to melting after pavement construction, the base thickness should be sufficient to
prevent thawing of these ice masses or major maintenance anticipated.
In addition to the conditions stated above, it will be necessary to consider all reliable information concerning
performance of airfield and highway' pavements constructed in the area being investigated with a view toward modifying
or increasing the design requirements. Local experience with soils of a particular frost-susceptible soil group as listed in
paragraph 4-04 may indicate that assigning the soils to the next highest or lowest grouping would best conform to actual
behavior. Due regard should be given to freezing and ground water conditions in considering local experience with the
(1) Flexible pavements. The curves of figures 4, 5, and 6 should be used to determine the combined
thickness of flexible pavement and non-frost-susceptible base required for various aircraft wheel loads and wheel
assemblies, and figure 7 should be used for highways. These curves reflect the reduction in strength of soil during the
frost melting period.
Since design thickness computed by this procedure does not prevent frost heaving, the annual depths of thaw and of
freeze should be estimated from figures 8 and 9. (These curves have been developed using the methods presented in
chapter 6.) If subsoils are nonuniform or contain pockets of ice, differential surface movements may be expected which
can be minimized by removal of such materials. The eventual development of abrupt surface irregularities in the finished
pavement may be reduced by the tapering out of all excavations and backfills so as to insure gradual transitions. If
annual thaw is greater than freeze, the permafrost will degrade and the depth of thaw may become progressively greater.
If the permafrost surface is at some depth beneath the designed pavement section, pockets of highly frost-susceptible
soils within the depth of annual freeze should be removed.
In estimating the depth of annual freeze by figure 9, the surface freezing index for bituminous surfaces (kept cleared of
snow) may be computed by multiplying the freezing index based on air temperatures by a correction factor of 0.7.
(2) Rigid pavements The thickness of concrete pavements should be determined in accordance with chapter
3, part XII, for airfields, and chapter 1, part XIII, for roads, using the subgrade modulus determined from figure 10, which
considers the reduced strength of the subgrade in conjunction with the modulus determined in the field. If the tested "k"
value is smaller than the subgrade modulus obtained from figure 10, the test value should govern the design.
Where thawing and freezing are permitted in a frost-susceptible subgrade of group F1 or group F2 soil beneath a
rigid pavement, the differential movement should generally small.