E. M.. PART XV
Consequently, a non-frost-susceptible base course having a minimum thickness equal to that of the concrete slab should
be used over these soils. In cases where rigid pavements are constructed over F3 soils, a non-frost-susceptible base
course equal to a minimum of one and one-half times the concrete slab thickness should be used. In areas where a
frost-susceptible subgrade soil of group F4 is present, appreciable total and differential movement may generally be
Therefore, a non-frost-susceptible base course of a thickness equal to one-half the depth of the subgrade that will be
subject to freezing and thawing should be used over F4 soils. In the application of this criterion the computed depth of
freeze or thaw, whichever is less, will govern the depth of base course to be employed. In no case should the base-
course thickness over an F4 soil be less than 24 inches nor greater than 48 inches.
The annual depths of thaw and freeze can be estimated by use of figures 8 and 9. The depths read from these
curves can be considered as depths below the top of the subgrade. In calculations of depth of freeze, correction factor
for portland cement concrete (kept clear of snow) may be taken as 0.6. If isolated pockets of ice or highly frost-
susceptible materials exist to these depths of thaw and freeze, they should be replaced to minimize differential surface
BASE COURSE DESIGN IN AREAS OF HIGHLY ORGANIC SOILS. Every effort should be made to avoid
construction on organic soils. However, because of the existence of extensive peat bogs and muskeg in the Arctic and
Subarctic, it is occasionally necessary to construct roads on such areas because of the lack of an alternate route. In this
case a thorough soil survey should be made to determine the depth and extent of the soft sediments in order that the
best possible route may be determined. From field experience it is considered that in bogs having a surficial peat deposit
less than 5 feet thick the peat should be removed and replaced with granular fill: in deep peat bogs fill may be placed
directly on the undisturbed cover. In either case the fill should be at least 5 feet thick regardless of surfacing and, in the
latter instance, it may be found that great quantities of fill may be necessary as the underlying organic material is
compressed or forced out to the sides. In any event continued differential settlement and almost constant maintenance
may be expected.
EXAMPLES OF PAVEMENT DESIGN.
Design an airfield taxiway for both flexible and rigid pavements to withstand a 40,000 pound single wheel load with
100 psi tire pressure, for the following conditions:
Mean air thawing index, 3000 degree days (F)
Mean air freezing index, 5500 degree days (F)
Subgrade = silt. 60 percent by weight of grains finer than 0.02 mm., moisture content = 45 percent group F4
Subgrade modulus, k (test made on top of gravel base) 300 lb./sq. in./in.
Base CBR = 80 percent
Concrete flexural strength = 650 Ib./sq. in.
(1) Flexible pavement.
(a) Design to restrict annual thaw to pavement and base depth..
Surface thawing index = (air thawing index) x (correction factor)
= 3000 x 1.6 = 4800 degree days (F)
From figure 3, for 5 percent moisture content, the depth of gravel base required is greater than 160 inches. This is an
impractical value, and therefore this design is not applicable.
(b) Design based on reduced strength of subgrade. According to figure 4, for an F4 soil, 36 inches of
pavement and base is required. The section may be a 4-inch bituminous wearing course plus a 32-inch gravel base.
(See paragraph 2-18, chapter 2, part XII for bituminous