E. M.. PART XV
Base moisture content expected=3 percent
Concrete flexural strength=650 Ibs./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,
=300 x 1.6=480 degree days (F)
Depth of gravel base required, from figure 3, for 3 percent moisture content in
base=70 x 1.19=83 in.
For the reason explained in paragraph 3-04a, the thickness of the bituminous surface is not included in the thickness
(b) Design based on reduced strength of subgrade. Subgrade is an F2 soil. By figure 5, 36 inches
combined thickness of pavement and base is required. If the bituminous surface is 4 inches thick, base thickness is 32
inches. The lower 4 inches of the base should be designed as a filter.
The depth of annual thaw many be approximate by interpolation in figure 8. For a surface thawing index of 480, depth of
annual thaw into subgrade= 11 inches. At least this depth in the subgrade should be investigated for non-uniformity and
pockets of highly frost-susceptible soils.
Such soils and pockets of clear ice should be removed to avoid excessive surface heaving or settlements.
By figure 9, it can be ascertained that the annual freeze is considerably greater than the annual thaw. Thus there ill be no
degradation of the permafrost below the new depth of annual thaw.
(2) Rigid payment.
(a) Design to restrict annual thaw to pavement and base depth. By use of figure 3 and a surface thawing
index of 450 degree days F, (300 x 1.5), a base thickness of 81 inches is required to prevent subgrade thawing. This
method neglects the thermal resistance of the pavement which, in the case of thick concrete slabs, may be significant.
Using formula (6) in chapter 6 and an assumed slab thickness of 15 inches, the estimate base course thickness to
prevent subgrade thaw is 67 inches.
(b) Design based on reduced strength of subgrade. By design curves for concrete pavements for dual
wheels in part XII, chapter 3, a first trial slab thickness of 15 inches is obtained.
With this a 15-inch base course would be required. By figure 10, the reduced subgrade modulus, k, with a 15-inch base
is 120 lbs. per sq. in. per in. Using this value for k in the design curve for slab thickness, a new value of 17 inches for
slab thickness is obtained. The base course should, therefore, also be 17 inches. The k value of 130 from figure 10 for a
17-inch base does not change the thickness of the concrete slab. The lower 4 inches of this base should be designed as
a filter. The depth of thaw for a 17-inch base thickness, by interpolation in figure 8, is 16 inches in the subgrade.
Investigation of the soils for at least this depth should be made so that undesirable pockets can be eliminated. The depth
of freeze from figure 9 would be more than 100 inches, which indicates that there will be no permafrost degradation.
DRAINAGE. Drainage ditches should be deep and narrow to minimize surface area.
They should preferably be lined to prevent seepage, which would increase the depth of thaw, and erosion, which might
cause blockage and ponding. Ponding must be avoided as it causes icing.
Frequent culverts of double the normal capacity are valuable; subdrains are of little use, however.
A steampipe should be installed in every culvert to permit artificial thawing.
CORDUROY ROADS. Where standing timber is available, corduroy may be used for temporary roads and
sometimes in permanent roads over poor soils if a better material is unobtainable. Corduroy must be protected by a
sand, gravel, or stone cover.