E. M. PART XV
abrupt surface subsidence. Thawing of such subgrades beneath completed pavements should be prevented by providing
sufficient base-course thickness, or the effects of thawing should be anticipated in planning future maintenance
requirements. Stage construction, where possible, such as for roads, should be considered so that final high-type
surfacing is placed after completion of the greater portion of subsidence due to thawing.
Where pavement construction is not going to result in a degrading condition, there may be advantage in placing the
base course directly on the existing vegetative cover as described in paragraph 1-04e(1), chapter 1, of this part. The
settlement due to compressibility of surface materials, and movement due to seasonal freezing and thawing of surface
cover deposit, and soils in the active zone prior to construction will generally be much less serious than thawing of
previously unthawed subgrade soils containing considerable segregated ice.
c. Effect of ground water. In areas of permafrost the location of the ground water surface is greatly influenced by
the existence of the underlying impervious permafrost layer. Surface infiltration and water released by the melting of
frozen soil supply water to the thawed zone above the permafrost layer during the summer period. Unless the soil is
relatively pervious and the terrain has sufficient slope to permit lateral drainage, the water table may rise to, or near, the
ground surface. During the early part of the freezing period lack of precipitation, cessation of thawing of the permafrost
layer, and reduction in infiltration by freezing of the ground surface combine to restrict the entrance of water to the
seasonally thawed layer. Where the terrain is sloping and the soil relatively pervious (coefficient of permeability greater
than approximately 1x 10-4 ft/min), the free water will tend to drain by gravity as freezing temperatures penetrate below
the surface, and in addition water is drawn to the zone of freezing in frost-susceptible soils. As a result the ground water
surface will approach the upper surface of the permafrost layer. However, where there is a considerable drainage area
above a specific location, the water level may remain above the permafrost table for a considerable part of the freezing
season, due to the time-lag in the subsurface flow, and in addition to supplying water for ice segregation in the active
zone, may cause the formation of surface icings and/or frost mounds.
In evaluating probable ground water conditions during the freezing period at a particular site, observations and
consideration of topographic position, slope of terrain, vegetative cover, and the soil type should be utilized. A potentially
troublesome water supply for ice segregation is present if the uppermost ground water surface is within 5 feet of the
plane of freezing, while if the ground water depth is in excess of 10 feet, appreciable ice segregation usually will not
occur. However, fine-grained, frost-susceptible subgrade soils in permafrost regions will usually become sufficiently
saturated to result in some ice segregation, even when the water table is remote. This applies to soils in groups CL, CH,
OL, OH, MH and to a lesser extent ML. In such cases, the total depth of the thawed layer as well as the degree of
saturation will influence the amount of ice segregation. Significant ice segregation will generally not occur in frost-
susceptible soils with a remote water table, when the degree of saturation is less than approximately 70 percent.
Where water for ice segregation is extracted from the voids of the soil below the zone of freezing, the surface
heaving generally will not be objectionable. When a frost-susceptible soil of group F3 or F4 is at or near full saturation,
the movement of water from the bottom to the top of the soil layer during the freezing process will tend to result in
weakening the soil at time of frost melting. In such cases the minimum base-course thickness should be determined by
use of reduced strength of subgrade method outlined in paragraph 3-04b.
a. Recommendations for base courses. All base-course materials lying within the depth of frost penetration should
be non-frost-susceptible. Where the combined thickness of pavement and base over a frost-susceptible subgrade is less
than the depth of seasonal freeze or thaw, the following additional design requirements apply: