TM-5-822-12
b. Frost areas. As previously stated, where frost is
Table 4. Maximum permissible values for subbases and
a consideration in the design of roads and airfields, a
select materials.
layered system should be used. The percentage of
Maximum Permissible Value
fines should be restricted in all the layers to facilitate
Gradation
Requirements
drainage and reduce the loss of stability and strength
Percent
during thaw periods. Gradation numbers 3 and 4 shown
Passing
in table 7 should be used with caution since they may be
Maximum
No.
No.
unstable in a freeze-thaw environment.
Design
Size
10
200
Liquid
Plasticity
Material
CBR
inch
Sieve
Sieve
Limit*
Index*
Subbase
50
2
50
15
25
5
9. Compaction requirements
Subbase
40
2
80
15
25
5
Compaction requirements for the subgrade and granular
Subbase
30
2
100
15
25
5
layers are expressed as a percent of maximum CE 55
Select
material
20
3
-
-
35
12
density as determined by using MIL-STD-621 Test
Method 100. For the granular layers, the material will
*Determinations of these values will be made in
be compacted to 100 percent of the maximum CE 55
accordance with ASTM D 4318.
density. Select materials and subgrades in fills shall
variable, and should be based on the required cover and
have densities equal to or greater than the values shown
economic considerations.
in tables 8 and 9 for roads and table 10 for airfields
except that fills will be placed at no less than 95 percent
The reduced subgrade
h. Alternate design.
compaction for cohesionless soils (PI<5; LL<25) or 90
strength design procedure provides the thickness of soil
percent compaction for cohesive soils (PI > 5; LL > 25).
required above a frost-susceptible subgrade to minimize
Subgrades in cuts shall have densities equal to or
frost heave. To provide a more economical design, a
greater than the values shown in tables 8 through 10.
frost susceptible select material or subbase may be
Subgrades occurring in cut sections will be either
used as a part of the total thickness above the frost-
compacted from the surface to meet the densities
susceptible subgrade. However, the thickness above
shown in tables 8 through 10, removed and replaced
the select material or subbase must be determined by
before applying the requirements for fills, or covered
using the FASSI of the select or subbase material.
with sufficient material so that the uncompacted
Where frost-susceptible soils are used as select
subgrade will be at a depth where the in-place densities
materials or subbases, they must meet the requirements
are satisfactory. The depths shown in tables 8 through
of current specifications except that the restriction on
10 are measured from the surface of the aggregate road
the allowable percent finer than 0.02 mm is waived.
or airfield and not the surface of the subgrade.
8. Surface course requirements
10. Drainage requirements
The requirements for the various materials to be used in
Adequate surface drainage should be provided in order
the construction of aggregate surfaced roads and
to minimize moisture damage. Expeditious removal of
airfields are dependent upon whether or not frost is a
surface water reduces the potential for absorption and
consideration in the design.
ensures more consistent strength and reduced
maintenance. Drainage, however, must be provided in
a. Nonfrost areas. The material used for gravel
a manner to preclude damage to the aggregate surfaced
surfaced roads and airfields should be sufficiently
road or airfield through erosion of fines or erosion of the
cohesive to resist abrasive action. It should have a
entire surface layer. Also, care must be taken to ensure
liquid limit no greater than 35 and a plasticity index of 4
that the change in the overall drainage regime as a
to 9. It should also be graded for maximum density and
result of construction can be accommodated by the
minimum volume of voids in order to enhance optimum
surrounding topography without damage to the
moisture retention while resisting excessive water
environment or to the newly constructed road or airfield.
intrusion. The gradation, therefore, should consist of the
optimum combination of coarse and fine aggregates that
will ensure minimum void ratios and maximum density.
should be designed so that drainage is provided at all
Such a material will then exhibit cohesive strength as
points. Depending upon the surrounding terrain, surface
well as intergranular shear strength. Recommended
drainage of the roadway can be achieved by a continual
gradations are as shown in table 7. If the fine fraction of
cross slope or by a series of two or more interconnecting
the material does not meet plasticity characteristics,
cross slopes. The entire area should consist of one or
modification by addition of chemicals might be required.
more cross slopes having a gradient that meet the
Chloride products can, in some cases, enhance
requirements of TM 5-820-1 and TM 5-8204.
moisture retention, and lime can be used to reduce
Judgement will be required to arrange the cross slopes
excessive plasticity.
in a manner to remove water from the road or airfield at
the
nearest
9
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