TM 5-822-12
(4) Using geotextiles. Either of the designs
shown above could be used by deducting 6 inches of
12.0 inches crushed
4 inches crushed
7 inches crushed
rock
rock
rock
well-graded sand subbase and replacing it with a
8.0 inches sand
Compacted
geotextile. The total thickness above the geotextile
subbase
subgrade CBR = 8
must be a minimum of 15 inches. Alternative designs
Subgrade CBR = 4
Subgrade CBR = 4
using a geotextile might be:
b. Determine the cross section in a severely cold
4 inches fine-graded stone
area where subgrade freezing is predicted.
11 inches coarse-graded crushed stone
geotextile
(1) Only the wearing surface and base course
Subgrade
layers will apply in this section. The sand subbase is not
or
required because the subgrade is not cohesive. The
7 inches fine-graded stone
filter fabric will not be used because the subgrade soil is
8 inches well-graded sand
an F2 material and the use of this fabric is restricted to
geotextile
Subgrade
F3 and F4 subgrade soils.
(2) In this case the natural subgrade CBR of
Notes:
4 is less than the frost-area soil support index and will
-All layer depths should be rounded up to the next
govern the design. The total thickness required above a
full inch for construction purposes.
subgrade CBR = 4 is 12.0 inches.
-The granular layers should be compacted to 100
(3) Therefore, the cross section for this
percent CE 55 maximum density.
condition will be:.
-The subgrade should be compacted to the density
required by table 8.
4 inches fine-graded stone
-The material should meet the gradation
8.0 inches coarse-graded crushed stone
requirements shown herein.
Subgrade CBR = 4
-The frost group soils F1l and F2 used as base and
c. Based on economic considerations, alternative
subbase materials should meet the requirements in
sections may be developed using frost group soils S1,
the appropriate guide specifications.
S2, and F1 with lower portion of the base material. An
-As previously stated, after all possible design
example using F1 soils is as follows:
sections are determined, the final section used for
the tank trail should be determined on the basis of
7.0 inches fine-graded stone
an economic analysis.
5 inches frost group soil Fl
Subgrade CBR = 4
14. Design Example No. 2. Assume the following
conditions:
15. Design Example No. 3. Assume the following
conditions:
CBR values.
-Natural subgrade = 4 (SM - silty sand material, frost
Design is for Army Class III airfield.
group F2).
Traffic protection = 10,000 passes of C-130 aircraft.
-Compacted subgrade = 8.
Design gross weight = 135 kips.
-Fine-graded crushed rock wearing surface = 80.
CBR values.
-Course-graded crushed rock base course = 80.
-Subgrade = 6
-Clean sand subbase = 15.
-Crushed stone = 80
Projected traffic.
Enter figure 4 with the subgrade CBR of 6, the 135 kip
-2,500 operations per day of Category IV traffic.
gross weight and 10,000 passes, and read the thickness
Calculations:
required above the 6 CBR of 13.5 inches which when
a. Determine the required thickness. From table 1
rounded to the next full inch will be 14.0 inches. The
determine the road to be a Class D road. From table 3,
section therefore would be:
select a design index = 5. From the design curves
(figure 1) the required thickness above the natural
14.0 inches of crushed stone
subgrade with a CBR of 4 is 11.5 inches (round to next
Subgrade CBR = 6
full inch of 12); the required cover over the compacted
subgrade (CBR = 8) is 7 inches. Therefore, the
hardstand might have the following cross sections:
14
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