TB 5-6635-228-10
d. In an area in which a hard crust (less
for smaller aircraft with tire-inflation pressure
than 4 in. thick) overlies a much softer soil,
20- to 30-p.s.i. range.
the readings in the crust will not be used in
c. All sands and gravels in a "quick" condi-
evaluating the airfield. For example, if a 3
tion (water percolating through them) must be
inch-thick crust results in an average reading
avoided. Evaluation of moist sands will be
of 10 at the 2 inch depth, while the average
based on the penetrometer readings obtained
reading is 5 below the 3 inch depth, the area
as described in paragraphs 10 and 11.
will be evaluated at 5. If the crust is more
than 4 inches thick, it will in all probability aid
Table 1. Required Airfield Index for Common Cargo
Aircraft
in supporting aircraft, but the readings below
the crust must be considered in the evaluation.
If the crust in the example above is 5 inches
thick, the rating of the field would then be
approximately halfway between the 10 of the
crust and the 5 of the underlying soil, or, con-
servatively, 7. Innumerable combinations of
crust thickness and strength and underlying
soil strength can occur. Therefore, sound rea-
soning and engineering judgment must be used
in evaluating such areas.
e. In an area where the top layer of soil is
very soft and thin with a firmer underlying
layer of soil, the evaluation is also a matter of
judgment. If, for example, 1 inch to 2 inches
of soil with the airfield index averaging 5 over-
lies a soil with an index of 10, the field can be
rated as 10. But if this soft layer is more than
4 inches thick, the field will be rated at 5.
Areas of fine-grained soils with very low read-
ings in the top 1 inch or more are likely to be
slippery or sticky, especially if the soil is a clay.
12. Precautions for Sands. a. Many sands
occur in a loose state. Such sands when rela-
tively dry will show increasing airfield indexes
with depth, but the 2-inch depth index will
often be low, possibly 3 or 4. Such sands usually
are capable of supporting aircraft whose re-
quirements are much higher than an airfield
index of 3 or 4, because the strength of the
sand actually increases under the confining
action of the aircraft tires.
b. Generally, any dry sand or gravel will be
adequate for aircraft in the C130 class (60-
to 80-p.s.i. tire-inflation pressure), regardless
of the penetrometer reading. This is also true
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