TB
5-6635-228-10
SECTION
III
SOIL-STRENGTH EVALUATION
widely spaced, e.g. approximately every 200
8. General. a. The number and location of
feet along the proposed center lime.
measurements to be made with the airfield
penetrometer vary with each area to be ex-
e. In areas of doubtful strength the pene-
amined and the time available. For this reason,
trations will be more closely spaced, and areas
hard and fast rules for evaluating an airfield
on both sides of the center line will be measured.
are not practicable, but the following instruc-
No less than three penetrations will be made at
tions will be helpful.
each location, and preferable five penetrations
are desired.
b. The information in this section applies
primarily to fine-grained soils (silts, clays, etc.)
f. If time permits or if inconsistencies are
except for paragraph 11d which pertains to
apparent, 10 penetrations will be made at each
coarse-grained soils (sands).
test location.
9. Range of Readings. a. The airfield pene-
11. Penetrometer Readings. a. Soil strength
trometer has a range of 0 to 15.
usually increases with depth, but in some cases
a thin, hard crust will overlie a deep, soft layer
b. A reading near 0 on the scale can occur
or the soil will contain thin layers of hard and
in a very wet soil, which, of course, cannot sup-
soft material. For this reason, penetrations
port aircraft traffic.
will be made to a depth of 24 inches unless pre-
c. A reading near 15 will occur in dry, com-
vented by a very firm condition at a lesser
pact clays or silts and in tightly packed sands
depth. When a penetration cannot be made to
or gravels.
the full depth of 24 "inches, a hole will be dug
d. Most aircraft that might be required to
or augered through the firm material and pene-
use an unpaved area could easily be supported
trometer readings will be taken in the bottom
for a substantial number of landings and take-
of the hole to ensure that no soft layer under-
offs by a soil having an airfield index of 15.
lies the firm layer.
b. Readings will be taken every 2 inches
10. Number of Measurements. a. Soil con-
from the surface to a depth of 24 inches. Gen-
ditions are extremely variable; therefore, as
erally, surface reading will not be used when
many penetrometer readings will be taken in
readings are averaged to obtain a representa-
a given area as time and circumstances will
tive airfield index for common cargo aircraft
permit.
b. The strength range and uniformity of the
c. In the normal soil condition, i.e. when
area will control the number of measurements
strength increases with depth, the readings at
required. Areas which are obviously too soft
the 2-inch to 8-inch depths (4 in. to 10 in. for
for emergency landing strips will be revealed
dry sands and for soil that will be trafficked
after a few measurements, as will areas with
by larger aircraft) will be used to designate
strengths that are more than adequate to sup-
t h e soil strength for airfield evaluation. If
port aircraft traffic.
readings in this critical layer at any one test
c. In all areas, it is advisable to test the
location do not differ more than 3 or 4 units,
softest-appearing spots first, since the softest
the arithmetic average of these readings can
condition controls the suitability of the area.
be taken as the airfield index for the area rep-
d. Soft spots are not always readily appar-
resented by the readings. When the range
ent, however, and if the first test results in-
between the highest and lowest readings is
more than 4, the interpretation of the data in
dicate barely adequate strength, the entire area
will be examined. Penetrations in areas that
arriving at a rating figure will be the lower
readings.
appear to be firm and uniform may be few and
5
