Civil engineer guide to fighting positions, shelters, obstacles

AFH 10-222 Volume 14 1 August 2008 148
A3.3.1.5. Undisturbed Soil Properties. Using characteristics determined up
to this point, evaluate the soil as it appeared in place. Gravels or sands can be
described qualitatively as loose, medium, or dense. Clays may be hard, stiff,
or soft. The ease or difficulty with which the sample was removed from the
ground is a good indicator. Soils that have been cultivated or farmed can be
further evaluated as loose or compressible. Highly organic soils can be
spongy or elastic. In addition, the moisture content of the soil influences the
in-place characteristics. This condition should be recognized and reported
with the undisturbed soil properties.
A3.3.2. Sedimentation Test. The visual observation test is used to approximate
the proportions of sand and gravel in a soil by spreading the dry sample
out on a flat surface and separating the gravel particles by hand. When No. 40
and No. 200 sieves are not available, separating the fines from the sand particles
is achieved by the sedimentation test. Place a small amount of the fine
fraction of soil, such as a heaping tablespoon, in a transparent container, such
as a jar. Break up any lumps of soil (especially the clays) by grinding the soil
in a canteen cup and add it back to the container. Cover the sample with
about 5 inches of water and agitate by stirring or shaking. The soil particles
will settle out in the time periods indicated in Table A3.3.
Table A3.3. Sedimentation Test Settling Times.
Approximate Settlement
Time (5 inches of water)
2 Seconds
Grain Diameter
Differentiates
(mm/inch)
Coarse Sand –
0.4 / 0.0157
Fine Sand
0.072 / 0.0028
Coarse Silt –
0.03 / 0.0118
Fine Silt
0.01 / 0.00039
30 Seconds Sand – Fines
10 Minutes
1 Hour Silt – Clay