Lesson 3/Learning Event 2
PLANNING THE SOIL SURVEY
The location of auger holes or test pits will depend upon the particular situation. In any case, the
method described locates the minimum amount of holes. The completeness of the exploration will
depend on the time available. A procedure for road, airfield, and borrow area investigation will be
described.
Subgrade and Borrow Areas
Subgrade. Since soil tests should be made on samples representing the major soil types in the area,
the first step in subgrade exploration is to develop a general picture of the subgrade conditions to assist
in determining the representative soils. Perform a field reconnaissance to study landforms and soil
conditions in ditches and cuts. Techniques have been developed whereby aerial photographs can be
used for delineating areas of similar soil conditions. Full use should be made of existing data in
agricultural soil maps for learning subsurface conditions.
The second step usually consists of preliminary borings spaced at strategic points. Arbitrary spacing of
these borings at regular intervals is not recommended because it does not give a true picture.
Intelligence use of the procedure described above, especially the technique of identifying soil boundaries
from aerial photographs, will permit strategic spacing of the preliminary borings to obtain the maximum
possible information with the least number of borings. In theater of operations cut areas, all holes
should extend four feet below final subgrade elevation, if possible. In theater of operations fill areas,
they should extend four feet below the natural ground elevation. The boring requirements stated above
usually will result in the borings penetrating beyond the depth of maximum frost penetration (or thaw in
permafrost areas). Where the above requirements do not achieve this result, the borings must extend to
the depth of maximum frost (or thaw) areas.
Step three is to obtain soil samples for classification purposes in these preliminary borings. After these
samples are classified, soil profiles should be developed, and representative soils should be selected for
detailed testing. Test pits, or large-diameter borings, should then be made to obtain the samples needed
for testing, or in-place pit tests. The types and number of samples required will depend on the
characteristics of the subgrade soils. Subsoil investigations in the areas of proposed pavement must
include measurements of the in-place water content, density, and strength. These are used to determine
the depth of compaction and to ascertain the presence of any soft layers in the subsoil.
Borrow Areas. When material is to be borrowed from adjacent areas, borings should be made in
these areas and carried two to four feet below the anticipated depth of borrow. Samples from the
borings should also be classified and tested for water content, density, and strength.
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