TM 5-822-5/AFM 88-7, Chap. 1
CHAPTER 2
PRELIMINARY INVESTIGATIONS
conditions in ditches, and cuts and tests of
2-1. General.
representative soils in the site. The survey should
The subgrade provides a foundation for supporting
be augmented with existing soil and geological
the pavement structure. As a result, the required
maps. Both natural and subsurface drainage of the
pavement thickness and the performance obtained
sub- grade must be considered.
from the pavement during its design life will depend
largely upon the strength and uniformity of the
b. Preliminary subsurface explorations. Prelimi-
subgrade. Therefore, insofar as is economically
nary subsurface explorations should be made at in-
feasible, a thorough investigation of the sub-grade
tervals selected to test each type of soil and topog-
should be made so that the design and construction
raphy identified in the general survey. Additional
will ensure uniformity of support for the pavement
subsurface explorations should be made in those
structure and realization of the maximum strength
areas where the preliminary investigation indicates
potential for the particular sub-grade soil type. The
unusual or potentially troublesome subgrade
importance of uniformity of soil and moisture
conditions. In determining subgrade conditions,
conditions under the pavement cannot be
borings will be carried to the depth of frost pene-
overemphasized with respect to frost action.
tration, but no less than 6 feet below the finished
grade. In the design of some high fills, it may be
2-2. Investigations of Site.
necessary to consider settlement caused by the
Characteristics of subgrade soils and peculiar fea-
weight of the fill. The depth requirements stated
tures of the site must be known to predict pave-
above will usually result in the subsurface explora-
ment performance. Investigations should determine
tions reaching below the depth of maximum frost
the general suitability of the subgrade soils based
penetration. If this is not the case, they should be
on classification of the soil, moisture-density
extended to the maximum depth of frost penetra-
relation, degree to which the soil can be compact-
tion below the design grade as determined from
ed, expansion characteristics, susceptibility to
chapter 10.
pumping, and susceptibility to detrimental frost
c. Soil. Soil samples from the preliminary bor-
action. Such factors as groundwater, surface infil-
ings should be classified and the data used to pre-
tration, soil capillarity, topography, rainfall, and
pare soil profiles and to select representative soils
drainage conditions also will affect the future sup-
for further testing. Measurements should include
port rendered by the subgrade by increasing its
moisture contents which indicate soft layers in the
moisture content and thereby reducing its
soil.
strength. Past performance of existing pavements
over a minimum of 5 years on similar local sub-
2-4. Borrow Areas.
rades should be used to confirm the proposed
Where material is to be borrowed from adjacent
design criteria. All soils should be classified ac-
areas, subsurface explorations should be made in
cording to the Unified Soil Classification Systems
these areas and carried 2 to 4 feet below the an-
(USCS) in ASTM D 2487.
ticipated depth of borrow. Samples from the explo-
rations should be classified and tested for moisture
2-3. Soil Conditions.
content and compactions characteristics.
a. General survey of subgrade conditions.
Sources of data should include the landforms, soil
2-1
Previous Page
