TM
5-822-14/AFJMAN
32-1019
CHAPTER 2
SELECTION OF ADDITIVE
2-1. Factors to be Considered. In the selection
restrictions based on grain size and/or plasticity
of a stabilizer, the factors that must be considered
index (PI). Also provided in the second column of
are the type of soil to be stabilized, the purpose for
table 2-1 is a listing of soil classification symbols
which the stabilized layer will be used, the type of
applicable to the area determined from figure 2-1.
soil improvement desired, the required strength
This is an added check to insure that the proper
and durability of the stabilized layer, and the cost
area was selected. Thus, information on grain size
and environmental conditions.
distribution and Atterberg limits must be known
a. Soil types and additives. There may be more
to initiate the selection process. Data required to
than one candidate stabilizer applicable for one
enter figure 2-1 are: percent material passing the
soil type, however, there are some general guide-
No. 200 sieve and percent material passing the
lines that make specific stabilizers more desirable
No. 4 sieve but retained on the No. 200 (i.e., total
based on soil granularity, plasticity, or texture.
percent material between the No. 4 and the No.
Portland cement for example is used with a vari-
200 sieves). The triangle is entered with these two
ety of soil types; however, since it is imperative
values and the applicable area (1A, 2A, 3, etc.) is
that the cement be mixed intimately with the
found at their intersection. The area determined
fines fraction (< .074 mm), the more plastic materi-
from figure 2-1 is then found in the first column
als should be avoided. Generally, well-graded gran-
of table 2-1 and the soil classification is checked
ular materials that possess sufficient fines to
in the second column. Candidate stabilizers for
produce a floating aggregate matrix (homogenous
each area are indicated in third column and
mixture) and best suited for portland cement sta-
restrictions for the use of each material are pre-
bilization. Lime will react with soils of medium to
sented in the following columns. These restrictions
high plasticity to produce decreased plasticity,
are used to prevent use of stabilizing agents not
increased workability, reduced swell, and in-
applicable for the particular soil type under consid-
creased strength. Lime is used to stabilize a
eration. For example, assume a soil classified as a
variety of materials including weak subgrade soils,
SC, with 93 percent passing the No. 4 and 25
transforming them into a "working table" or
percent passing the No. 200 with a liquid limit of
subbase; and with marginal granular base materi-
20 and plastic limit of 11. Thus 68 percent of the
als, i.e., clay-gravels, "dirty" gravels, to form a
material is between the No. 4 and No. 200 and the
strong, high quality base course. Fly ash is a
plasticity index is 9. Entering figure 2-1 with the
pozzolanic material, i.e. it reacts with lime and is
values of 25 percent passing the No. 200 and 68
therefore almost always used in combination with
percent between the No. 4 and No. 200, the
lime in soils that have little or no plastic fines. It
intersection of these values is found in area 1-C.
has often been found desirable to use a small
Then going to the first column of table 2-1, we
amount of portland cement with lime and fly ash
find area 1-C and verify the soil classification, SC,
for added strength. This combination of lime-
in the second column. From the third column all
cement-flyash (LCF) has been used successfully in
four stabilizing materials are found to be potential
base course stabilization. Asphalt or bituminous
candidates. The restrictions in the following col-
materials both are used for waterproofing and for
umns are now examined. Bituminous stabilization
strength gain. Generally, soils suitable for asphalt
is acceptable since the PI does not exceed 10 and
stabilization are the silty sandy and granular
the amount of material passing the No. 200 does
materials since it is desired to thoroughly coat all
not exceed 30 percent. However it should be noted
the soil particles.
that the soil only barely qualifies under these
b. Selection of candidate additives. The selection
criteria and bituminous stabilization probably
of candidate/stabilizers is made using figure 2-1
would not be the first choice. The restrictions
and table 2-1. The soil gradation triangle in figure
under portland cement indicate that the PI must
2-1 is based upon the soil grain size characteris-
be less that the equation indicated in footnote b.
Since the PI, 9, is less than that value, portland
tics and the triangle is divided into areas of soils
with similar grain size and therefore pulverization
cement would be a candidate material. The restric-
characteristics. The selection process is continued
tions under lime indicate that the PI not be less
with table 2-1 which indicates for each area
than 12 therefore lime is not a candidate material
for stabilization, The restrictions under LCF stabi-
shown in figure 2-1 candidate stabilizers and
2-1
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