CHAPTER 9 Soil Stabilization for Roads and Airfields

81 Pages·1996·2.61 MB·English

by  Mark Duncan

FM 5-410
CHAPTER 9
S o i l S t a b i l i z a t i on
f o r R o a d s a n d A i r f i e l ds

Soil stabilization is the alteration of one or troops (and equipment), and time available
more soil properties, by mechanical or chemi- (METT-T).
cal means, to create an improved soil material
Mission. What type of facility is to be
possessing the desired engineering proper-
constructed—road, airfield, or build-
ties. Soils may be stabilized to increase
ing foundation? How long will the
strength and durability or to prevent erosion
facility be used (design life)?
and dust generation. Regardless of the pur-
Enemy. Is the enemy interdicting
pose for stabilization, the desired result is the
lines of communications? If so, how
creation of a soil material or soil system that
will it impact on your ability to haul
will remain in place under the design use con-
stabilizing admixtures delivered to
ditions for the design life of the project.
your construction site?
Terrain, Assess the effect of terrain
Engineers are responsible for selecting or
specifying the correct stabilizing method, on the project during the construction
technique, and quantity of material required. phase and over the design life of the
This chapter is aimed at helping to make the facility. Is soil erosion likely? If so,
correct decisions. Many of the procedures what impact will it have? Is there a
outlined are not precise, but they will “get you slope that is likely to become unstable?
in the ball park.” Soils vary throughout the Troops (and equipment). Do you have
world, and the engineering properties of soils or can you get equipment needed to
are equally variable. The key to success in perform the stabilization operation?
soil stabilization is soil testing. The method Time available. Does the tactical situa-
of soil stabilization selected should be verified
tion permit the time required to stabi-
in the laboratory before construction and
lize the soil and allow the stabilized
preferably before specifying or ordering
soil to cure (if necessary)?
materials.
There are numerous methods by which
Section I. Methods of
soils can be stabilized; however, all methods
Stabilization
fall into two broad categories. They are—
Mechanical stabilization.
BASIC CONSIDERATIONS Chemical admixture stabilization.
Deciding to stabilize existing soil material
in the theater of operations requires an as- Some stabilization techniques use a com-
sessment of the mission, enemy, terrain, bination of these two methods. Mechanical
Soil Stabilization for Roads and Airfields 9-1
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stabilization relies on physical processes to To obtain uniform bearing capacity, uniform
stabilize the soil, either altering the physical mixture and blending of all materials is es-
composition of the soil (soil blending) or plac- sential. The mixture will normally be
ing a barrier in or on the soil to obtain the compacted at or near OMC to obtain satisfac-
desired effect (such as establishing a sod tory densities.
cover to prevent dust generation). Chemical
stabilization relies on the use of an admixture The primary function of the portion of a
to alter the chemical properties of the soil to mechanically stabilized soil mixture that is
achieve the desired effect (such as using lime retained on a Number 200 sieve is to con-
to reduce a soil’s plasticity). tribute internal friction. Practically all
materials of a granular nature that do not sof-
Classify the soil material using the USCS. ten when wet or pulverize under traffic can be
When a soil testing kit is unavailable, classify used; however, the best aggregates are those
that are made up of hard, durable, angular
the soil using the field identification
particles. The gradation of this portion of the
methodology. Mechanical stabilization
mixture is important, as the most suitable ag-
through soil blending is the most economical
gregates generally are well-graded from
and expedient method of altering the existing
coarse to fine. Well-graded mixtures are
material. When soil blending is not feasible
preferred because of their greater stability
or does not produce a satisfactory soil
when compacted and because they can be
material, geotextiles or chemical admixture
compacted more easily. They also have
stabilization should be considered. If chemi-
greater increases in stability with cor-
cal admixture stabilization is being
responding increases in density. Satisfactory
considered, determine what chemical admix-
materials for this use include—
tures are available for use and any special
Crushed stone.
equipment or training required to successfully
Crushed and uncrushed gravel.
incorporate the admixture.
Sand.
Crushed slag.
MECHANICAL STABILILIZATION
Mechanical stabilization produces by com-
Many other locally available materials
paction an interlocking of soil-aggregate
have been successfully used, including disin-
particles. The grading of the soil-aggregate tegrated granite, talus rock, mine tailings,
mixture must be such that a dense mass is
caliche, coral, limerock, tuff, shell, slinkers,
produced when it is compacted. Mechanical
cinders, and iron ore. When local materials
stabilization can be accomplished by
are used, proper gradation requirements can-
uniformly mixing the material and then com-
not always be met.
pacting the mixture. As an alternative,
additional fines or aggregates maybe blended NOTE: If conditions are encountered in
before compaction to form a uniform, well-
which the gradation obtained by blend-
graded, dense soil-aggregate mixture after ing local materials is either finer or
compaction. The choice of methods should be
coarser than the specified gradation, the
based on the gradation of the material. In
size requirements of the finer fractions
some instances, geotextiles can be used to im-
should be satisfied and the gradation of
prove a soil’s engineering characteristics (see the coarser sizes should be neglected.
Chapter 11).
The portion of the soil that passes a Num-
The three essentials for obtaining a
ber 200 sieve functions as filler for the rest of
properly stabilized soil mixture are—
the mixture and supplies cohesion. This aids
Proper gradation. in the retention of stability during dry
A satisfactory binder soil. weather. The swelling of clay material serves
Proper control of the mixture content. somewhat to retard the penetration of
Soil Stabilization for Roads and Airfields 9-2
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moisture during wet weather. Clay or dust granular materials. The objectives here are
from rock-crushing operations are commonly to—
used as binders. The nature and amount of Increase the drainability of the soil.
this finer material must be carefully con- Increase stability.
trolled, since too much of it results in an unac-
Reduce volume changes.
ceptable change in volume with change in Control the undeirable effects associated
moisture content and other undesirable with clays.
properties. The properties of the soil binder
are usually controlled by controlling the plas-
Objective
ticity characteristics, as evidenced by the LL
The objective of mechanical stabilization is
and PI. These tests are performed on the por-
to blend available soils so that, when properly
tion of the material that passes a Number 40
compacted, they give the desired stability. In
sieve. The amount of fines is controlled by
certain areas, for example, the natural soil at
limiting the amount of material that may
a selected location may have low load-bearing
pass a Number 200 sieve. When the stabi-
strength because of an excess of clay, silt, or
lized soil is to be subjected to frost action, this
fine sand. Within a reasonable distance,
factor must be kept in mind when designing
suitable granular materials may occur that
the soil mixture.
may be blended with the existing soils to
markedly improve the soil at a much lower
Uses
cost in manpower and materials than is in-
Mechanical soil stabilization may be used volved in applying imported surfacing.
in preparing soils to function as—
Subgrades. The mechanical stabilization of soils in
Bases. military construction is very important. The
Surfaces. engineer needs to be aware of the possibilities
of this type of construction and to understand
Several commonly encountered situations the principles of soil action previously
may be visualized to indicate the usefulness presented. The engineer must fully inves-
of this method. One of these situations occurs tigate the possibilities of using locally
available materials.
when the surface soil is a loose sand that is in-
capable of providing support for wheeled
vehicles, particularly in dry weather. If Limitations
suitable binder soil is available in the area, it Without minimizing the importance of
may be brought in and mixed in the proper mechanical stabilization, the limitations of
proportions with the existing sand to provide this method should also be realized. The
an expedient all-weather surface for light principles of mechanical stabilization have
traffic. This would be a sand-clay road. This frequently been misused, particularly in
also may be done in some cases to provide a areas where frost action is a factor in the
“working platform” during construction design. For example, clay has been added to
operations. A somewhat similar situation “stabilize” soils, when in reality all that was
may occur in areas where natural gravels needed was adequate compaction to provide a
suitable for the production of a well-graded strong, easily drained base that would not be
sand-aggregate material are not readily susceptible to detrimental frost action. An
available. Crushed stone, slag, or other understanding of the densification that can
materials may then be stabilized by the addi- be achieved by modern compaction equip-
tion of suitable clay binder to produce a ment should prevent a mistake of this sort.
satisfactory base or surface. A common Somewhat similarly, poor trafficability of a
method of mechanically stabilizing an exist- soil during construction because of lack of
ing clay soil is to add gravel, sand, or other fines should not necessarily provide an excuse
Soil Stabilization for Roads and Airfields 9-3
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for mixing in clay binder. The problem may penetration of precipitation. To some extent,
possibly be solved by applying a thin surface moisture lost by evaporation can be replaced
treatment or using some other expedient by capillarity.
method.
Emergency airfields that have surfaces of
this type require a mixture with a PI between
Soil Base Requirements
5 and 10. Experience indicates that road sur-
Grading requirements relative to mechani-
faces of this type should be between 4 and 9.
cally stabilized soil mixtures that serve
The surface should be made as tight as pos-
as base courses are given in Table 7-3 of
sible, and good surface drainage should be
TM 5-330 /Air Force Manual (AFM) 86-3,
provided. For best results, the PI of a stabi-
Volume II. Experience in civil highway con-
lized soil that is to function first as a wearing
struction indicates that best results are
surface and then as a base, with a bituminous
obtained with this type of mixture if the frac-
surface being provided at a later date, should
tion passing the Number 200 sieve is not
be held within very narrow limits. Con-
greater than two-thirds of the fraction pass-
sideration relative to compaction, bearing
ing the Number 40 sieve. The size of the
value, and frost action are as important for
largest particles should not exceed two-thirds
surfaces of this type as for bases.
of the thickness of the layer in which they are
incorporated. The mixture should be well-
Proportioning
graded from coarse to fine.
Mixtures of this type are difficult to design
A basic requirement of soil mixtures that and build satisfactorily without laboratory
are to be used as base courses is that the PI control. A rough estimate of the proper
should not exceed 5. Under certain cir- proportions of available soils in the field is
cumstances, this requirement may be relaxed possible and depends on manual and visual
inspection. For example, suppose that a loose
if a satisfactory bearing ratio is developed,
sand is the existing subgrade soil and it is
Experience also indicates that under ideal
desired to add silty clay from a nearby borrow
circumstances the LL should not exceed 25.
source to achieve a stabilized mixture. Each
These requirements may be relaxed in
soil should be moistened to the point where it
theater-of-operations construction. The re-
is moist, but not wet; in a wet soil, the mois-
quirements may be lowered to a LL of 35 and
ture can be seen as a shiny film on the surface,
a PI of 10 for fully operational airfields. For
What is desired is a mixture that feels gritty
emergency and minimally operational air-
and in which the sand grains can be seen.
fields, the requirements may be lowered to a
Also, when the soils are combined in the
LL of 45 and a PI of 15, when drainage is good.
proper proportion, a cast formed by squeezing
the moist soil mixture in the hand will not be
Soil Surface Requirements
either too strong or too weak; it should just be
Grading requirements for mechanically able to withstand normal handling without
stabilized soils that are to be used directly as breaking. Several trial mixtures should be
surfaces, usually under emergency condi- made until this consistency is obtained. The
tions, are generally the same as those indi-
proportion of each of the two soils should be
cated in Table 7-3 of TM 5-330/AFM 86-3, carefully noted. If gravel is available, this
Volume II. Preference should be given to mix- may be added, although there is no real rule of
tures that have a minimum aggregate size thumb to tell how much should be added. It is
equal to 1 inch or perhaps 1 ½ inches. Ex- better to have too much gravel than too little.
perience indicates that particles larger than
this tend to work themselves to the surface Use of Local Materials. The essence of
over a period of time under traffic. Somewhat mechanical soil stabilization is the use of lo-
more fine soil is desirable in a mixture that is cally available materials. Desirable require-
to serve as a surface, as compared with one for ments for bases and surfaces of this type were
a base. This allows the surface to be more given previously. It is possible, especially
resistant to the abrasive effects of traffic and under emergency conditions, that mixtures of
Soil Stabilization for Roads and Airfields 9-4
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local materials will give satisfactory service, numerical example (see Table 9-1, page 9-6).
even though they do not meet the stated re- Two materials are available, material B in the
quirements. Many stabilized mixtures have roadbed and material A from a nearby borrow
been made using shell, coral, soft limestone, source. The mechanical analysis of each of
cinders, marl, and other materials listed ear- these materials is given, together with the LL
lier. Reliance must be placed on—
and PI of each. The desired grading of the
Experience. combination is also shown, together with the
An understanding of soil action. desired plasticity characteristics.
The qualities that are desired in the
finished product. Specified Gradation. Proportioning of
Other factors of local importance in trial combinations may be done arithmetical-
proportioning such mixtures in the ly or graphically. The first step in using
field. either the graphical or arithmetical method is
to determine the gradation requirements.
Blending. It is assumed in this discussion Gradation requirements for base course, sub-
that an existing subgrade soil is to be stabi- course, and select material are found in
lized by adding a suitable borrow soil to Tables 7-1 and 7-3, TM 5-330/AFM 86-3,
produce a base course mixture that meets the Volume II. In the examples in Figures 9-1
specified requirements. The mechanical and 9-2, page 9-7, abase course material with
analysis and limits of the existing soil will a maximum aggregate size of 1 inch has been
usually be available for the results of the sub- specified. In the graphical method, the
grade soil survey (see Chapter 3). Similar gradation requirements are plotted to the
information is necessary concerning the bor- outside of the right axis. In the arithmetical
row soil. The problem is to determine the method, they are plotted in the column
proportions of these two materials that labelled “Specs.” Then the gradations of the
should be used to produce a satisfactory mix- soils to be blended are recorded. The graphi-
ture. In some cases, more than two soils must cal method has the limitation of only being
be blended to produce a suitable mixture. capable of blending two soils, whereas the
However, this situation is to be avoided when arithmetical method can be expanded to
possible because of the difficulties frequently blend as many soils as required. At this point,
encountered in getting a uniform blend of the proportioning methods are distinctive
more than two local materials. Trial com- enough to require separate discussion.
binations are usually made on the basis of the
mechanical analysis of the soil concerned. In Graphical Proportioning. The actual
other words, calculations are made to deter- gradations of soil materials A and B are
mine the gradation of the combined materials plotted along the left and right axes of the
and the proportion of each component ad- graph, respectively. As shown in Figure 9-1,
justed so that the gradation of the page 9-7, material A has 92 percent passing
combination falls within specified limits. The the 3/4-inch sieve while material B has 72
PI of the selected combination is then deter- percent passing the same sieve. Once plotted,
mined and compared with the specification. a line is drawn across the graph, connecting
If this value is satisfactory, then the blend the percent passing of material A with the
may be assumed to be satisfactory, provided percent passing of material B for each sieve
that the desired bearing value is attained. If size.
the plasticity characteristics of the first comb-
ination are not within the specified limits, NOTE: Since both materials A and B had
additional trials must be made. The propor- 100 percent passing the l-inch sieve, it
tions finally selected then may be used in the was omitted from the graph and will
field construction process. not affect the results.
Numerical Proportioning. The process of Mark the point where the upper and lower
proportioning will now be illustrated by a limits of the gradation requirements intersect
Soil Stabilization for Roads and Airfields 9-5
FM 5-410
the line for each sieve size. In Figure 9-1, the B is selected for a trial mixture, A similar
allowable percent passing the Number 4 sieve diagram can be prepared for any two soils.
ranges from 35 to 65 percent passing. The
point along the Number 4 line at which 65 Arithmetical Proportioning. Record the
percent passing intersects represents 82 per- actual gradation of soils A and B in their
cent material A and 18 percent material B. respective columns (Columns 1 and 2, Figure
The 35 percent passing intersects the Num- 9-2). Average the gradation limits and record
ber 4 line at 19 percent material A and 81 in the column labelled "S". For example, the
percent material B. The acceptable ranges of allowable range for percent passing a 3/8-inch
material A to be blended with material B is sieve in a 1-inch minus base course is 50 to 80
the widest range that meets the gradation re- percent. The average, 50±80/2, is 65 percent.
quirements for all sieve sizes. The shaded As shown in Figure 9-2, S for 3/8 inch is 65.
area of the chart represents the combinations Next, determine the absolute value of S-A
of the two materials that will meet the and S-B for each sieve size and record in the
specified gradation requirements. The columns labelled “(S-A)“ and “ (S-B), res-
boundary on the left represents the combina- pectively. Sum columns (S-A) and ( S-B).
tion of 44 percent material A and 56 percent To determine the percent of soil A in the final
material B. The position of this line is fixed mix, use the formula—
by the upper limit of the requirement relating
In the example in Figure 9-2:
to the material passing the Number 200 sieve
(15 percent). The boundary on the right rep-
resents the combination of 21 percent
material A and 79 percent material B. This
line is established by the lower limit of the re-
quirement relative to the fraction passing the
Number 40 sieve (15 percent). Any mixture
falling within these limits satisfies the grada-
tion requirements. For purposes of
illustration, assume that a combination of 30 103 103
x 100% = 43.5%
percent material A and 70 percent material 134 + 103 = 237
Soil Stabilization for Roads and Airfieids 9-6
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Soil Stabilization for Roads and Airfields 9-7
FM 5-410
The percent of soil B in the final mix can be approximate values is shown in Figure 9-3.
determined by the formula: The values shown in Figure 9-3 require addi-
tional explanation, as follows. Consider 500
pounds of the mixture tentatively selected (30
percent as material A and 70 percent as
material B). Of this 500 pounds, 150 pounds
are material A and 350 pounds material B.
or
Within the 150 pounds of material A, there
are 150 (0.52) = 78 pounds of material passing
100% - %A = %B
the Number 40 sieve. Within the 350 pounds
of material B, there are 150 (0.05) = 17.5
NOTE: If three or more soils are to be
pounds of material passing the Number 40
blended, the formula would be—
sieve. The total amount of material passing
the Number 40 sieve in the 500 pounds of
blend = 78+ 17.5= 95.5 pounds, The percent-
%C = age of this material that has a PI of 9
(material A) is (78/95.5) 100= 82. As shown in
Figure 9-3, the approximate PI of the mixture
This formula can be further expanded as of 30 percent material A and 70 percent
necessary. material B is 7.4 percent. By similar reason-
ing, the approximate LL of the blend is 28,4
Multiply the percent passing each sieve for percent. These values are somewhat higher
than permissible under the specification. An
soil A by the percentage of soil A in the final
increase in the amount of material B will
mix; record the information in column 4 (see
somewhat reduce the PI and LL of the com-
Figure 9-2, page 9-7), Repeat the procedure
bination.
for soil B and record the information in
column 5 (see Figure 9-2, page 9- 7). Complete
the arithmetical procedure by adding Field Proportioning. In the field, the
columns 4 and 5 to obtain the percent passing materials used in a mechanically stabilized
each sieve in the blended soil. soil mixture probably will be proportioned by
loose volume. Assume that a mixture incor-
Both the graphical and arithmetical porates 75 percent of the existing subgrade
methods have advantages and disad- soil, while 25 percent will be brought in from
vantages. The graphical method eliminates a nearby borrow source. The goal is to con-
the need for precise blending under field con- struct a layer that has a compacted thickness
ditions and the methodology requires less of 6 inches. It is estimated that a loose thick-
effort to use, Its drawback becomes very com- ness of 8 inches will be required to form the
6-inch compacted layer. A more exact
plex when blending more than two soils. The
relationship can be established in the field as
arithmetical method allows for more precise
construction proceeds, Of the 8 inches loose
blending, such as mixing at a batch plant, and
thickness, 75 percent (or 0.75(8) = 6 inches)
it can be readily expanded to accommodate
will be the existing soil, The remainder of the
the blending of three or more soils. It has the
mix will be mixed thoroughly to a depth of
drawback in that precise blending is often un-
8 inches and compacted by rolling. The
attainable under field conditions. This
proportions may be more accurately control-
reduces the quality assurance of the perfor-
led by weight, if weight measurements can be
mance of the blended soil material.
made under existing conditions.
Plasticity Requirements. A method of
Waterproofing
determining the PI and LL of the combined
soils serves as a method to indicate if the The ability of an airfield or road to sustain
proposed trial mixture is satisfactory, pend- operations depends on the bearing strength of
ing the performance of laboratory tests. This the soil. Although an unsurfaced facility may
may be done either arithmetically or graphi- possess the required strength when initially
cally. A graphical method of obtaining these constructed, exposure to water can result i n a
Soil Stabilization for Roads and Airfields 9-8
FM 5-410
loss of strength due to the detrimental effect Compaction.
of traffic operations. Fine-grained soils or Drainage.
granular materials that contain an excessive
amount of fines generally are more sensitive Objectives of Waterproofers. The objec-
to water changes than coarse-grained soils. tive of a soil-surface waterproofer is to protect
Surface water also may contribute to the a soil against attack by water and thus
development of dust by eroding or loosening preserve its in-place or as-constructed
material from the ground surface that can be- strength during wet-weather operations.
come dust during dry weather conditions. The use of soil waterproofers generally is
limited to traffic areas. In some instances,
Sources of Water. Water may enter a soil soil waterproofers may be used to prevent ex-
either by the percolation of precipitation or cessive softening of areas, such as shoulders
ponded surface water, by capillary action of or overruns, normally considered nontraffic
underlying ground water, by a rise in the or limited traffic areas.
water-table level, or by condensation of water
vapor and accumulation of moisture under a Also, soil waterproofers may prevent soil
vapor-impermeable surface. As a general erosion resulting from surface water runoff.
rule, an existing groundwater table at shal- As in the case of dust palliative, a thin or
low depths creates a low load-bearing shallow-depth soil waterproofing treatment
strength and must be avoided wherever pos- loses its effectiveness when damaged by ex-
sible. Methods to protect against moisture cessive rutting and thus can be used
ingress from sources other than the ground efficiently only in areas that are initially firm.
surface will not be considered here. In most Many soil waterproofers also function well as
instances, the problem of surface water can be dust palliatives; therefore, a single material
lessened considerably by following the proper might be considered as a treatment in areas
procedures for— where the climate results in both wet and dry
Grading. soil surface conditions. Geotextiles are the
Soil Stabilization for Roads and Airfields 9-9
FM 5-410
primary means of waterproofing soils when With 95 percent passing the Number
grading, compaction, and drainage practices sieve, the PI is 14.
are insufficient. Use of geotextiles is dis- With 14 percent passing the Number
cussed in detail in Chapter 11. 200 sieve, the LL is 21.
CHEMICAL ADMIXTURE STABILIZATION Therefore the soil is 5 percent gravel, 81
Chemical admixtures are often used to sta- percent sand, and 14 percent fines. Figure
9-4, page 9-12, shows this soil in Area 1C.
bilize soils when mechanical methods of
stabilization are inadequate and replacing an
Table 9-3, page 9-13, shows that the
undesirable soil with a desirable soil is not
stabilizing agents recommended for Area 1C
possible or is too costly. Over 90 percent of all
soils include bituminous material, portland
chemical admixture stabilization projects
cement, lime, and lime-cement-fly ash. In
use—
this example, bituminous agents cannot be
Cement.
used because of the restriction on PI, but any
Lime.
of the other agents can be used if available.
Fly ash.
Bituminous materials.
Cement
Cement can be used as an effective stabi-
Other stabilizing chemical admixtures are
lizer for a wide range of materials. In general,
available, but they are not discussed in this
however, the soil should have a PI less than
manual because they are unlikely to be avail-
30. For coarse-grained soils, the percent
able in the theater of operations.
passing the Number 4 sieve should be greater
than 45 percent.
WARNING
If the soil temperature is less than 40
Chemical admixtures may contain haz-
degrees Fahrenheit and is not expected to in-
ardous materials, Consult Appendix C
to determine the necessary safety crease for one month, chemical reactions will
precautions for the selected admixture. not occur rapidly. The strength gain of the ce-
ment-soil mixture will be minimal. If these
environmental conditions are anticipated,
When selecting a stabilizer additive, the the cement may be expected to act as a soil
factors that must be considered are the— modifier, and another stabilizer might be con-
Type of soil to be stabilized. sidered for use. Soil-cement mixtures should
Purpose for which the stabilized layer be scheduled for construction so that suffi-
will be used. cient durability will be gained to resist any
Type of soil quality improvement freeze-thaw cycles expected.
desired.
Required strength and durability of Portland cement can be used either to
the stabilized layer. modify and improve the quality of the soil or
Cost and environmental conditions. to transform the soil into a cemented mass,
which significantly increases its strength and
Table 9-2 lists stabilization methods most durability. The amount of cement additive
suitable for specific applications. To deter- depends on whether the soil is to be modified
mine the stabilizing agent(s) most suited to a or stabilized. The only limitation to the
particular soil, use the gradation triangle in amount of cement to be used to stabilize or
Figure 9-4, page 9-12, to find the area that cor- modify a soil pertains to the treatment of the
responds to the gravel, sand, and fine content base courses to be used in flexible pavement
of the soil. For example, soil D has the follow- systems. When a cement-treated base course
ing characteristics: for Air Force pavements is to be surfaced with
Soil Stabilization for Roads and Airfields 9-10