CHAPTER TWO
LITERATURE REVIEW
2.1
Introduction
Soil compaction changes pore-space size, particle distribution and soil strength.
One way to quantify the change is by measuring the bulk density. As the pore space
is decreased within a soil, the bulk density is increased. Soils with a higher
percentage of clay and silt, which naturally have more pore space, have a lower bulk
density than sandy soils.
Theories of soil compaction seek to explain the typical moisture-density
relationships as represented by the compaction curve obtained in laboratory tests or
field compaction (Hausmann, 1990). Many interpretations of the basic phenomena
have been put forward since Proctor (1933) did his pioneering studies. They began
with the lubrication concept and proceeded to examining pore water and air pressures,
and finally, the soil microstructure. Each of the theories has its merits, although it
may have to be placed in the context of the state of development of soil mechanics at
the time and the soil types and methods of compaction used in obtaining the
experimental data.
7
2.2
Soil Compaction
Compaction is a common process in urban areas (Kelsey, 2000). Typically, to
build stable infrastructure, soils are compressed until they have been compacted to
more than 90% or 95% of a laboratory maximum dry density. Urban horticulture
relies on these compacted materials for the rooting space of trees, shrubs, and other
plants. Compaction is generally difficult to alleviate and perhaps even more difficult
to measure. Without effective methods for measuring soil compaction, it is difficult
to assess whether the soil is compacted or not or and whether a treatment has
reversed any of the compaction in a soil. Bulk density (weight per unit volume) is the
most common measure of compaction.
Table 2.1 Definitions of parameters of a compaction test
TERMS
Compaction
DEFINITION
the process of packing soil particles more
closely together, usually by mechanical means,
increasing the density of the soil
Optimum moisture content (OMC)
The moisture content of the soil at which a
specified amount of compaction will produce
the maximum dry density
Maximum dry density (MDD)
The dry density obtained using a specified
amount of compaction at the optimum
moisture content
Dry density – moisture content
The relationship between dry density and
relationship
moisture content of a soil under a given
compactive effort
Percentage air voids (Va)
The volume of air voids in a soil expressed as
a percentage of the total volume of the soil
Air voids line
Aline
showing
the
dry density-moisture
content relationship for a soil containing a
constant percentage of air voids
8
Saturation line (Zero air void line)
The line showing the dry density-moisture
content relationship for a soil containing no air
voids
2.2.1 Compaction characteristics of soils
The density at which a soil can be placed as fill or backfill depends on the
placement water content and the compaction effort. Figure 2.1 presents typical
engineering properties of compacted soils.
Figure 2.1 Typical Engineering Properties of Compacted Materials (U. S. Army Corps of Engineers)
9
10
2.3
Compaction Theory
The performance of a standard laboratory compaction test on material from
each field density test usually give the most accurate relationship of the in-place
material to optimum water content and maximum density. But it is not generally
feasible to do so in-field because the testing could not keep pace with the rate of fill
placement. However, standard compaction tests should be performed during
construction when an insufficient number of the compaction curves were developed
during the design phase, when borrow material is obtained from a new source, and
when material similar to that being placed has not been tested previously. In any
event, laboratory compaction tests should be performed periodically on each type of
fill material (preferably one for every ten field density tests) to check the optimum
water content and maximum dry density values being used for correlation with field
density test results.
Mitchell et al. (1965) state, the nature and magnitude of compaction in finegrained soil significantly influences their mechanical behavior. It is generally known
that when a clayey soil is compacted to a given dry density (or relative compaction),
it is stiffer if it is compacted dry-of-optimum than if it is compacted wet-of-optimum.
Lambe and Whitman (1969), Hilf (1975), and Mitchell (1976), attribute this effect to
soil fabric, as a result of different remolding water contents. However, these
references imply that for sand, the drained shear strength and compressibility are
independent of the remolding water content; i.e., these properties are uniquely
determined, once the relative compaction, or void ratio, is specified.
Soil consists of organic matter, minerals and pore space. The mineral fraction
of the soil is made up of a combination of sand, silt, and clay particles. These
particles do not fit together tightly, but are surrounded by open pore spaces. This
open space is important because it allows soil to hold air and water. Spaces between
the particles are filled with air in dry soil, water in saturated soil, or both in moist soil.
Soil compaction occurs when soil particles are pressed together, limiting the space
11
for air and water. The amount of soil water is a critical factor in soil compaction
potential. A dry soil, which has friction between the soil particles, is not easily
compacted. Water acts as a lubricant between the particles, making the soil easier to
compact. However, as soil water content increases, a point is reached where most
pore spaces in the soil are filled with water, not air. Water cannot be compressed, so
water between the soil particles carries some of the load of the soil, resisting
compaction. Therefore, a very wet soil will not compact as much as a moderately
moist soil.
Compaction can be applied to improve the properties of an existing soil or in
the process of placing fill. There are three main objectives:
i. to increase shear strength and therefore bearing capacity
ii. to increase stiffness and therefore reduce future settlement
iii. to decrease the voids ratio and so permeability, thus reducing potential frost
heave
Similar to Mitchell et al. (1965), Carrier (2000) also found that the samples of
compacted dry-of-optimum were to be stiffer than samples compacted wet-ofoptimum at the same relative compaction. This difference in stress-strain behavior is
not generally expected for sand; fabric and/or overconsolidation may explain these
results. Thus, for the case of shallow depth (such as backfill for a flexible conduit
located within a few meters of the ground surface) it is important to consider the
water content and the method of compaction, as the degree of compaction by itself
will not necessarily achieve the desired modulus.
12
2.4
Influence Factors For Compaction
Soil texture (the percentage of sand, silt, and clay in a soil) has some effects
on compaction, although compaction can be a problem to one degree or another in
almost all soil types (Kok et al., 1996). Soils made up of particles of about the same
size compact less than soil with a variety of particle sizes. Smaller particles can fill
the pores between larger particles making for a more dense soil. A sandy loam soil
(67 percent sand, 24 percent silt, and 9 percent clay) is the most susceptible to
compaction. Soil texture is not easily changed. The structure of a soil (how well the
soil breaks up into small, cohesive clumps when crumbled) also plays a role in its
potential for compaction. A soil with higher levels of organic matter generally has
better structure and resists compaction better than soils with lower organic matter
levels. Organic matter helps create larger and stronger soil aggregates. Hard, dense,
low organic-matter soils suffer more from compaction than loose, friable, highorganic matter soils.
Whitlow (1999) states that the effectiveness of compaction process is
dependent on several factors:
i. The nature and type of soil, i.e. sand or clay, uniform or well-graded, plasticity.
ii. The water content at the time of compaction.
iii. Site conditions, i.e. weather, type of site, layer thickness.
iv. Amount of compactive effort: type of hammer or compaction tools (weight,
vibration, number of passes).
13
2.5
Dry-Density Versus Water-Content Relationship
The relationship between dry density and moisture content for soil subjected
to a given compactive effort, established by laboratory compaction test, provides
reference data for the specification and control of soil placed as fill. In many projects
the laboratory compaction tests are supplemented by field compaction trials by using
the actual placing and compacting equipment which is to be employed for
construction (Williams, 1949).
Sometimes it is necessary to adjust the natural moisture content of a soil to a
value at which it can be most effectively compacted, or at which it has the highest
strength. The required moisture content, and dry density to be achieved, can be
assessed on the basis of the dry density-moisture content relationship derived fro
laboratory compaction tests on samples taken fro the borrow area.
The state of compaction of a soil is conveniently measured using the dry
density, the attainable values of which are related to the water content. As water is
added to a dry soil, film of adsorbed water form around the particles. As the adsorbed
water films increase in thickness the particles become lubricated and are able to pack
more closely together, thus the density increases. At a certain point, the pore-water
pressure in adsorbed film tend to push the particles apart and so with further
increases in water content the density decreases. The maximum dry density therefore
occurs at optimum water content as shown in Figure 2.2. Curves for different air
contents also can be added to the d/w plot using this expression:
ρd 
Gs ρw
1 A v 
1 w G s
Where:
d = dry density
(2.1)
14
Gs = specific gravity of soil particles
w = density of water
Av = air voids content
w = water content
The air-void content corresponding to the maximum dry density and optimum
water content can be read off the d/w plot or calculated from this expression.
Figure 2.2 Dry density versus water content plot with air void curves
2.5.1
Explanation of the shape of the curve
As illustrated in the typical compaction curve of Figure 2.3, water has an
important effect on soil compaction. Even at low water content, the soil grains are
surrounded by a thin film of water. A small increase in water content tends to
15
increase the repulsion of particles and to facilitate their orderly arrangement. Until
the optimum water content is reached, the addition of water expels more air from
soils, and enables to reach larger dry unit weight. The maximum-densed soil is
obtained at the optimum water content. When the water content exceeds the optimum
value, the water pushes the grains apart. Since water is much more incompressible
than the grain assembly and has no time to drain, the dry unit weight starts to
decrease. There are three conditions to be considered:
i.
At low water content in clays (<wopt)
The material being compacted is generally recently excavated saturated
lumps having relatively high undrained strength; too stiff to compact. As the water
increases the lumps soften and weaken. So it can be compacted easily.
ii.
At low water content in sands (<wopt)
At low water content, the soil is unsaturated and derives strength from pore
water suction at grain contacts. As the water content increases this suction
decreases and the soil grains are more easily displaced into a denser arrangement,
and therefore easily compacted.
iii.
At high water content (clays and sands) (> wopt)
At relatively high water content the compacted soil is nearly saturated (nearly
all of the air has been removed) and so the compactive effort is in effect undrained
loading and so the void volume does not decrease. As the water content further
increases the air content remains almost constant and the extra water increases the
volume thus the compacted density achieved decreases.
16
Figure 2.3 Dry density versus water content plot
2.5.2
Dry density and air-void content
A fully saturated soil has zero air content. However, in practice, even quite
wet soil will have small air content.
Air voids content, A v 
Volume of air
Total volume
(2.2)
17
2.5.3
Effect of compactive effort
Whitlow (1999) states that the compactive effort will be greater when using
either a heavier roller on site or a heavier rammer in the laboratory. With greater
compactive effort:
· maximum dry density increases
· optimum water content decreases
· air-void content remains almost the same.
Figure 2.4 Dry density versus water content curves for different compactive efforts
18
Table 2.2 Specification of BS 1377:1990 (Whitlow, 1999)
Name of test
Light compaction
Heavy compaction
Vibrating
hammer
Rammer:
mass
2.5 kg
4.5 kg
300—400 N
face diameter
50 mm
50 mm
150 mm
drop height
300 mm
450 mm
Soil size
<20 mm
>20 mm
<20 mm
>20 mm
>20 mm
5 kg
25 kg
5 kg
25 kg
25 kg
Volume
1000 ml
2300 ml
1000 ml
2300 ml
2300 ml
Internal diam.
105 mm
152 mm
105 mm
152 mm
152 mm
115.5 mm
127 mm
115.5 mm
127 mm
127 mm
3
3
5
5
3
27
62
27
62
(60 s)
Soil quantity
Mould:
Internal
height
No. of layers
No. of blows per
layer
2.5.4
Effect of soil type and condition
Poulos (1988) states that there are two errors that affect degree of compaction,
which are the mismatch and the oversize correction. The mismatch error arises when
the compaction test and the field density test are not performed on the same soil
specimen. It is common practice in land-development projects to determine the
19
percent compaction of hundreds of field-density tests based on a few laboratory
compaction curves on representative samples. But minor variations in soil gradation
can cause significant errors in the degree of compaction.
The soil compaction has become more of a problem in recent years due to
increased equipment size and lack of crop rotations (Kok et. al., 1996). In continuous
mono-cropping, more tillage passes may be needed to control weeds and bury crop
residue that could foster diseases. Increased vehicle traffic increases the potential for
compaction. Increase in field size can contribute to compaction, too. Larger fields
may contain more variation in soil conditions. When working in a large field, some
sections might be dry while others are still too wet. When fields are smaller, each
field is in more uniform condition and tilled only when ready.
Whitlow (1999) explains that well-graded granular soils can be compacted to
higher densities than uniform or silty soils. Clays of high plasticity may have water
contents over 30% and achieve similar densities (and therefore strengths) to those of
lower plasticity with water contents below 20%.
Figure 2.5 Dry density versus water content curves for range of soil types
20
2.6
Atterberg Limits
Smith (1981) states that as moisture removed from fine-grained soil it passes
through a series of states, which are liquid, plastic, semi-solid and solid. The
moisture contents of a soil at the points where it passes from one stage to the next are
known as consistency limits (Atterberg limit). These limits are defined as:
liquid limit (LL) – the minimum moisture content at which the soil will
i.
flow under its own weight.
ii.
plastic limit (PL) – the minimum moisture content at which the soil can
be rolled into a thread 3 mm diameter without breaking up.
iii.
shrinkage limit (SL) – the maximum moisture content at which further
loss of moisture does not cause a decrease in the volume of the soil.
Das (2003) mentions that when a clayey soil is mixed with an excessive
amount of water, it may flow like a semi- liquid. If the soil is gradually dried, it will
behave like a plastic, semisolid, or solid material, depending on its moisture content.
The moisture content, in percent, at which the soil changes from a liquid to a plastic
state is defined as the liquid limit (LL). Similarly, the moisture content, in percent, at
which the soil changes from a plastic to a semisolid state and from a semisolid to a
solid state are defined as the plastic limit (PL) and the shrink age limit (SL),
respectively. These limits are referred to as Atterberg limits (Figure 2.6).
The range of moisture content over which a soil is plastic is known as the
plasticity index (PI).
PI = LL – PL
(2.3)
21
Figure 2.6 Changes of volume of soil with moisture content with respect to Atterberg
limits
Consistency varies with the water content of the soil. The consistency of a
soil can range from (dry) solid to semi-solid to plastic to liquid (wet). The water
contents at which the consistency changes from one state to the next are called
consistency limits (or Atterberg limits). Measures of liquid and plastic limit values
can be obtained from laboratory tests. Two of these are utilized in the classification
of fine soils:
i.
Liquid limit (LL) - change of consistency from plastic to liquid.
ii.
Plastic limit (PL) - change of consistency from brittle/crumbly to plastic.
22
Figure 2.7 Plasticity Chart
A plasticity chart (Figure 2.7) is provided to aid classification. In the British
Standard Soil Classification fine soils are divided into ten classes based on their
measured plasticity index and liquid limit values: CLAYS are distinguished from
SILTS, and five divisions of plasticity are defined:
2.7
Low plasticity
LL = < 35%
Intermediate plasticity
LL = 35 - 50%
High plasticity
LL = 50 - 70%
Very high plasticity
LL = 70 - 90%
Extremely high plasticity
LL = > 90%
Correlation Between Compaction Parameters With Atterberg Limit
Boutwell (1961) reported that a linear relationship existed between the
maximum dry unit weight (d max) and the base 10 logarithm of the compaction
energy (log E), based on tests he conducted on a micaceous silty fine sand.
23
Hammond (1980) studied three groups of soils in Ghanna and performed a
linear regression analysis of the relationships between wopt and either wp, wL, Ip, ws,
or (% fines). Some of the expressions derived are:
For lateritic soils (predominantly clayey and sandy gravels):
wopt = 0.42 wp + 5
(2.4)
For micaceous soils (clayey silty sands, with Atterberg limits of the fines
plotted below the A line):
wopt = 0.45wp + 3.58
(2.5)
wopt = 0.5wL – 6
(2.6)
For black cotton soils (silty clays):
wopt = 0.96wp - 7.7
(2.7)
A discussion of Atterberg limits correlations and comparison of results with
the compaction parameters were given by Torrey (1970). In order to determine a
mathematical relationship between the variables of interest (that is liquid limit,
plastic limit, optimum water content, maximum dry density) using the methods of
statistics, it is necessary to assume a frequency distribution between the variables. It
was assumed that there is a normal or Gaussian distribution between the variables. A
normal distribution has a very specific mathematical definition and, although the
assumption of normal distribution is reasonable, it must be pointed out that there is
no insurance that the assumption is valid. Additionally, it was assumed that the
24
relationship between the variables of interest is linear. The results of the analysis of
the data by Torrey (1970) are presented in Figures 2.8 and 2.9. It showed that the
linear correlations between optimum water content and liquid limit (shown in Figure
2.8 a) and maximum density and liquid limit (shown in Figure 2.8 b) explain only
77.6 percent and 76.3 percent, respectively, of variation between the regression line
and the data points.
The equations derived by Torrey (1970) were :
wopt = 0.240LL + 7.549
(2.8)
d max = -0.414LL + 125.704
(2.9)
wopt = 0.263PI + 12.282
(2.10)
d max = -0.449PI + 117.372
(2.11)
25
Figure 2.8
Plots of Optimum Water Content and Maximum Dry Density versus
Liquid Limit (Torrey, 1970)
26
Figure 2.9
Plots of Optimum Water Content and Maximum Dry Density versus
Plasticity Index (Torrey, 1970)
27
Hausmann (1990) states that the optimum water content increases and the
maximum dry density decreases with the increasing plasticity of the soil, as defined
by the Atterberg limits. The “Design Manual” (U.S. Navy (1960)), gives the
following rules of thumb in the relation to the parameters determined in standard
laboratory compaction (not modified proctor compaction):
wopt = (std.) =
wp – 5 at wopt = 10%
(2.12)
wp – 2 at wopt = 30%
(2.13)
Where wp is the plastic limit. Alternatively wopt and dmax for standard
compaction can be estimated from the liquid limit wL and the plasticity index Ip
defined as the difference between the liquid limit and plastic limit:
wopt = 6.77 + 0.43 wL – 0.21 Ip
(2.14)
dmax = 20.48 – 0.13 wL + 0.05 Ip
(2.15)
Al-Khafaji (1993) examined the relationship between the Atterberg limits and
soil compaction as measured by the use of the standard proctor compaction test. The
relationship of liquid limit, wL, and plastic limit, wp,, to proctor maximum dry
density, d, and optimum moisture content, wopt, were determined quantitatively for
soils from Iraq and USA. Using the curve fitting techniques, empirical equations
were derived and charts were prepared. From these it is possible to estimate the
potential optimum moisture content and maximum dry density for standard proctor
compaction from the knowledge of Atterberg limits only. The accuracy of these
charts (refer to Figures 2.10 and 2.11) is considered in relation to the basic data. He
also did the comparison for the compaction parameters of the Iraqi and the United
States soils.
28
For Iraqi soils, the following equations were derived:
dmax = 2.44 - 0.02 wp - 0.008 wL
(2.16)
wopt = 0.24 wL + 0.63 wp - 3.13.
(2.17)
and
For the US soils, the following equations were derived:
dmax = 2.27 - 0.0 19 wp - 0.003 wL
(2.18)
wopt = 0.14 wL + 0.54 wp
(2.19)
and
Where;
dmax = maximum dry density
wopt = optimum moisture content
wL = liquid limit
wp = plastic limit
Figure 2.10 Estimation of maximum dry density (ρd) and optimum moisture content (wopt) from Atterberg limits based on Iraqi data (AlKhafaji , 1993)
29
Figure 2.11 Estimation of maximum dry density (ρd) and optimum moisture content (wopt) from Atterberg limits based on U.S data (AlKhafaji, 1993)
30
31
Blotz et al. (1998) used an empirical method to describe the estimation of
maximum dry unit weight and optimum water content of clayey soils at any rational
compactive effort E. All soils were compacted using two to four compactive efforts
including Standard Proctor (ASTM D 698), Modified Proctor (ASTM D 1557),
“Reduced” Proctor and Super-Modified Proctor”. One variation of the method uses
the liquid limit (LL) and one compaction curve, whereas the other uses only the LL.
Linear relationships between d max and the logarithm of compaction energy (log E),
and wopt and log E, both of which are a function of the LL, are used to extrapolate to
different compactive energies. Data for twenty two (22) clayey soils were used to
develop the empirical relationship, and data for five additional soils were used for the
validation of the empirical relationship. The variation employing the LL and one
compaction curve is slightly more precise, with typical errors of about ± 1% for wopt
and ± 2% on d max. For the variation employing only the LL, typical errors are about
± 2% for wopt and ± 6% on d max. The equations obtained were:
 E 
γ d max, E  γ d max, k  2.27 LL  0.94log  
 Ek 
and
2.20
 E 
w opt,E  w opt,k  12.39 12.21LL log  
 Ek 
2.21
Where :
E = Compaction energy (unknown) (kJ/m3)
Ek = Compaction energy (known) (kJ/m3)
Figure 2.12 shows relationships between dmax, wopt and LL with Reduced
Proctor (RP), Standard Proctor (SP) and Modified Proctor (MP) corresponding to
reduced, standard and modified Proctor efforts. From the research, wopt increases and
dmax decreases when the LL becomes larger. These curves can be used directly to
estimate the optimum point for standard or modified Proctor effort if the LL is
known.
32
Figure 2.12 Maximum Dry Unit Weight (Eq. 2.20) and Optimum Water
Content (Eq. 2.21) Versus Liquid Limit For Reduced (RP), Standard (SP)
and Modified (MP) Proctor Compactive Efforts (Blotz et al., 1998)
Teo (2000) has developed a relationship between compaction parameters with
liquid limit and plasticity index. Both relationships are linear single regression
equations. The equation derived are:
(a) Correlation of optimum moisture content and Atterberg limits with
corresponding coefficient of determination, R2
OMC = 0.1411 LL + 0.0725
(R2 = 0.3755)
(2.22)
OMC = 0.1703 PI + 0.1073
(R2 = 0.1933)
(2.23)
and
33
(b) Correlation of maximum dry density and Atterberg limits with corresponding
coefficient of determination, R2
MDD = 2.0403 – 0.4872 LL
(R2 = 0.5089)
(2.24)
MDD = 1.9331 – 0.6523 PI
(R2 = 0.3219)
(2.25)
and
Redzuan (2002) stated that the correlation of maximum dry density and
Atterberg limits is more suitable compared to optimum moisture content. The linear
relationship derived as follows:
(a) Correlation of maximum dry density and Atterberg limits with corresponding
coefficient of determination, R2

dmax = -0.058LL + 2.0532
(R2 = 0.5091)
(2.26)
dmax = -0.0082PI + 1.9262
(R2 = 0.3603)
(2.27)
and


(b) Correlation of optimum moisture content and Atterberg limits with
corresponding coefficient of determination, R2
wopt = 0.1821LL + 5.9869
(R2 = 0.4236)
(2.28)
wopt = 0.2494PI + 10.127
(R2 = 0.2882)
(2.29)
and
34
(c) Correlation of maximum dry density and optimum moisture content with
corresponding coefficient of determination, R2
dmax = -0.026 wopt + 2.1526
(R2 = 0.7935)
(2.30)
Khainoriyani (2002) also found that maximum dry density has a strong
relationship with Atterberg limits compared to optimum moisture content. Following
are the linear relationships that have been developed:
(a)
Correlation of maximum dry density and Atterberg limits corresponding
coefficient of determination, R2

dmax = 2.0398 – 0.0057LL
(R2 = 0.5101)
(2.31)
dmax = 1.9178 - 0.0084PI
(R2 = 0.3890)
(2.32)
and
 
(b)
Correlation of optimum moisture content and Atterberg limits with
corresponding coefficient of determination, R2
wopt = 0.168LL + 6.8215
(R2 = 0.4320)
(2.33)
wopt = 0.2316PI + 10.792
(R2 = 0.3031)
(2.34)
and
35
(c)
Correlation of maximum dry density and optimum moisture content with
corresponding coefficient of determination, R2
dmax = -0.0258 wopt + 2.1388
(R2 = 0.7942)
(2.35)
2.8 Summary of Literature Review
Thorough the literature review, there are several relationships exist between
maximum dry density, ρdmax and optimum moisture content, wopt with the Atterberg
limits parameters. The review of data in the literature also reveals that a linear
relationship between maximum dry density, ρdmax and Atterberg limits parameters as
well as optimum moisture content, wopt with the Atterberg limits parameters exists
for cohesive soil. There are also exist linear relationships between d max and the
logarithm of compaction energy (log E), and wopt and log E for compacted cohesive
soil.
The literature also shows that there are negative regression relationships for
maximum dry density versus liquid limit and plasticity index. The maximum dry
density decreases with increasing liquid limit and plasticity index. However, the
relationships between the optimum moisture content versus liquid limit and plasticity
index are positive-regression correlated. In other words, the optimum moisture
content increases with increasing liquid limit and plasticity index.
36
Based on these findings, one can make a conclusion that there is a simple
method to predict compaction parameters, maximum dry density, ρdmax and optimum
moisture content, wopt based on the liquid limit, plastic limit and plasticity index. The
regression analysis maybe in a linear relationship or curve relationship. Thus it
means that this study has its own significance and very useful for others. However
the success of the results obtained or correlation produced depends on the method of
compaction used and the quality of the collected data.