CHAPTER TWO

PAGE 36

CHAPTER TWO

LITERATURE REVIEW

2.1IntroductionSoil 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 moisturedensity 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.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 testTERMSDEFINITION

Compaction 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 relationshipThe relationship between dry density and 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 lineAline showing the dry density-moisture content relationship for a soil containing a constant percentage of air voids

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 soilsThe 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.

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 fine-grained 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 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-of-optimum 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.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, high-organic 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).

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:

(2.1)

Where:

d = dry density

Gs = specific gravity of soil particles

w = density of water

Av = air voids content

w = water contentThe 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 curveAs 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 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 (20 mm

Soil quantity5 kg25 kg5 kg25 kg25 kg

Mould:

Volume

Internal diam.

Internal height1000 ml

105 mm

115.5 mm2300 ml

152 mm

127 mm1000 ml

105 mm

115.5 mm2300 ml

152 mm

127 mm2300 ml

152 mm

127 mm

No. of layers33553

No. of blows per layer27622762(60 s)

2.5.3 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 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 types2.6 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:i. liquid limit (LL) the minimum moisture content at which the soil will 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)

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:

Liquid limit (LL) - change of consistency from plastic to liquid.Plastic limit (PL) - change of consistency from brittle/crumbly to plastic.

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:

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%

2.7 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.

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 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)

Figure 2.8 Plots of Optimum Water Content and Maximum Dry Density versus Liquid Limit (Torrey, 1970)

Figure 2.9 Plots of Optimum Water Content and Maximum Dry Density versus Plasticity Index (Torrey, 1970)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):

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. For Iraqi soils, the following equations were derived:

(dmax = 2.44 - 0.02 wp - 0.008 wL

(2.16)and

wopt = 0.24 wL + 0.63 wp - 3.13.

(2.17)For the US soils, the following equations were derived:

(dmax = 2.27 - 0.0 19 wp - 0.003 wL

(2.18)and

wopt = 0.14 wL + 0.54 wp

(2.19)Where;

(dmax = maximum dry density

wopt = optimum moisture content

wL = liquid limit

wp = plastic limit

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:

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.

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)and

OMC = 0.1703 PI + 0.1073

(R2 = 0.1933)

(2.23)(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)

and

MDD = 1.9331 0.6523 PI

(R2 = 0.3219)

(2.25)

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)and

(dmax = -0.0082PI + 1.9262

(R2 = 0.3603)

(2.27)(b) Correlation of optimum moisture content and Atterberg limits with corresponding coefficient of determination, R2wopt = 0.1821LL + 5.9869

(R2 = 0.4236)

(2.28)and

wopt = 0.2494PI + 10.127

(R2 = 0.2882)

(2.29)(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) and

(dmax = 1.9178 - 0.0084PI

(R2 = 0.3890)

(2.32)(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)

and

wopt = 0.2316PI + 10.792

(R2 = 0.3031)

(2.34)(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, dmax 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, dmax 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.

Based on these findings, one can make a conclusion that there is a simple method to predict compaction parameters, maximum dry density, dmax 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. EMBED Equation.3

Figure 2.10 Estimation of maximum dry density (d) and optimum moisture content (wopt) from Atterberg limits based on Iraqi data (Al-Khafaji , 1993)

Figure 2.11 Estimation of maximum dry density (d) and optimum moisture content (wopt) from Atterberg limits based on U.S data (Al-Khafaji, 1993)

wopt = (std.) =

wp 5 at wopt = 10% (2.12)

EMBED Equation.3

wp 2 at wopt = 30% (2.13)

Figure 2.1 Typical Engineering Properties of Compacted Materials (U. S. Army Corps of Engineers)

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