GEOLOGICAL CHARACTERISTICS AND PARTICLE SIZES OF SOILS
4. Clays are composed of three main types of mineral—kaolinite, illite, and montmorillonite
5. The clay minerals consist of silica and alumina sheets that are combined to form layers. The bonds between layers play a very important role in the mechanical behavior of clays. The bond between the layers in montmorillonite is very weak compared with kaolinite and illite. Water can easily enter between the layers in montmorillonite, causing swelling.
6. A thin layer of water, called adsorbed water, is bonded to the mineral surfaces of soils. This layer signifi cantly infl uences the physical and mechanical characteristics of fi ne-grained soils.
What’s next . . . In most soils, there is a distribution of particle sizes that infl uences the response of soils to loads and to the fl ow of water. We will describe methods used in the laboratory to fi nd particle sizes of soils.
2.5 D E T E R M I N AT I O N O F PA R T I C L E S I Z E O F S O I L S — AST M D 4 2 2
2.5.1 Particle Size of Coarse-Grained Soils
The distribution of particle sizes or average grain diameter of coarse-grained soils—gravels and sands—
is obtained by screening a known weight of the soil through a stack of sieves of progressively fi ner mesh size. A typical stack of sieves is shown in Figure 2.11.
FIGURE 2.11 Stack of sieves.
2.5 DETERMINATION OF PARTICLE SIZE OF SOILS—ASTM D 422 15
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16 CHAPTER 2 GEOLOGICAL CHARACTERISTICS AND PARTICLE SIZES OF SOILS
Each sieve is identifi ed by either a number that corresponds to the number of square holes per linear inch of mesh or the size of the opening. Large sieve (mesh) openings (25.4 mm to 6.35 mm) are designated by the sieve opening size, while smaller sieve sizes are designated by numbers. The particle diameter in the screening process, often called sieve analysis, is the maximum dimension of a particle that will pass through the square hole of a particular mesh. A known weight of dry soil is placed on the largest sieve (the top sieve) and the nest of sieves is then placed on a vibrator, called a sieve shaker, and shaken. The nest of sieves is dismantled, one sieve at a time. The soil retained on each sieve is weighed, and the percentage of soil retained on each sieve is calculated. The results are plotted on a graph of percent of particles fi ner than a given sieve size (not the percent retained) as the ordinate versus the logarithm of the particle sizes, as shown in Figure 2.12. The resulting plot is called a particle size distribution curve or, simply, the gradation curve. Engineers have found it convenient to use a logarithmic scale for particle size because the ratio of particle sizes from the largest to the smallest in a soil can be greater than 104.
Let Wi be the weight of soil retained on the ith sieve from the top of the nest of sieves and W be the total soil weight. The percent weight retained is
% retained on ith sieve5 Wi
W 3100 (2.1)
The percent fi ner is
% finer than ith sieve 5 100 2a
i i51
1% retained on ith sieve2 (2.2)
You can use mass instead of weight. The unit of mass is grams or kilograms.
2.5.2 Particle Size of Fine-Grained Soils
The screening process cannot be used for fi ne-grained soils—silts and clays—because of their extremely small size. The common laboratory method used to determine the size distribution of fi ne-grained soils is a hydrometer test (Figure 2.13). The hydrometer test involves mixing a small amount of soil into a sus- pension and observing how the suspension settles in time. Larger particles will settle quickly, followed by smaller particles. When the hydrometer is lowered into the suspension, it will sink into the suspension until the buoyancy force is suffi cient to balance the weight of the hydrometer.
Poorly graded
0.001 0 10 20 30 40 50 60 70 80 90 100
0.01 0.1 1 10 100 Particle size (mm) – logarithmic scale
% Finer
Gap graded
Well graded
FIGURE 2.12 Particle size distribution curves.
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The length of the hydrometer projecting above the suspension is a function of the density, so it is possible to calibrate the hydrometer to read the density of the suspension at different times. The calibration of the hydrometer is affected by temperature and the specifi c gravity of the suspended solids. You must then apply a correction factor to your hydrometer reading based on the test temperatures.
Typically, a hydrometer test is conducted by taking a small quantity of a dry, fi ne-grained soil (ap- proximately 50 grams) and thoroughly mixing it with distilled water to form a paste. The paste is placed in a 1-liter glass cylinder, and distilled water is added to bring the level to the 1-liter mark. The glass cylinder is then repeatedly shaken and inverted before being placed in a constant-temperature bath. A hydrometer is placed in the glass cylinder and a clock is simultaneously started. At different times, the hydrometer is read. The diameter D (cm) of the particle at time tD (seconds) is calculated from Stokes’s law as
D5 Å
18mz
1Gs212rwgtD (2.3)
where m is the viscosity of water [0.01 gram/(cm.s) at 208C], z is the depth (cm), rw is the density of water (1 gram/cm3), g is the acceleration due to gravity (981 cm/s2), and Gs is the specifi c gravity of the soil particles. For most soils, Gs<2.7.
In the application of Stokes’s law, the particles are assumed to be free-falling spheres with no col- lision. But the mineral particles of clays are platelike, and collision of particles during sedimentation is unavoidable. Also, Stokes’s law is valid only for laminar fl ow with Reynolds number (Re5 vDgw
mg , where v is velocity, D is the diameter of the particle, gw is the unit weight of water, m is the dynamic viscosity of water at 208C, and g is the acceleration due to gravity) smaller than 1. Laminar fl ow prevails for particle sizes in the range 0.001 mm , Ds, 0.1 mm. By using the material passing the No. 200 sieve (aver- age particle size ,0.075 mm), laminar fl ow is automatically satisfi ed for particles less than 0.001 mm.
Particles smaller than 0.001 mm are colloids. Electrostatic forces infl uence the motion of colloids, and Stokes’s law is not valid. Brownian motion describes the random movement of colloids.
The results of the hydrometer test suffi ce for most geotechnical engineering needs. For more accurate size distribution measurements in fi ne-grained soils, other, more sophisticated methods are available (e.g., light-scattering methods). The dashed line in Figure 2.12 shows a typical particle size distribution for fi ne-grained soils.
2.5.3 Characterization of Soils Based on Particle Size
The grading curve is used for textural classifi cation of soils. Various classifi cation systems have evolved over the years to describe soils based on their particle size distribution. Each system was developed for
Hydrometer
Soil suspension
Increasing density
FIGURE 2.13
Hydrometer in soil–water suspension.
2.5 DETERMINATION OF PARTICLE SIZE OF SOILS—ASTM D 422 17
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18 CHAPTER 2 GEOLOGICAL CHARACTERISTICS AND PARTICLE SIZES OF SOILS
a specifi c engineering purpose. In Figure 2.14, four systems are compared. These are the Unifi ed Soil Classifi cation System (USCS), the American Society for Testing and Materials (ASTM) (a modifi cation of the USCS system), the American Association of State Highway and Transportation Officials (AASHTO), and the British Standards (BS). We will discuss soil classifi cation in more detail in Chapter 4.
In this book we will use the ASTM system. Soils will be separated into two categories. One cat- egory is coarse-grained soils that are delineated if more than 50% of the soil is greater than 0.075 mm (No. 200 sieve). The other category is fi ne-grained soils that are delineated if more than 50% of the soil is fi ner than 0.075 mm. Coarse-grained soils are subdivided into gravels and sands, while fi ne-grained soils are divided into silts and clays. Each soil type—gravel, sand, silt, and clay—is identifi ed by grain size, as shown in Table 2.1. Clays have particle sizes less than 0.002 mm. Real soils consist of a mixture of particle sizes.
The selection of a soil for a particular use may depend on the assortment of particles it contains. Two coeffi cients have been defi ned to provide guidance on distinguishing soils based on the distribution of the particles. One of these is a numerical measure of uniformity, called the uniformity coeffi cient, Cu, defi ned as
Cu5 D60
D10
(2.4) where D60 is the diameter of the soil particles for which 60% of the particles are fi ner, and D10 is the diameter of the soil particles for which 10% of the particles are fi ner. Both of these diameters are obtained from the grading curve.
TABLE 2.1 Soil Types, Descriptions, and Average Grain Sizes According to ASTM D 2487
Soil type Description Average grain size Gravel Rounded and/or angular bulky hard rock, Coarse: 75 mm to 19 mm
coarsely divided Fine: 19 mm to 4.75 mm
Sand Rounded and/or angular hard rock, Coarse: 4.75 mm to 2.0 mm (No. 10) fi nely divided Medium: 2.0 mm to 0.425 mm (No. 40)
Fine: 0.425 mm to 0.075 mm (No. 200) Silt Particle size between clay and sand. Exhibit 0.075 mm to 0.002 mm
little or no strength when dried.
Clay Particles are smooth and mostly clay ,0.002 mm minerals. Exhibit signifi cant strength
when dried; water reduces strength.
FIGURE 2.14 Comparison of four systems for describing soils based on particle size.
Sand BS
USCS
AASHTO
ASTM
Gravel
Gravel Silt
Sand Sand
Gravel
Gravel Fine Medium Coarse Fine Medium
Medium Coarse
Fine
Fine
Fine
Fine
Medium Coarse
Coarse Fines (silt, clay)
Coarse
Coarse
Fine Medium Coarse
Clay Cobbles
Cobbles Boulders
Boulders
Cobbles Boulders Boulders
Clay Silt
Clay Silt
0.001 0.01 0.1 1 10 100 1000
0.005 0.075 2
0.425 4.75 30075
Sand
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The other coeffi cient is the coeffi cient of curvature, Cc (other terms used are the coeffi cient of gra- dation and the coeffi cient of concavity), defi ned as
Cc5 1D3022
D10D60
(2.5) where D30 is the diameter of the soil particles for which 30% of the particles are fi ner. The average par- ticle diameter is D50.
A soil that has a uniformity coeffi cient of ,4 contains particles of uniform size (approximately one size). The minimum value of Cu is 1 and corresponds to an assemblage of particles of the same size. The gradation curve for a poorly graded soil is almost vertical (Figure 2.12). Humps in the gradation curve indi- cate two or more poorly graded soils. Higher values of uniformity coeffi cient (.4) indicate a wider assort- ment of particle sizes. A soil that has a uniformity coeffi cient of .4 is described as a well-graded soil and is indicated by a fl at curve (Figure 2.12). The coeffi cient of curvature is between 1 and 3 for well-graded soils.
The absence of certain grain sizes, termed gap-graded, is diagnosed by a coeffi cient of curvature outside the range 1 to 3 and a sudden change of slope in the particle size distribution curve, as shown in Figure. 2.12.
Poorly graded soils are sorted by water (e.g., beach sands) or by wind. Gap-graded soils are also sorted by water, but certain sizes were not transported. Well-graded soils are produced by bulk transport processes (e.g., glacial till). The uniformity coeffi cient and the coeffi cient of concavity are strictly appli- cable to coarse-grained soils.
The diameter D10 is called the effective size of the soil and was described by Allen Hazen (1892) in connection with his work on soil fi lters. The effective size is the diameter of an artifi cial sphere that will produce approximately the same effect as an irregularly shaped particle. The effective size is particularly important in regulating the fl ow of water through soils, and can dictate the mechanical behavior of soils since the coarser fractions may not be in effective contact with each other; that is, they fl oat in a matrix of fi ner particles. The higher the D10 value, the coarser the soil and the better the drainage characteristics.
Particle size analyses have many uses in engineering. They are used to select aggregates for concrete, soils for the construction of dams and highways, soils as fi lters, and material for grouting and chemical injection. In Chapter 4, you will learn about how the particle size distribution is used with other physical properties of soils in a classifi cation system designed to help you select soils for particular applications.
T H E E S S E N T I A L P O I N T S A R E :