Phase Relationships of Soil

Transcription

1 AMRC 2011 MODULE 4 Phase Relationships of Soil CONTENTS Overview Objectives Procedures Mind Experiment Introduction Phase Relationships of Soil Basic Terms Relating to Soil Composition and Condition Porosity Void Ratio Degree of Saturation Water Content Soil Unit Weights and Densities Moist (or Bulk) Unit Weight Dry Unit Weight Saturated Unit Weight Moist or Wet Density Dry Density Saturated Density Typical Values of Basic Terms for Various Soil Types Soil Classification Determination of Grain Size and Distribution Sieve Test Wash Test Hydrometer Test WPC # /09

2 4.9 Soil Water Content and Plasticity Water Content Soil Plasticity Classification Systems Engineering Use of Soils Example Problems on Engineering Use of Soils Self-Test ii WPC # /09

3 Module 4 Phase Relationships of Soil Overview This module introduces the mass-volume and weight-volume phase relationships of a soil. These relationships define the differences in structure between one soil and another and are important in determining the uses and suitability of a soil for construction purposes. This module introduces the Unified Soil Classification System. This system is the most widely used system of soil classification in the engineering field. The naming of a soil gives an indication of the soil properties. This information, with reference to a chart, gives the indication of appropriate engineering use of a soil. Objectives Upon successful completion of this module, you will be able to: define and describe the mass-volume and weight-volume phase relationships of a soil define the basic terms relating to soil composition and condition describe the sieve test, produce a grading curve for a given soil sample, and discuss the types of possible material gradations describe the method used to determine the moisture content define what a soil classification system is and state its purpose identify soils using the Unified Soil Classification System relate the classification grouping of a soil to its properties and potential engineering uses for roads. Procedures Study the module materials and make notes as required. Perform the self-test on these principles and review the course materials in such a manner as to be able to successfully complete similar questions upon examination. WPC # /09 AMRC

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5 SECTION 4.1 Mind Experiment Mind experiments are often described in Physics and other sciences in order to illustrate a concept or to make a point. These experiments are either impossible to carry out in practice or are possible but would be unnecessary, if not very expensive. The following mind experiment is designed to illustrate the concepts of phase relationships of soils. It is never actually carried out but could be carried out, in practice. However, a description of the possible experiment helps us to understand the concepts that are important in defining the phases of a soil. Imagine that a sample of soil with a volume of one cubic metre is placed in a very strong steel box and that the lid is securely bolted and sealed. This box is then heated at a temperature that would melt the solid contents but not the steel box, supposing that were possible. When the box has been heated for a sufficient length of time to ensure that everything is melted, it is allowed to cool down and opened. The box would now hold three different types of material. At the bottom we would expect to see a solid glassy rectangular object like cooled lava. This would be the result of melting all the grains of gravel, sand, silt, and clay that might have been in the original sample. It would be the same volume as the original solid particles that were in the soil sample. Above this solid there could very well be and most probably would be a liquid that has the same volume as the original amount of liquid in the original soil sample. If the original soil sample was not totally saturated with water and other possible liquids, then in addition to the solid and the liquids in the cooled box we would expect to see a volume of gas. This gas would very probably be air. The three substances that appear in the cooled sample following this mind experiment are what we refer to as the three phases of the soil solid, liquid, and gas. We usually call them simply the solids, the water, and the air. Figure 4.1 illustrates these ideas. WPC # /09 AMRC

6 Figure 4.1 Three Phases of Soil In actual lab tests on soils, we label the volume of solids as V s and the volume of both the air and the water together as V v which stands for the volume of the voids. The volume of the water is labelled V w. We seldom refer to the volume of the air alone for this has no actual use of application. Naturally, the total volume which is generally labelled just V or sometimes V t is simply the sum of V v and V s. V = V v + V s The measurement of these quantities is done indirectly as well as directly. We can easily measure the total volume, for this is the volume of the container which is totally filled with the soil sample = V. If the sample of soil which would fill the container is weighed, we can obtain the mass of the solid and water together = M. If the sample is entirely dried out and weighed, we can obtain the mass of the solids = M s. Subtracting the mass of the solids from the original total mass would give the mass of water M w = M M s. (We never obtain the mass of the air even though the physicists assure us that it has a mass. The reason for this is illustrated by asking if you would be concerned about the weight or mass of the air surrounding corn flakes in a box when you check its weight against what it says on the label.) If the sample is now totally saturated (all the little holes called voids between the grains of solids are filled with water) and then weighed again the new mass could be labelled M sat for saturated 4-4 AMRC 2011 WPC # /09

7 mass. The difference between this mass and the dry mass or mass of solids alone would give the mass of the water M ws where the subscript ws refers to the mass of water to saturate the sample. That is, M ws = M sat M s. If this mass M ws is in grams, then we simply assume that the volume of the voids is numerically equal to the mass since we assume the density of water is one gram per cubic centimetre. Subtracting the volume of the voids from the total volume will give the volume of the solids V s = V V v. It can be seen that the above-mentioned data can be easily obtained. It is, in fact, all we need to obtain the facts concerning the soil s phase relationships. The actual formulae and calculations for the lab tests that we do are illustrated in the following pages. WPC # /09 AMRC

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9 SECTION 4.2 Introduction Mass (M) expresses the quantity of matter in a body and is measured in kilograms (kg). The mass of a body is independent of the gravitational force acting upon a mass at a particular location. Weight (W) is a measure of the gravitational force acting upon a mass at a particular location. Newton s Law states: W = Mg where g is the acceleration of gravity and is usually taken as metres/second/second. (Be aware that the letter g is also used to represent gram, a unit of mass.) Hence weight is measured in newtons ; a newton (N) being the force required to give a mass of one kg an acceleration of 1 metre/second/second. Normally in your work you will encounter SI units (the International System of Units is abbreviated SI in all languages) but you may for a while still occasionally have to use Imperial units. To avoid confusion, this module explains the mass-volume and weight-volume relationships in both units, with the emphasis given to SI units. Imperial units used in soil mechanics are normally pounds (lb) and cubic feet. In Imperial units the pound is a unit of weight or force. Note that we cannot measure mass. Laboratory scales often use units of grams. However, the scale must measure weight (not mass) due to the gravitational attraction of the earth. We will talk about density (g/cm 3 ) and unit weight (kn/m 3 or lb/ft 3 (pcf) ), but practically they mean the same thing since we can only measure weight. The main thing is to work in a consistent set of units. At the end of a problem, it is easy to convert since 1 g/cm 3 is equivant to 9.81 kn/m 3 = 62.4 pcf. WPC # /09 AMRC

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11 SECTION 4.3 Phase Relationships of Soil The influence of the soil structure on the soil properties is profound and there is obviously a need to know these properties when conducting design or construction. It is important, therefore, to be able to clearly define the difference in structure from one soil to another. For instance, a soil with the same size grains could have many different structures depending on how closely or loosely the grains were packed. Figure 4.2 illustrates the difference in void space (i.e., irregular spaces between soil particles) in the same soil as the particle orientation changes. Figure 4.2 Soil Settlement Caused by Applied Load The interrelationships between the weight and mass of solid soil particles and the water in the void space to a given volume of soil are usually referred to as soil phase relationships. In order to define these relationships it is necessary to represent the constituent parts of the soil as shown in Figure 4.3. WPC # /09 AMRC

12 Figure 4.3 Mass-Volume and Weight-Volume Relationships V = Total volume M = Total mass V v = Volume of voids M w = Mass of water = V w + V a V s = Volume of solids M s = Mass of solids V w = Volume of water V a = Volume of air 4-10 AMRC 2011 WPC # /09

13 In Imperial units W = Total weight W w = Weight of water W s = Weight of solid The total volume of the soil includes the volume occupied by solids, liquids, and air. V = V v + V s = V w + V a + V s The total mass M of a soil volume is taken as the sum of the mass of the solids M s and the mass of water M w. In practical problems, the mass of air is ignored. M = M w + M s Similarly, for measurement of weight (in Imperial units): W = W w + W s In terms of mass, the Specific Gravity, G s, of the soil solids is the ratio of the mass of a given volume of soil solids compared to the mass of an equal volume of water. G s = = G s = Specific Gravity of soil solids Mass of Solids Density of Water Volume of Solids Ms V w s or M s = G s ρ w. V s where the symbol ρ w is the density of water = M w / V w A typical value of specific gravity of solid soil particles is This means that the soil particle is 2.65 times heavier than a drop of water of the same volume as the particle. The density of water, ρ w, has the following values: 1.0 g/cm 3 (grams per cubic centimetre), or kg/m 3 (kilograms per cubic metre) WPC # /09 AMRC

14 In terms of weight, the Specific Gravity, G s, of soil solids is the ratio of the weight of a given volume of soil solids compared to the weight of an equal volume of water. G s = G s = Weight of Solids Unit Weight of Water Volume of Solids Ws V w s or W s = G s γ w V s where the symbol γ w is the unit weight of water = W w / V w. The unit weight of water, γ w, is equal to 62.4 lb/ft 3 or 9.81 kn/m 3 Similarly, W w = G w γ w V w = γ w V w where G w is the Specific Gravity of water (G w = 1.0 for water) AMRC 2011 WPC # /09

15 SECTION 4.4 Basic Terms Relating to Soil Composition and Condition The relationships between void space and the volume occupied by the soil particles are fundamental and are characterized by the following definitions: Porosity n = volume of voids / total volume n V v V, usually expressed as a percentage Void Ratio e = volume of voids / volume of solids e V V v s, usually expressed as a decimal Hence, e n 1 n then, n e 1 e Degree of Saturation S = volume of water / volume of voids S V V w v, usually expressed as a percentage Water Content In terms of mass, w = mass of water / mass of solids w M M w s, usually expressed as a percentage WPC # /09 AMRC

16 In terms of weight, w = weight of water / weight of solids w W W w s, usually expressed as a percent Note, sometimes water content is called moisture content and the symbol m is used. These terms have been included because you may come across them in geotechnical or soils investigation reports. You are not expected to use the formula (i.e., calculate), just be aware of the existence of the terms AMRC 2011 WPC # /09

17 SECTION 4.5 Soil Unit Weights and Densities Unit weight and density are not identical, but they are related by the expression γ = ρ g, where g is the acceleration due to gravity, but practically they are used interchangeably. Comparative values of unit weight or density are normally used to relate to other properties this practise does not normally complicate matters. To overcome any confusion between γ and ρ, unit weights based on a gram measurement can be expressed as gram-force (gf) per unit volume. The unit weight of a soil is the force or weight per unit volume and is conventionally expressed as pounds per cubic foot, grams per cubic centimetre, or kilonewtons per cubic metre. Soil density may be defined as mass per unit volume and is given the symbol ρ. There are three modes of reporting soil unit weights or densities: moist (also called bulk unit weight or wet density) dry saturated Moist (or Bulk) Unit Weight W w(gs S e) V 1 e Moist (or bulk) unit weight includes the weight of water in addition to the weight of solids since, W = W w + W s Dry Unit Weight Ws w Gs d V 1 e since S (the degree of saturation) in the formula of Section is 0 for a dry soil. Dry unit weight includes only the weight of solid soil particles since W w = 0. WPC # /09 AMRC

18 4.5.3 Saturated Unit Weight W sat w (Gs e) sat V 1 e since S (the degree of saturation) is 1 for a saturated soil Moist or Wet Density M w (Gs S e) V 1 e Moist or wet density includes the mass of water in addition to the mass of solids since M = M w + M s. This term is also sometimes referred to as in situ density, i.e., the density of the soil as it is in its natural or original situation Dry Density Ms Gs w d V 1 e since S (the degree of saturation) in the formula of Section is 0 for a dry soil. Dry density includes only the mass of solid soil particles since M w = AMRC 2011 WPC # /09

19 4.5.6 Saturated Density M sat w (Gs e) sat V 1 e since S (the degree of saturation) is 1 for a saturated soil. Unit weight and density are therefore not identical, but they are related by the expression γ = ρ g, where g is the acceleration due to gravity. If all work in the laboratory is carried out using centimetres-gramsseconds system (cgs), then right at the end of the calculation; and γ = ρ 9.81 to convert ρ in g/cm 3 to γ in kn/m 3 γ = ρ 62.4 to convert ρ in g/cm 3 to γ in lb/ft 3 Given some of the factors, you can use the above formulae to calculate the other unknowns. Some manipulation of the formulae may be necessary. A summary of weight-volume and mass-volume equations is given in Figure 4.4. V v = ev s V = V s (1 +e) V w = SeV s =SV v W w = V w w = ww s W = W s + W w M = M s + M w V M W T s s Vs 1 e G s w G s w W s = V s G s w = W W w M s = V s G s w V M V v w w e w% 100% s% 100 Vs Ms Vv W Or w w% 100% Ws Figure 4.4 Summary of Weight-Volume and Mass-Volume Equations WPC # /09 AMRC

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21 SECTION 4.6 Typical Values of Basic Terms for Various Soil Types When an accurate value of density, void ratio, or any of the other phase relationships is required, then tests and measurements have to be conducted on the soil. However, a general idea of the range of values can be obtained from Figure 4.5. Note: The values are not intended to be exact, but are typical for the types of soil described. Source: Terzaghi, K. & Peck, R. (1967). Soil Mechanics in Engineering Practice. John Wiley & Sons. Figure 4.5 Typical Values of Basic Terms WPC # /09 AMRC

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23 SECTION 4.7 Soil Classification Classification of soils on the basis of field observation in the early stages of an investigation greatly enhances understanding of soil types and potential uses or problems. However, once it has been decided to proceed with design, more detailed soil properties and classification are required: The grain size and range must be accurately determined. The consistency of the soil and change in consistency with changing moisture content must also be determined in a precise manner. Tests have been developed for this purpose. When a designated test procedure is conducted carefully, then the results of the test will be virtually identical for a particular soil irrespective of the personnel conducting the test. It is for this reason that the tests should be conducted exactly in the manner stated in the testing manuals. No deviation from the stated procedure should be contemplated unless it is clearly understood what effect this change will have on the final test results. In order to determine grain size and distribution, a wash test, a sieve (mechanical analysis) test and, occasionally, a hydrometer test are required if the soil contains a full range of sizes. The test procedures which indicate the degree of consistency of cohesive soils and how it varies with moisture content are called Atterberg Limit tests. Only some of these tests are described in this module. If the reader is interested in knowing more, they should consult an introductory soils/geotechnical text. WPC # /09 AMRC

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25 SECTION 4.8 Determination of Grain Size and Distribution The range in the size of grains is almost limitless. The largest grains are by definition the largest that can be moved by hand, while the fines grains are so small that they cannot be seen by an ordinary microscope Sieve Test To determine the grain size and distribution, the soil is passed through batteries of sieves (see Figure 4.6 and Table 4.1). Typically, meshes with openings 4.75 mm and larger are fixed in large frames while meshes 4.75 mm and smaller are fixed in small sieve frames like those shown in Figure 4.6. No. 4 or 4.75 mm mesh is found in both frame sizes and is, in fact, the dividing line between gravel and sand. Figure 4.6 A Bank of Typical Sieves in a Rotap Mechanical Sieve Shaker In a sieve test, the selected sieves are stacked in order with the largest openings at the top and a pan at the bottom. The sample is placed in the top, the lid is added, and the mechanical shaker operates for a specified time. The weight of material retained on each sieve is determined and recorded on a form like the one shown in Figure 4.7. The recording form is set up this way because sieve analysis is done first with a large (approximately 10 kg) sample through the large sieves and then a representative (approximately 500 g) of the material in the pan below the No. 4 sieve is passed through the series of smaller sieves. WPC # /09 AMRC

26 Table 4.1 Typical sieve sizes 1 used in soils and aggregate analysis for roadwork Metric 75 mm 63 mm 37.5 mm 25.0 mm 19.0 mm 12.5 mm 9.5 mm 4.75 mm 2.36 mm 1.18 mm 600 :m 300 :m 150 :m 75 :m PAN Imperial 3 2½ 1½ 1 ¾ ½ 3/8 No. 4 No. 8 No. 16 No. 30 No. 50 No. 100 No. 200 PAN 1 Standard sieves are sized according to the space between each pair of wires. However, in the Imperial series the larger sizes (3" to 3/8") indicate the spacing between the wires, and the small sizes (No. 4 and less) indicate the number of wires per inch AMRC 2011 WPC # /09

27 Figure 4.7 Typical Sieve Analysis Form WPC # /09 AMRC

28 Typical mechanical analysis test results and associated calculations are shown in Table 4.2. These calculations are usually done right on the sieve analysis form. Columns 1 and 2 of Table 4.2 contain the data obtained from the test; columns 3, 4, and 5 are the calculations which must be made to express the test results in the conventional form: the percentage of material passing through each size of sieve. The information in columns 1 and 5 has been plotted on a gradation chart (Figure 4.8) and the points connected with a smooth curve. This curve is the particle size distribution curve (or gradation curve) of the soil tested. The sieve size opening is plotted on a horizontal logarithmic scale which may be laid out with increasing particle size left to right, as shown in Figure 4.8 and commonly used for analysis of aggregates, or with decreasing particle size left to right, as shown in Figure 4.9 and commonly used for soils analysis. The figures at the top of the chart in Figure 4.8 indicate the sieve names, 3 inch to one quarter inch and then the smaller sieve openings are listed as No. 4, No. 8 to No The scale at the bottom of the chart indicates the grain size in millimetres (mm) corresponding to each sieve. Also at the bottom of the chart is a description of the soil type in relation to grain size. For instance, a fine sand can be seen to be a soil with particles which range from mm to 0.4 mm. The vertical arithmetic scale, percent finer by mass, indicates the total percentage by mass of the soil which would be smaller than any particular sieve. A sieve with an opening smaller than that of the No. 200 would be impractical for sieve work; particle size analysis of such fine material is done by hydrometer as discussed in Subsection Table 4.2 Mechanical Analysis: Results and Calculations 1 Sieve No. 2 Wt. Retained (g) 3 % Retained on each sieve 4 Cumulative % retained 5 Cumulative % passing / = = / = = = = = = = = 0 PAN = 0 Total Weight = 500 grams Total = 100.0% 4-26 AMRC 2011 WPC # /09

29 The percentage of each division of particle size in the example shown can be quoted thus: Coarse Sand Fine Sand Medium Sand 18 percent 39 percent 43 percent The gradation curves of three different soils are shown in Figure When the curve indicates a well-distributed range of particle sizes (curve a), the soil is referred to as well graded. Curve b, which is almost vertical, indicates that there is a narrow range of particles in the soil; it is called poorly graded uniformly graded. A soil which has a gap in the range of particle sizes, as in curve c, is called a poorly graded gap graded soil. The uniformity coefficient of a soil is the ratio of the effective size of the particles, shown by the 60% finer point to the 10% finer point on the gradation curve, i.e., C u = D 60 /D 10 The soil gradation curve in Figure 4.11 has a D 60 of 0.6 mm and a D 10 of 0.12 mm and the uniformity coefficient is: C u = 0.6/0.12 = 5.0 A high value of uniformity coefficient indicates a well-graded soil, and a low value, a uniformly-graded soil. A value of 4 or under signifies a uniform soil. The coefficient of curvature of the soil is defined as: C c = D 30 2 /(D 10 D 60 ) A value outside the range of 1 to 3 indicates the soil to be poorly graded. The coefficients C c and C u are used together in the Unified Soil Classification System described later. WPC # /09 AMRC

30 Figure 4.8 Grading Curve 4-28 AMRC 2011 WPC # /09

31 Source: Lambe, T.W. & Whitman, R.V. (1979). Soils Mechanics, SI Version. Whitman, Wiley. Figure 4.9 Soil Particle Size Gradation WPC # /09 AMRC

32 Figure 4.10 Typical Grading Curves 4-30 AMRC 2011 WPC # /09

33 Figure 4.11 Grading Curve WPC # /09 AMRC

34 4.8.2 Wash Test Sieve test results can be made inaccurate by the tendency of dust and very fine particles to adhere to the larger particles and by the tendency for the No. 200 sieve to become blocked by dry sieving. This can be eliminated by oven drying the soil, breaking it as fine as possible, washing it through a No. 200 sieve (see Figure 4.12), and then by sieving the reduced residue through a stack of sieves. The washing ensures that little dust will adhere to the layer particles and that all the reducible lumps are water softened so that they can be reduced to their elemental soil particles. Figure 4.12 Wash Test Hydrometer Test A hydrometer test is used when it is necessary to determine the grain size of particles which are finer than a No. 200 sieve: silts and clays. The test will not be described in detail but it is worth noting when and why it is undertaken AMRC 2011 WPC # /09

35 SECTION 4.9 Soil Water Content and Plasticity Water Content Water content is determined by weighing a small empty tin, then by placing the moist soil in the tin and weighing the moist soil plus the tin. The tin and soil are then placed in an oven at 100 Celsius (±5 C) until all the water has left the soil and the weight is constant. The tin plus the dry soil is then weighed. It is usually sufficient to leave the tin in the oven overnight, hours. Unless the sample tins with the moist soil are weighed immediately, a lid should be placed on the tins to prevent moisture loss. The lid should be opened before placing the tin in the oven. From the data obtained, the moisture content can be determined: Mass of water in soil, M w = (mass of tin + moist soil) (mass of tin + dry soil) Mass of dry soil, M s = (mass of tin + dry soil) (mass of tin) Water content, w = (M w 100)/ M s (expressed as a percentage) Note: You should review the comments re weight and mass in Section 4.5. Table 4.3 shows a typical data sheet used in the calculation of water content. WPC # /09 AMRC

36 Table 4.3 Water Content Calculation Sheet Project Station Ground Elev. Boring No. Tested By Offset Method Date Hole No. Depth Sample No. Container No. Mass of Wet Sample + Tare Mass of Dry Sample + Tare Tare of Container Mass of Water Mass of Dry Soil Moisture Content Hole No. Depth Sample No. Container No. Mass of Wet Sample + Tare Mass of Dry Sample + Tare Tare of Container Mass of Water Mass of Dry Soil Moisture Content Hole No. Depth Sample No. Container No. Mass of Wet Sample + Tare Mass of Dry Sample + Tare Tare of Container Mass of Water Mass of Dry Soil Moisture Content Remarks 4-34 AMRC 2011 WPC # /09

37 4.9.2 Soil Plasticity Increasing water content changes the plasticity (indicative of the consistency) of a fine-grained soil such as silts and clays (see Table 4.4). Table 4.4 Plasticity of a Fine-Grained Soil Increasing Moisture Content (w %) Solid Semi-Solid Plastic Liquid Hard Firm Soft Very soft to slurry Shrinkage Limit Plastic Limit Liquid Limit SL PL LL A clay soil is said to be plastic if it behaves like chewing gum or modelling clay. That is, if an object is pushed into a lump of plastic soil, then the shape made by that object will remain in the clay. A clay soil is said to be firm if a shape can be made in it but with a lot of effort. A clay soil is said to be hard if an object such as a pencil cannot be pushed into it. A clay soil is said to be liquid if when a shape is made in it the shape quickly disappears. That is, the soil quickly closes up after an object like a pencil has been pulled out of it. It is like dairy cream. The above terms hard, firm, plastic, and liquid are not good terms in themselves for defining the state of a soil. We need a more precise way of defining these terms by the use of definite tests that will give us numbers that will define these conditions. These tests were devised by a soil scientist named Atterberg. Atterberg Limits are fully described in introductory soil/geotechnical texts. Mind Experiment The following experiment cannot be carried out for practical reasons, but it can be done in the imagination. Suppose a very clever laboratory technician could mix water with a clay sample and place it in the trough shown in Figure 4.13 so that its moisture content (w%) increases steadily from zero at the left end of the trough to, say, 200% at the right end of the trough. If a pencil is now pushed into the clay starting at the left side and moving after each trial push to the right side, the operator will WPC # /09 AMRC

38 be able to identify four zones. Moving from left to right, these zones could be named: hard no mark can be made in the clay. firm a mark can be made but with difficulty. plastic a mark can be easily made and stays in the clay. liquid a hole made with a pencil quickly closes up. Figure 4.13 Defining Atterberg Limits Atterberg, using his experience of the way soils behave, defined the boundaries between these zones as follows: Shrinkage limit is a moisture content percent. Soils with moisture contents below this limit will not change in volume with any change in moisture content. Soils with moisture contents above the shrinkage limit will change in volume with changes in moisture content. Plastic limit is a moisture content percent. It defines a boundary at which the clay soil will change from a plastic condition to a firm condition as the moisture content is decreased. It is defined as that moisture content percent at which a 3 mm thread or cylinder rolled by hand will crack when bent around a finger. If the thread does not break or crack, it is plastic. If the thread breaks or cracks, it is firm. This boundary is identified by taking a small amount of the clay which is plastic that is, it behaves like chewing gum or modelling clay and rolling it into a 3 mm thread then hanging it over a finger. If it does not break or crack, it is still plastic. By rerolling (and so drying it) and bending it over a finger its moisture content can be gradually reduced to the point where it just begins to 4-36 AMRC 2011 WPC # /09

39 crack. If this moisture content is now calculated, the plastic limit will have been found. WPC # /09 AMRC

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41 SECTION 4.10 Classification Systems A soil classification system is an arrangement of different soils into groups which have similar properties. Its purpose is to make it possible to estimate soil properties or capabilities by association with soils of the same class whose properties are known, and to provide an accurate method of soil description. However, there are so many different soil properties of interest, and so many combinations of these properties in any soil deposit, that any universal system of classification seems impractical. Instead, the groups or classes are based on those properties which are most important in that particular phase of engineering for which the classification was developed. The two most commonly used systems in civil engineering are the Unified Soil Classification System and that of the American Association of State Highway Officials (AASHO). The BC Ministry of Forests (MoF) and Ministry of Transportation (MoT) use the Unified Soil Classification System. The Unified System is summarized in Figure 4.14 while the MoT soil types summary chart is shown in Figure The results of the classification tests are used to allocate the soil to a particular position in the table, and to assign a group symbol to it. For instance, the classification tests on a particular soil might show that it had 96% by weight above the No. 200 sieve (4% below) and more than half of the coarse fraction (the plus No. 200) greater than a No. 4 sieve, a C u of 6 and a C c of 2. This soil would then be allocated a group symbol, GW, according to Figure The plasticity chart in the previous module is used in conjunction with the classification table in order to assign group symbols to the finer grained soils. The symbols used with the unified system have the following meanings: G Gravel S Sand M Silt or silty (from the Swedish word for silt, mho) C Clay or clayey W Well graded P Poorly graded (uniformly graded, or gap graded) L Low plasticity H High plasticity O Organic silt or clay WPC # /09 AMRC

42 Therefore, a group symbol CH means a high plasticity clay. GW means a well-graded gravel. When you review Figures 4.14 and 4.15, you will come across the Atterburg Limits. You are not expected to describe or use the Atterberg Limits in this course AMRC 2011 WPC # /09

43 C u D D C c 2 30 D D D Figure 4.14 Unified Soil Classification System WPC # /09 AMRC

44 Figure 4.15 is a chart used by the British Columbia Ministry of Transportation which summarizes the various types of soils and their group symbols. Figure 4.15 Soil Types Summary and Their Group Symbols 4-42 AMRC 2011 WPC # /09

45 SECTION 4.11 Engineering Use of Soils Once a soil has been classified, its suitability as a construction material can be estimated on the basis of prior performance of soils of the same classification. For example, when soils of the OL group classification on the Unified System have been used to build embankments, experience has shown that these embankments are unstable. The practical lesson to be learned from this is that OL soils are usually unsuitable materials with which to build embankments. Practical experience of this nature has been combined to make up tables of the types shown in Tables 4.5 and 4.6. For many engineering projects, the soil is classified and then the suitability of the soil for the particular project is based almost entirely upon knowledge of previous behaviour of similar soils. These tables are based upon experience of this behaviour. Some of the properties mentioned in these tables have not yet been defined. They are covered in subsequent modules. It should be noted that with adequate investigation, design, and supervision, functional fills can be constructed with material that is less useful for other purposes. For instance, if there were a choice between a well-graded gravel and a sandy clay mixture for a fill, then considering only the short-term factors of availability of material, it would be better to use the gravel. However, in the longer term and in the interests of attempting to conserve materials, if it were possible with good engineering and construction to obtain adequate performance of the fill made from the less suitable sandy clay, then this should be done. This would preserve the gravel for some future use such as asphaltic concrete aggregate. WPC # /09 AMRC

46 Table 4.5 Engineering Use Chart Unified System 4-44 AMRC 2011 WPC # /09

47 Table 4.6 Engineering Characteristics Unified System WPC # /09 AMRC

48 The CH soil* is not suitable as a highway fill, and its use in this capacity should be avoided, unless there is no other economic alternative. If it is used for highway fill, then detailed testing and design would be necessary. The British Columbia Ministry of Transportation has built fills up to 7.6 m high out of ML and CL soils (poor suitability according to Table 4.6). This has been done, where the water content was not too high, by alternating it with layers of clean granular material (GW, GP, SW gravelly, or SP gravelly) to help stabilize the fill. This aided drainage and helped it to settle to a stable mass (see Figure 4.16). Figure 4.16 Fill of Poor Soil Facilitated by Alternating Layers with Stable High Drainage Soil * Note: In particular, the field water content and compaction characteristics should be determined for such soils. If the CH soil is damp (not too wet) and can still be compacted, it can be used for low fills (say less than 4.6 metres); however, the base course constructed on such a fill would have to be built to a high standard AMRC 2011 WPC # /09

49 SECTION 4.12 Example Problems on Engineering Use of Soils Example 4.1 A roadway is to be built in an area where there is a cheap source of fill material available. This soil has been classified as a GW on the Unified System. Is this soil suitable? Answer From Table 4.5, as a roadway fill, a GW is rated excellent. It is pervious, has excellent shearing strength, low compressibility, and has excellent workability as a construction material. From Table 4.6, a GW soil has excellent suitability as a subgrade when not subject to frost. It has good drainage, almost no compressibility and expansion, and good compaction characteristics with a rubber-tired steel wheel or vibratory roller. The GW soil is suitable for use as a road fill. * Note: This does not guarantee a successful job; it just means that with this material, it is possible to achieve good results. Should the fill be placed without provision being made for water to drain away from the fill, then strength reduction may occur despite the suitability of the soil. Good design and construction methods are as necessary as suitable materials. Example 4.2 A highway is to be constructed in a region where there is a large quantity of CH (unified group symbol) soil available for the fill sections. Is this soil suitable? Answer From Table 4.5, a CH soil is rated as having very low suitability as a roadway fill (13 or 8). It is impervious, has poor shear strength, high compressibility, and poor workability. WPC # /09 AMRC

50 Table 4.6 rates it as poor to very poor as a subgrade. It has very high compressibility, no drainage ability, and fair to poor compaction characteristics with a sheepsfoot roller. The CH soil is not suitable for the fill sections AMRC 2011 WPC # /09

51 SECTION 4.13 Self-Test 1. What are the constituent parts of a soil mass? 2. Define the following terms: A. water content B. degree of saturation C. moist or bulk density of soil. 3. Fill in typical values for the following: A. Soft, glacial clay has a saturated unit weight of grams/cm 3 and a natural water content of. B. Loose, uniform sand has a dry unit weight of grams/cm 3 and a porosity of. C. Dense, mixed-grained sand has a bulk unit weight of grams/cm 3 and a void ratio of. 4. What is the purpose of a sieve test? 5. Define the uniformity coefficient (C u ) and the coefficient of curvature (C c ). 6. Given in the table below are the test data results from sieve analyses on two soils. Do the appropriate calculations and plot the gradation curve for each soil on the blank chart, (page 4 49) Calculate the coefficient of uniformity and the coefficient of curvature for each soil. WPC # /09 AMRC

52 Sieve Soil 1 Soil 2 Weight retained (grams) Sieve Weight retained (grams) 3/ / / Pan Pan 14.0 (should properly show initial weight) 7. What is the purpose of a soils classification system? 8. What do the following symbols mean when used in conjunction with the United Classification System? A. G B. C C. O D. ML E. SW 4-50 AMRC 2011 WPC # /09

53 WPC # /09 AMRC

54 4-52 AMRC 2011 WPC # /09