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  • 1. 12.1 CLASSIFICATION AND SOIL BEHAVIOR 12.1.1 Classification and Engineering Properties Soil properties that are of most concern in engineering are strength and volume change under existing and future anticipated loading conditions. Various tests have been devised to determine these behaviors, but the tests can be costly and time-consuming, and often a soil can be accepted or rejected for a particular use on the basis of its classification alone. For example, an earth dam constructed entirely of sand would not only leak, it would wash away. Classification can reveal if a soil may merit further investigation for founding a highway or building foundation, or if it should be rejected and either replaced, modified, or a different site selected. Important clues can come from the geological and pedological origin, discussed in preceding chapters. Another clue is the engineering classification, which can be useful even if the origin is obscure or mixed, as in the case of random fill soil. 12.1.2 Classification Tests Engineering classifications differ from scientific classifications because they focus on physical properties and potential uses. Two tests devised in the early 1900s by a Swedish soil scientist, Albert Atterberg, are at the heart of engineering classifications. The tests are the liquid limit or LL, which is the moisture content at which a soil become liquid, and the plastic limit or PL, which is the moisture content at which the soil ceases to become plastic and crumbles in the hand. Both limits are strongly influenced by the clay content and clay mineralogy, and generally as the liquid limit increases, the plastic limit tends to decrease. 12 Soil Consistency and Engineering Classification 246 Source: GEOTECHNICAL ENGINEERING Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 2. The numerical difference between the two limits therefore represents a range in moisture contents over which the soil is plastic, and is referred to as the plasticity index or PI. By definition, PI ¼ LL À PL ð12:1Þ where PI is the plasticity index and LL and PL the liquid limit and plastic limit, respectively. This relationship is shown in Fig. 12.1. Because the plasticity index is a difference in percentages and not in itself a percentage, it is expressed as a number and not a percent. Also shown in the figure is the shrinkage limit, which is discussed later in the chapter. 12.1.3 Preparation of Soil for Testing As discussed in relation to clay mineralogy, drying a soil can change its adsorptive capacity for water and therefore can change the liquid and plastic limits. If the soil contains the clay mineral halloysite, dehydration from air-drying is permanent, so to obtain realistic data the soil must not be dried prior to testing. A similar change can occur in soils that have a high content of organic matter. Air-drying nevertheless is still an approved method because it is more convenient for storing soil samples and for dry sieving, because only the portion of a soil passing the No. 40 (425 mm) sieve is tested. Also, many existing correlations were made on the basis of tests of air-dried samples. If a soil has been air-dried it should be mixed with water for 15 to 30 minutes, sealed and stored overnight, and re-mixed prior to testing. Details are in ASTM D-4318. Figure 12.1 Schematic representation of transitions between solid, plastic, and viscous liquid behaviors defined by liquid and plastic limits. These tests are basic to engineering classifications and emphasize influences of clay mineralogy and capillarity. Soil Consistency and Engineering Classification 247 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 3. 12.1.4 Liquidity Index The liquidity index indicates how far the natural soil moisture content has progressed between the plastic and liquid limits. If the soil moisture content is at the plastic limit, the liquidity index is 0; if it is at the liquid limit, it is 1.0. The formula for the liquidity index is LI ¼ w À PL LL À PL ð12:2Þ where LI is the liquidity index, w is the soil moisture content, PL is the plastic limit, and LL is the liquid limit. The liquidity index also is called the relative consistency. 12.2 MEASURING THE LIQUID LIMIT 12.2.1 Concept The concept of the liquid limit is simple: keep adding water to a soil until it flows, and measure the moisture content at that point by oven-drying a representative sample. Two difficulties in application of this concept are (1) the change from plastic to liquid behavior is transitional, and (2) flow can be prevented by thixotropic setting. In order to overcome these limitations, Atterberg suggested that wet soil be placed in a shallow dish, a groove cut through the soil with a finger, and the dish jarred 10 times to determine if the groove closes. While this met the challenge of thixotropy, it also introduced a personal factor. Professor A. Casagrande of Harvard University therefore adapted a cog arrangement invented by Leonardo da Vinci, such that turning a crank drops a shallow brass cup containing wet soil 10 mm onto a hard rubber block, shown in Fig. 12.2. The crank is turned at 2 revolutions per second, and the groove is standardized. Casagrande defined the liquid limit as the moisture content at which the groove would close after 25 blows, which increased the precision of the blow count determination. Different amounts of water are added to a soil sample and stirred in, and the test repeated so that the blow counts bracket the required 25. As it is unlikely that the exact number will be achieved at any particular moisture content, a graph is made of the logarithm of the number of blows versus the moisture content, a straight line is drawn, and the liquid limit read from the graph where the line intersects 25 blows (Fig. 12.3). 12.2.2 Procedure for the Liquid Limit Test A quantity of soil passing the No. 40 sieve is mixed with water to a paste consistency and stored overnight. It is then re-mixed and placed in a standardized 248 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 4. round-bottomed brass cup, and the surface is struck off with a spatula so that the maximum thickness is 10 mm. The soil pat then is divided into two segments by means of a grooving tool of standard shape and dimensions. The brass cup is mounted in such a way that, by turning a crank, it can be raised and allowed to fall sharply onto a hard rubber block or base. The shock produced by this fall causes the adjacent sides of the divided soil pat to flow together. The wetter the mixture, the fewer shocks or blows will be required to cause the groove to close, and the drier the mixture, the greater will be the number of blows. The number of blows required to close the groove in the soil pat is determined at three or more moisture contents, some above the liquid limit and some below it. The logarithm of the number of blows is plotted versus the moisture content and a straight line is drawn through the points, as shown in Fig. 12.3. The moisture content at which 25 blows cause the groove to close is defined Figure 12.2 Casagrande-da Vinci liquid limit device. Figure 12.3 Semilogarithmic plot for determining a soil liquid limit. Soil Consistency and Engineering Classification 249 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 5. as the liquid limit. Tests usually are performed in duplicate and average results reported. A ‘‘one-point’’ test may be used for routine analyses, in which the number of blows is between 20 and 30 and a correction that depends on the departure from 25 is applied to the moisture content. See AASHTO Specification T-89 or ASTM Specification D-423 for details of the liquid limit test. 12.3 MEASURING THE PLASTIC LIMIT 12.3.1 Concept Soil with a moisture content lower than the liquid limit is plastic, meaning that it can be remolded in the hand. An exception is clean sand, which falls apart on remolding and is referred to as ‘‘nonplastic.’’ It is the plasticity of clays that allows molding of ceramics into statues or dishes. At a certain point during drying, the clay can no longer be remolded, and if manipulated, it breaks or crumbles; it is a solid. The moisture content at which a soil no longer can be remolded is the plastic limit, or PL. The standard procedure used to determine the plastic limit of a soil is deceptively simple. The soil is rolled out into a thread, and if it does not crumble it is then balled up and rolled out again, and again, and again . . . until the thread falls apart during remolding. It would appear that a machine might be devised to perform this chore, but several factors make the results difficult to duplicate. First, the soil is continuously being remolded, and second, it gradually is being dried while being remolded. A third factor is even more difficult—the effort required to remold the soil varies greatly depending on the clay content and clay mineralogy. Despite these difficulties and the lack of sophistication, the precision is comparable to or better than that of the liquid limit test. 12.3.2 Details of the Plastic Limit Test The plastic limit of a soil is determined in the laboratory by a standardized procedure, as follows. A small quantity of the soil-water mixture is rolled out with the palm of the hand on a frosted glass plate or on a mildly absorbent surface such as paper until a thread or worm of soil is formed. When the thread is rolled to a diameter of 3 mm (1 8 in.), it is balled up and rolled out again, the mixture gradually losing moisture in the process. Finally the sample dries out to the extent that it becomes brittle and will no longer hold together in a continuous thread. The moisture content at which the thread breaks up into short pieces in this rolling process is considered to be the plastic limit (Fig. 12.4). The pieces or crumbs therefore are placed in a small container for weighing, oven-drying, and re-weighing. Generally at least two determinations are made and the results 250 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 6. averaged. See AASHTO Specification T-90 or ASTM Specification D-424 for details of the plastic limit test. 12.4 DIRECT APPLICATIONS OF LL AND PL TO FIELD SITUATIONS 12.4.1 When a Soil Moisture Content Exceeds the Plastic Limit The liquid limit and plastic limit tests are more diagnostic than descriptive of soil behavior in the field because the tests involve continual remolding. However, there are some important situations where remolding occurs more or less con- tinuously in the field. One example is soil in the basal zone of a landslide. As a landslide moves, it shears and mixes the soil. This mixing action can occur if the soil moisture exceeds the plastic limit. If through chemical treatment such as with drilled lime (quicklime) the plastic limit is increased, the landslide stops. 12.4.2 When a Soil Moisture Content Exceeds the Liquid Limit Exceeding the soil liquid limit in the field can generate harmful and potentially devastating results, as the soil may appear to be stable and then when disturbed can suddenly break away, losing its thixotropic strength and becoming transformed into a rapid churning, flowing mudslide that takes everything in its way. The rate of sliding depends on the slope angle and viscosity of the mud; the lower the viscosity and steeper the slope, the faster the slide. The most devastating mudslides in terms of loss of life therefore occur in mountainous terrain where the mud moves faster than people can get out of the way and escape almost certain Figure 12.4 The plastic limit test. As the soil thread crumbs, pieces are collected in a metal container for oven- drying to determine the moisture content. Soil Consistency and Engineering Classification 251 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 7. death. Quick clays that appear stable can turn into a soup that can be poured like pancake batter. 12.4.3 Liquefaction Another example where a liquid limit may be exceeded is when a saturated sand or silt suddenly densifies during an earthquake so that all of its weight goes to pore water pressure. This is liquefaction, which can cause a sudden and complete loss of shear strength so that landslides develop and buildings may topple. The consequences, diagnosis, and prevention of liquefaction are discussed in more detail in a later chapter. 12.5 THE PLASTICITY INDEX 12.5.1 Concept The plasticity index, or PI, is the numerical difference between the liquid and plastic limit moisture contents. Whereas the two limits that are used to define a PI are directly applicable to certain field conditions, the plasticity index is mainly used to characterize a soil, where it is a measure of cohesive properties. The plasticity index indicates the degree of surface chemical activity and hence the bonding properties of clay minerals in a soil. The plasticity index is used along with the liquid limit and particle size gradation to classify soils according to their engineering behavior. An example of a direct application of the plasticity index is as an indicator of the suitability of the clay binder in a soil mixture used for pavement subgrades, base courses, or unpaved road surfaces. If the PI of the clay fraction of a sand-clay or clay-gravel mixture is too high, the exposed soil tends to soften and become slippery in wet weather, and the road may rut under traffic. On the other hand, if the plasticity index is too low, the unpaved road will tend to ‘‘washboard’’ in response to resonate bouncing of wheels of vehicular traffic. Such a road will abrade under traffic and antagonize the public by producing air-borne dust in amounts that have been measured as high as one ton per vehicle mile per day per year. That is, a rural unpaved road carrying an average of 40 vehicles per day can generate up to 40 tons of dust per mile per year. Most collects in roadside ditches that periodically must be cleaned out. 12.5.2 A PI of Zero Measurements of the LL and PL may indicate that a soil has a plasticity index equal to zero; that is, the numerical values of the plastic limit and the liquid limit may be the same within the limits of accuracy of measurement. Soil with a plasticity index of zero therefore still exhibits a slight plasticity, but 252 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 8. the range of moisture content within which it exhibits the properties of a plastic solid is not measured by the standard laboratory tests. 12.5.3 Nonplastic Soils Drying and manipulating a truly nonplastic soil such as a clean sand will cause it to abruptly change from a liquid state to an incoherent granular material that cannot be molded. If it is not possible to roll soil into a thread as small as 3 mm in diameter, a plastic limit cannot be determined and the soil is said to be nonplastic, designated as NP in test reports. 12.6 ACTIVITY INDEX AND CLAY MINERALOGY 12.6.1 Definition of Activity Index The activity index was defined by Skempton to relate the PI to the amount of clay in a soil, as an indication of the activity of the clay and therefore the clay mineral- ogy: the higher the activity index, or AI, the more active the clay. The activity index was defined as the PI divided by the percent 0.002 mm (or 2 mm) clay: AI ¼ PI C002 ð12:3Þ where AI is the activity index, PI the plasticity index, and C002 the percent 2 mm clay determined from a particle size analsysis. The basis for this relationship is shown in Fig. 12.5. 12.6.2 Relation to Clay Mineralogy The relationship between activity index and clay mineralogy is shown in Table 12.1. Clay mineral mixtures and interlayers have intermediate activities. Data in the table also show how the activity of smectite is strongly influenced by the adsorbed cation on the plasticity index. 12.6.3 Modified Activity Index The linear relationships in Fig. 12.5 do not necessarily pass through the origin. This is shown in Fig. 12.6, where about 10 percent clay is required to generate plastic behavior. This also has been found in other investigations (Chen, 1988). It therefore is recommended that for silty soils eq. (12.3) be modified as follows: A ¼ PI C002 À k ð12:4Þ where A is the activity, C is the percent of the soil finer than 0.002 mm, and k is a constant that depends on the soil type. For silty soils k ¼ 10. If this equation Soil Consistency and Engineering Classification 253 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 9. is applied to the data in Fig. 12.6 with k ¼ 10, then A ¼ 1.23 Æ 0.04 where the Æ value is the standard error for a number of determinations n ¼ 81. This identifies the clay mineral as calcium smectite, which is confirmed by X-ray diffraction. 12.7 LIQUID LIMIT AND COLLAPSIBILITY 12.7.1 Concept A simple but effective idea was proposed in 1953 by a Russian geotechnical engineer, A. Y. Denisov, and later introduced into the U.S. by Gibbs and Bara of Figure 12.5 Linear relations between PI and percent clay. Ratios were defined by Skempton (1953) as activity indices, which are shown in parentheses. The Shellhaven soil clay probably is smectite. Table 12.1 Activity indices of selected clay minerals (after Grim, 1968) Naþ smectite (montmorillonite) 3–7 Ca2þ smectite (montmorillonite) 1.2–1.3 Illite 0.3–0.6 Kaolinite, poorly crystallized 0.3–0.4 Kaolinite, well crystallized 50.1 254 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 10. the U.S. Bureau of Reclamation. Denisov argued that if the moisture content upon saturation exceeds the liquid limit, the soil should be collapsible—that is, it should collapse and densify under its own weight if it ever becomes saturated. The most common collapsible soil is loess, which is a widespread surficial deposit in the U.S., Europe, and Asia. Because loess increases in density with depth and with distance from a source, only the upper material close to a source may be collapsible, so this is a valuable test. 12.7.2 Moisture Content Upon Saturation The soil unit weight and specific gravity of the soil mineral grains are required to calculate the moisture content upon saturation, which are entered into the following equations: SI: ws ¼ 100 9:807=d À 1=Gð Þ ð12:5Þ English: ws ¼ 100 62:4=d À 1=Gð Þ ð12:5aÞ where ws is the percent moisture at saturation, d is the dry unit weight in kN/m3 or lb/ft3 , and G is the specific gravity of the soil minerals. Solutions of this equation with G ¼ 2.70 are shown in Fig. 12.7. 12.8 CONSISTENCY LIMITS AND EXPANSIVE SOILS 12.8.1 Measuring Expandability Expandability can be determined with a consolidometer, which is a device that was developed to measure compression of soil but also can be used to measure expansion under different applied loads. Samples are confined between porous ceramic plates, loaded vertically, wet with water, and the amount of expansion measured. An abbreviated test measures expansion under only applied pressures that can simulate a floor or a foundation load. Figure 12.6 Data suggesting a modification to eq. (12.3) for silty soil. Soil Consistency and Engineering Classification 255 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 11. Many investigations have been made relating expansion to various para- meters, including activity, percent finer than 0.002 mm, percent finer than 0.001 mm, plasticity index, and liquid limit. For the most part the studies have used artificially prepared soil mixtures with varying amounts of different clay minerals. 12.8.2 Influence of Surcharge Pressure Generally the higher the vertical surcharge pressure, the lower the amount of expansion. This leads to a common observation in buildings founded on expansive clay: floors in contact with the soil are lifted more than foundations that are supporting bearing walls and columns and therefore are more heavily loaded. Partition walls that are not load-bearing are lifted with the floor. 12.8.3 Lambe’s PVC Meter A rapid method for measuring clay expandability was developed by T. W. Lambe and his coworkers at MIT. In this device, soil expands against a spring-loaded plate and the expansion is measured. Because the vertical stress increases as the soil expands, results are useful for classification but do not directly translate into expansion amounts that may be expected in the field. 12.8.4 Influence of Remolding Chen (1988) emphasizes that expansion is much lower for undisturbed than for disturbed soil samples subjected to the same treatment, indicating an impor- tant restraining influence from soil fabric. Therefore the expansive clay that is inadvertently used for fill soil, as sometimes happens, may expand much Figure 12.7 Denisov criterion for collapsibility with G ¼ 2.70. Data are for loess at 3, 40, and 55 ft (1, 12, and 16.8 m) depths in Harrison County, Iowa. The deepest soil was mottled gray, suggesting a history of wet conditions, and is indicated to be noncollapsible. 256 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 12. more than if the clay were not disturbed. The reason for this has not been investigated, but an argument may be made for a time-related cementation effect of edge-to-face clay particle bonding, which would prevent water from entering and separating the clay layers. 12.8.5 Relation to PI Figure 12.8 shows the conclusions of several researchers who related clay expand- ability to the plasticity index or PI. Curves A and B show results for remolded samples, and curves C and D are from undisturbed samples where surcharges were applied to more or less simulate floor and foundation loads, respectively. It will be seen that the lowest expandability is shown by curve D, which is for undisturbed soil under the foundation load. 12.8.6 Relation to Moisture Content Generally expansion pressure decreases as the soil moisture content increases, and expansion stops when the smectite clay is fully expanded. This depends on the relative humidity of the soil air and occurs well below the point of saturation of the soil itself. This is illustrated in Fig. 12.9, where seasonal volume change occurs between 30 and 70 percent saturation. Expansion will not occur in a clay that already is wet, which is not particularly reassuring because damaging shrinkage still can occur if and when the clay dries out. Figure 12.8 Data indicating that remolding and a loss of structure greatly increase swelling pressures. All but curve D, which has a surcharge pressure of 1000 lb/ft2 (44 kPa) have a surcharge pressure of 1 lb/in.2 (6.9 kPa). (Modified from Chen, 1988.) Soil Consistency and Engineering Classification 257 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 13. 12.8.7 Summary of Factors Influencing Expandability The amount of expansion that can be anticipated depends on at least seven variables: (1) clay mineralogy, and (2) clay content, both of which are reflected in the consistency limits; (3) existing field moisture content; (4) surcharge pressure; (5) whether or not the soil is remolded; (6) thickness of the expanding layer; and (7) availability of water. 12.8.8 Thickness of the Active Layer One of the most extensive expansive clay areas in the world is in India, but detailed field investigations indicate that only about the upper 90 cm (3 ft) of the expansive soil actually experiences seasonal volume changes. Below that depth the clay is volumetrically stable, even though, as seen in the second graph of Fig. 12.9, the moisture content is not. As previously indicated, saturation is not required for full expansion of Ca-smectite, which is the most common expansive clay. The surficial layer involved in seasonal volume change is called the active layer, and determines the depth of shrinkage cracking and vertical mixing, which by disrupting the soil structure tends to increase its expandability. Example 12.1 Calculate the seasonal ground heave from data in Fig. 12.9. Answer: If the soil is divided into three layers, 0–30, 30–60, and 60–90 cm, average increases in density from the left-hand graph are approximately 0.5/1.22 ¼ 41%; 0.2/1.3 ¼ 15%; and 0.1/1.3 ¼ 8%, respectively. Multiplying these percentages by the layer thicknesses gives total volume changes from the dry to the wet seasons of (0.41 þ 0.15 þ 0.08) Â 30 ¼ 19 cm (7.5 in.). However, part of this will go toward closing open ground cracks, in which case one-third of the volume change will be directed vertically, about 6 cm or 2.5 in. The answer Figure 12.9 Seasonal volume changes in Poona clay, India. Left graph shows that expandability is limited to the upper meter despite deeper variations in moisture content. (From Katti and Katti, 1994.) 258 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 14. therefore is between 6 and 19 cm, depending on filling of the shrinkage cracks and amount of lateral elastic compression of the soil. 12.8.9 Controlling Volume Change with a Nonexpansive Clay (n.e.c.) Layer One of the most significant discoveries for controlling expansive clay was by Dr. R. K. Katti and his co-workers at the Indian Institute of Technology, Mumbai. Katti’s group conducted extensive full-scale laboratory tests to confirm field measurements, such as shown in Fig. 12.9, and found that expansion can be controlled by a surficial layer of compacted non-expansive clay. A particularly severe test for the design was the canal shown in Fig. 12.10. The most common application of Katti’s method is to stabilize the upper meter (3 ft) of expansive clay by mixing in hydrated lime, Ca(OH)2. If only the upper one-third, 30 cm (1 ft), is stabilized, volume change will be (0.15 þ 0.08) Â 30 ¼ 7 cm (3 in.), a reduction of about 60 percent. If the upper 60 cm (2 ft) is stabilized, the volume change will be 0.08 Â 30 ¼ 2.4 cm (1 in.), a reduction of over 85 percent. Stabilization to the full depth has been shown to eliminate volume change altogether. The next question is, why? An answer may be in the curves in Fig. 12.8, as a loss of clay structure greatly increases clay expandability. As a result of shrink-swell cycling and an increase in horizontal stress, expansive clays are visibly sheared, mixed, and remolded, so by destroying soil structure expansion probably begets more expansion. According to this hypothesis, substituting a layer of nonexpansive clay for the upper highly expansive layer may help to preserve the structure and integrity of the underlying layer. It was found that using a sand layer or a foundation load instead of densely Figure 12.10 The Malaprabha Canal in India was successfully built on highly expansive clay using Katti’s method, by replacing the upper 1 m of soil with compacted nonexpansive clay (n.e.c.) to replicate the conditions shown in Fig. 12.9 for the underlying soil. Soil Consistency and Engineering Classification 259 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 15. compacted clay is less effective, perhaps because it interrupts the continuum (Katti and Katti, 2005). 12.9 PLASTICITY INDEX VS. LIQUID LIMIT 12.9.1 Concept The relationship between PI and LL reflects clay mineralogy and has an advan- tage over the activity index because a particle size analysis is not required. Because the liquid limit appears on both sides of the relationship, data can plot only within a triangular area defined by the PL ¼ 0 line shown in Fig. 12.11. 12.9.2 The A-Line A line that approximately parallels the PI versus LL plot for particular soil groups is called the A-line, which was proposed by A. Casagrande and for the most part separates soils with and without smectite clay minerals. However, the separation is not always consistent, as can be seen in Fig. 12.11 where loess crosses the line. At low clay contents loessial soils also are more likely to show collapse behavior instead of expanding. 12.10 A SOIL CLASSIFICATION BASED ON THE A-LINE 12.10.1 Background During World War II, Arthur Casagrande devised a simplified soil classification system for use by the armed forces. The objective was a system that could be used to classify soils from visual examination and liquid/plastic behavior. In 1952 Figure 12.11 Representative relationships between PI and LL. (Adapted from U.S. Dept. of Interior Bureau of Reclamation, 1974.) 260 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 16. the system was adopted for civilian uses by the U.S. Bureau of Reclamation and the U.S. Army Corps of Engineers and became known as the Unified Classification. The ASTM Designation is D-2487. The system applies not only to fine-grained soils but also to sands and gravels, and is the most widely used system for soil investigations for building foundations and tunneling. 12.10.2 ‘‘S’’ Is for Sand One advantage of the Unified Classification system is its simplicity, as it uses capital letters to represent particular soil properties: S stands for sand, G for gravel, and C for clay. Because S already is used for sand, another letter, M, was selected for silt, from the German word Moh. A sand or gravel can either be well graded, W, or for poorly graded, designated by P, respectively indicating broad or narrow ranges of particle sizes. Thus, SP is a poorly graded sand, GW a well-graded gravel. Fine-grained soils are characterized on the basis of liquid limit and the PI and LL relationships to the A-line. A silt or clay with a liquid limit higher than 50 percent is designated by H, meaning high liquid limit, and if the data plot above the A-line the soil is CH, clay with a high liquid limit. If the liquid limit is higher than 50 percent and the data plot below the A-line, the designation is MH, silt with a high liquid limit. The Unified Classification system therefore distinguishes between silt and clay not on the basis of particle size, but on relationships to the liquid limit and plasticity index. In order to avoid confusion, clay and silt that are defined on the basis of size now usually are referred to as ‘‘clay-size’’ or ‘‘silt-size’’ material. If the silt or clay liquid limit is lower than 50, the respective designations are ML and CL, silt with a low liquid limit or clay with a low liquid limit. However, if the plasticity index is less than 4, silt dominates the soil behavior and the soil is designated ML. This is shown in the graph in Table 12.2. Soils with a plasticity index between 4 and 7 show properties that are intermediate and are designated CL-ML. 12.10.3 Details of the Unified Classification System Letter abbreviations for the various soil characteristics are as follows: G ¼ Gravel O ¼ Organic S ¼ Sand W ¼ Well graded M ¼ Nonplastic or low plasticity P ¼ Poorly graded C ¼ Plastic fines L ¼ Low liquid limit Pt ¼ Peat, humus, swamp soils H ¼ High liquid limit Soil Consistency and Engineering Classification 261 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 17. Table 12.2 Unified soil classification system 262 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 18. Soil Consistency and Engineering Classification 263 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 19. These letters are combined to define various groups as shown in Table 12.2. This table is used to classify a soil by going from left to right and satisfying the several levels of criteria. 12.10.4 Equivalent Names Some equivalent names for various combined symbols are as follows. These names are appropriate and should be used in reports that may be read by people who are not familiar with the soil classification symbols. For example, describing a soil as an ‘‘SC clayey sand’’ will be more meaningful than to only refer to it as ‘‘SC,’’ and illustrates the logic in the terminology. The meanings of the symbols for coarse-grained soils are fairly obvious. Names used for fine-grained soils are as follows: CL ¼ lean clay CH ¼ fat clay ML ¼ silt MH ¼ elastic silt OL ¼ organic silt OH ¼ organic clay If a fine-grained soil contains over 15 percent sand or gravel, it is referred to as ‘‘with sand or with gravel;’’ if over 30 percent, it is ‘‘sandy’’ or ‘‘gravelly.’’ If over 50 percent it goes into a coarse-grained classification. If a soil contains any cobbles or boulders it is referred to as ‘‘with cobbles’’ or ‘‘with boulders.’’ Other more detailed descriptors will be found in ASTM D-2487. 12.10.5 Detailed Descriptions Descriptions of the various groups that may be helpful in classification are as follows: GW and SW Soils in these groups are well-graded gravelly and sandy soils that contain less than 5 percent nonplastic fines passing the No. 200 sieve. The fines that are present do not noticeably affect the strength characteristics of the coarse-grained fraction and must not interfere with its free-draining characteristic. In areas subject to frost action, GW and SW soils should not contain more than about 3 percent of soil grains smaller than 0.02 mm in size. GP and SP GP and SP soils are poorly graded gravels and sands containing less than 5 percent of nonplastic fines. The soils may consist of uniform gravels, uniform sands, or nonuniform mixtures of very coarse material and very fine sand with intermediate sizes lacking, referred to as skip-graded, gap-graded, or step-graded. GM and SM In general, GM and SM soils are gravels or sands that contain more than 12 percent fines having little or no plasticity. In order to qualify as M, the 264 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 20. plasticity index and liquid limit plot below the A-line on the plasticity chart. Because of separation of coarse particles gradation is less important, and both well-graded and poorly graded materials are included in these groups. Some sands and gravels in these groups may have a binder composed of natural cementing agents, so proportioned that the mixture shows negligible swelling or shrinkage. Thus, the dry strength is provided either by a small amount of soil binder or by cementation of calcareous materials or iron oxide. The fine fraction of noncemented materials may be composed of silts or rock-flour types having little or no plasticity, and the mixture will exhibit no dry strength. GC and SC These groups consist of gravelly or sandy soils with more than 12 percent fines that can exhibit low to high plasticity. The plasticity index and liquid limit plot above the A-line on the plasticity chart. Gradation of these materials is not important, as the plasticity of the binder fraction has more influence on the behavior of the soils than does variation in gradation. The fine fraction is generally composed of clays. Borderline G and S Classifications It will be seen that a gap exists between the GW, SW, GP, and SP groups, which have less than 5 percent passing the No. 200 sieve, and GM, SM, GC, and SC soils, which have more than 12 percent passing the No. 200 sieve. Soils containing between 5 and 12 percent fines are considered as borderline and are designated by a dual symbol such as GW-GM if the soil is a well-graded gravel with a silt component, or GW-GC if well-graded with a clay component. Many other dual symbols are possible, and the meaning should be evident from the symbol. For example, SP-SC is a poorly graded sand with a clay component, too much clay to be SP and not enough to be SC. ML and MH ML and MH soils include soils that are predominantly silts, and also include micaceous or diatomaceous soils. An arbitrary division between ML and MH is established where the liquid limit is 50. Soils in these groups are sandy silts, clayey silts, or inorganic silts with relatively low plasticity. Also included are loessial soils and rock flours. Micaceous and diatomaceous soils generally fall within the MH group but may extend into the ML group when their liquid limit is less than 50. The same is true for certain types of kaolin clays and some illitic clays having relatively low plasticity. CL and CH The CL and CH groups embrace clays with low and high liquid limits, respectively. These are mainly inorganic clays. Low-plasticity clays are classified as CL and are usually lean clays, sandy clays, or silty clays. The medium-plasticity and high-plasticity clays are classified as CH. These include the fat clays, gumbo Soil Consistency and Engineering Classification 265 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 21. clays, certain volcanic clays, and bentonite. The glacial clays of the northern U.S. cover a wide band in the CL and CH groups. ML-CL Another type of borderline classification that already has been commented on is when the liquid limit of a fine-grained soil is less than 29 and the plasticity index lies in the range from 4 to 7. These limits are indicated by the shaded area on the plasticity chart in Fig. 12.11, in which case the double symbol, ML-CL, is used to describe the soil. OL and OH OL and OH soils are characterized by the presence of organic matter and include organic silts and clays. They have plasticity ranges that correspond to those of the ML and MH groups. Pt Highly organic soils that are very compressible and have very undesirable construction characteristics are classified in one group with the symbol Pt. Peat, humus, and swamp soils with a highly organic texture are typical of the group. Particles of leaves, grass, branches of bushes, or other fibrous vegetable matter are common components of these soils. 12.11 FIELD USE OF THE UNIFIED SOIL CLASSIFICATION SYSTEM 12.11.1 Importance of Field Identitication A detailed classification such as indicated in Table 12.2 requires both a gradation and plasticity analysis. Even after this information is available from laboratory tests of soil samples, it is important to be able to identify the same soils in the field. For example, if a specification is written based on the assumption that a soil is an SC, and the borrow excavation proceeds to cut into ML, it can be very important that somebody serves notice and if necessary issues a stop order. This is a reason why all major construction jobs include on-site inspection. Suggestions for conduct of a field identification using the Unified Classification system are in ASTM D-2488. 12.11.2 Granular Soils A dry sample of coarse-grained material is spread on a flat surface to determine gradation, grain size and shape, and mineral composition. Considerable skill is required to visually differentiate between a well-graded soil and a poorly graded soil, and is based on visual comparisons with results from laboratory tests. The durability of coarse aggregate is determined from discoloration of weathered materials and the ease with which the grains can be crushed. Fragments of shale or 266 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 22. other rock that readily breaks into layers may render a coarse-grained soil unsuitable for certain purposes, since alternate wetting and drying may cause it to disintegrate partially or completely. This characteristic can be determined by submerging thoroughly dried particles in water for at least 24 hours and observing slaking or testing to determine a loss of strength. 12.11.3 Fine-Grained vs. Coarse-Grained Soils As shown in Table 12.2, fine-grained soils are defined as having over 50 percent passing the No. 200 sieve. The percentage finer can be estimated without the use of a sieve and weighing device, by repeatedly mixing a soil sample with water and decanting until the water is clear, and then estimating the proportion of material that has been removed. Another method is to place a sample of soil in a large test tube, fill the tube with water and shake the contents thoroughly, and then allow the material to settle. Particles retained on a No. 200 sieve will settle out of suspension in about 20 to 30 seconds, whereas finer particles will take a longer time. An estimate of the relative amounts of coarse and fine material can be made on the basis of the relative volumes of the coarse and fine portions of the sediment. 12.11.4 Fine-Grained Soils Field identification procedures for fine-grained soils involve testing for dilatancy, or expansion on shaking, plasticity, and dry strength. These tests are performed on the fraction of soil finer than the No. 40 sieve. In addition, observations of color and odor can be important. If a No. 40 sieve is not available, removal of the fraction retained on this sieve may be partially accomplished by hand picking. Some particles larger than this sieve opening (0.425 mm, or nominally 0.5 mm) may remain in the soil after hand separation, but they probably will have only a minor effect on the field tests. DiIatancy For the dilatancy test, enough water is added to about 2 cm3 (1 2 in.3 ) of the minus-40 fraction of soil to make it soft but not sticky. The pat of soil is shaken horizontally in the open palm of one hand, which is struck vigorously against the other hand several times. A fine-grained soil that is nonplastic or has very low plasticity will show free water on the surface while being shaken, and then squeezing the pat with the fingers will cause the soil structure to dilate or expand so that the soil appears to dry up. The soil then will stiffen and finally crumble under increasing pressure. Shaking the pat again will cause it to flow together and water to again appear on the surface. A distinction should be made between a rapid reaction, a slow reaction, or no reaction to the shaking test, the rating depending on the speed with which the pat changes its consistency and the water on the surface appears or disappears. Rapid Soil Consistency and Engineering Classification 267 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 23. reaction is typical of nonplastic, uniform fine sand, of silty sand (SP or SM), of inorganic silt (ML), particularly the rock-flour type, and of diatomaceous earth (MH). The reaction becomes more sluggish as the uniformity of gradation decreases and the plasticity increases, up to a certain degree. Even a small amount of colloidal clay will impart some plasticity to the soil and will materially slow the reaction to the shaking test. Soils that react in this manner are somewhat plastic inorganic and organic silts (ML or OL), very lean clays (CL), and some kaolin-type clays (ML or MH). Extremely slow reaction or no reaction to the shaking test is characteristic of typical clays (CL or CH) and of highly plastic organic clays (OH). Field Estimate of Plasticity The plasticity of a fine-grained soil or the binder fraction of a coarse-grained soil may be estimated by rolling a small sample of minus-40 material between the palms of the hand in a manner similar to the standard plastic limit test. The sample should be fairly wet, but not sticky. As it is rolled into 1 8-inch threads, folded and re-rolled, the stiffness of the threads should be observed. The higher the soil above the A-line on the plasticity chart (CL or CH), the stiffer the threads. Then as the water content approaches the plastic limit, the tougher are the lumps after crumbling and remolding. Soils slightly above the A-line (CL or CH) form a medium-tough thread that can be rolled easily as the plastic limit is approached, but when the soil is kneaded below the plastic limit, it crumbles. Soils below the A-line (ML, NH, OL, or OH) form a weak thread and with the exception of an OH soil, such a soil cannot be lumped into a coherent mass below the plastic limit. Plastic soils containing organic material or much mica form threads that are very soft and spongy near the plastic limit. In general, the binder fraction of a coarse-grained soil with silty fines (GM or SM) will exhibit plasticity characteristics similar to those of ML soils. The binder fraction of a coarse-grained soil with clayey fines (GC or SC) will be similar to CL soils. Field Estimate of Dry Strength Dry strength is determined from a pat of minus-40 soil that is moistened and molded to the consistency of putty, and allowed to dry in an oven or in the sun and air. When dry the pat should be crumbled between the fingers. ML or MH soils have a low dry strength and crumble with very little finger pressure. Also, organic siIts and lean organic clays of low plasticity (OL) and very fine sandy soils (SM) also have low dry strength. Most clays of the CL group and some OH soils, as well as the binder fraction of gravelly and sandy clays (GC or SC), have medium dry strength and require considerable finger pressure to crumble the sample. Most CH clays and some organic clays (OH) having high liquid limits and located near the A-line have high dry strength, and the test pat can be broken with the fingers but cannot be crumbled. 268 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 24. Color and Odor Dark or drab shades of gray or brown to nearly black indicate fine-grained soils containing organic colloidal matter (OL or OH), whereas brighter colors, including medium and light gray, olive green, brown, red, yellow, and white, are generally associated with inorganic soils. An organic soil (OL or OH) usually has a distinctive odor that can be helpful for field identification. This odor is most obvious in a fresh sample and diminishes on exposure to air, but can be revived by heating a wet sample. The details of field identification are less imposing and more easily remembered if they are reviewed in relation to each particular requirement. For example, if a specification is for SC, the criteria for an SC soil should be reviewed and understood and compared with those for closely related soils, SM and SP and respective borderline classifications. 12.12 THE AASHTO SYSTEM OF SOIL CLASSIFICATION 12.12.1 History A system of soil classification was devised by Terzaghi and Hogentogler for the U.S. Bureau of Public Roads in the late 1920s, predating the Unified Classification system by about 20 years. The Public Roads system was subsequently modified and adopted by the American Association of State Highway Officials (now Highway and Transportation Officials) and is known as the AASHTO system (AASHTO Method M14S; ASTM Designation D-3282). As in the Unified Classification system, the number of physical properties of a soil upon which the classification is based is reduced to three—gradation, liquid limit, and plasticity index. Soil groups are identified as A-1 through A-8 for soils ranging from gravel to peat. Generally, the higher the number, the less desirable the soil for highway uses. 12.12.2 Using the AASHTO Chart The process of determining the group or subgroup to which a soil belongs is simplified by use of the tabular chart shown in Table 12.3. The procedure is as follows. Begin at the left-hand column of the chart and see if all these known properties of the soil comply with the limiting values specified in the column. If they do not, move to the next column to the right, and continue across the chart until the proper column is reached. The first column in which the soil properties fit the specified limits indicates the group or subgroup to which the soil belongs. Group A-3 is placed before group A-2 in the table to permit its use in this manner even though A-3 soils normally are considered less desirable than A-2 soils. Soil Consistency and Engineering Classification 269 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 25. The ranges of the liquid limit and the plasticity index for fine-grained soils in groups A-4, A-5, A-6, and A-7 are shown in Fig. 12.12, which has been arranged to be comparable to the Unified chart. Example 12.2 Classify a soil containing 65% of material passing a No. 200 sieve and having a liquid limit of 48 and a plasticity index of 17. Answer: Since more than 35% of the soil material passes the No. 200 sieve, it is a silt-clay material and the process of determining its classification can begin by examining the specified limits for group A-4, where the maximum is 40. Since the liquid limit of the soil being classified is 48%, it cannot be an A-4 soil so we proceed to the columns to the right, where it will be seen that it meets the liquid limit requirement of A-6 and A-7, but meets the plasticity index requirements of A-7. The soil therefore is A-7. This procedure is simplified by reference to Fig. 12.12, which also is used to separate A-7 soils into two subgroups, A-7-5 and A-7-6. 12.12.3 Size Grade Definitions AASHTO definitions of gravel, sand, and silt-cIay are as follows: Gravel Material passing a sieve with 75 mm (3 in.) square openings and retained on a No. 10 (2 mm) sieve. Coarse Sand Material passing the No. 10 sieve and retained on the No. 40 (425 mm) sieve. Fine Sand Material passing the No. 40 sieve and retained on the No. 200 (75 mm) sieve. Figure 12.12 Chart for classifying fine-grained soils by the AASHTO system. 270 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 26. Table12.3 SoilclassificationbytheAASHTOsystem General ClassificationGranularMaterials(35%orlesspassingNo.200) Silt-ClayMaterials (Morethan35%passingNo.200) A-7 A-1A-2A-7-5 GroupClassificationA-1-aA-1-bA-3A-2-4A-2-5A-2-6A-2-7A-4A-5A-6A-7-6 Sieveanalysis,percentpassing: No.1050max. No.4030max.50max.51min. No.20015max.25max.10max.35max.35max.35max.35max.36min.36min.36min.36min. Characteristicsoffractionpassing No.40: Liquidlimit40max.41min.40max.41min.40max.41min.40max.41min.b Plasticityindex6max.NP10max.10max.11min.11min.10max.10max.11min.11min. Usualtypesof significant constituent materials Stonefragments, gravelandsand FinesandSiltyorclayeygravelandsandSiltysoilsClayeysoils Generalrating assubgrade ExcellenttogoodFairtopoor a Classificationprocedure:Withrequiredtestdataavailable,proceedfromlefttorightonabovechartandcorrectgroupwillbefoundbytheprocessof elimination.Thefirstgroupfromtheleftintowhichthetestdatawillfitisthecorrectclassification. b PlasticityindexofA-7-5subgroupisequaltoorlessthanLLminus30.PlasticityindexofA-7-6subgroupisgreaterthanLLminus30(seeFig.12.12) Soil Consistency and Engineering Classification 271 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 27. Silt-Clay or Combined Silt þ Clay Material passing the No. 200 sieve. Boulders Boulders retained on the 75 mm (3 in.) sieve are excluded from the portion of the sample being classified, but the percentage of such material is recorded. The term ‘‘silty’’ is applied to fine material having a plasticity index of 10 or less, and the term ‘‘clayey’’ is applied to fine material having a plasticity index of 11 or more after rounding to the nearest whole percent. 12.12.4 Descriptions of Groups The following generalized observations may be applied to the various AASHTO soil groups: A-1 Typical of this group are well-graded mixtures of stone fragments or gravel, volcanic cinders, or coarse sand. They do not contain a soil binder or have a nonplastic or feebly plastic binder. Subgroup A-1-a is mainly stone fragments or gravel, and A-1-b is mainly coarse sand. A-3 Typical of this group is fine beach sand or fine desert blow sand without silty or clayey fines, or with a very small amount of nonplastic silt. The group also includes stream-deposited mixtures of poorly graded fine sand with limited amounts of coarse sand and gravel. A-2 This group includes a wide variety of granular materials that are at the borderline between A-1 and A-3 and silt-clay materials of groups A-4 through A-7. A-2 includes materials with less than 35 percent passing a No. 200 sieve that do not classify as A-1 or A-3, because either the fines content or plasticity, or both, are in excess of the amounts allowed in those groups. Subgroups A-2-4 and A-2-5 include various granular materials with not more than 35 percent passing a No. 200 sieve and containing a minus No. 40 portion that has characteristics of the A-4 and A-5 groups, respectively. These subgroups include such materials as gravel and coarse sand with silt content or plasticity index in excess of those allowed in A-1, and fine sand with nonplastic silt content in excess of the limitations of group A-3. Subgroups A-2-6 and A-2-7 include materials similar to those described under subgroups A-2-4 and A-2-5, except that the fine portion contains plastic clay 272 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 28. having the characteristics of the A-6 or the A-7 group. The group index, described below, is 0 to 4. A-4 The typical material of this group is a nonplastic or moderately plastic silty soil, 75 percent or more of which passes the No. 200 sieve. However, the group also can include mixtures of fine silty soil with up to 64 percent retained on the No. 200 sieve. A5 Typical of this group is soil that is similar to that described under group A-4, but has a diatomaceous or micaceous content that makes it highly elastic, indicated by a high liquid limit. These soils are ‘‘springy’’ and may be difficult to compact. A-6 The material of this group typically is plastic clay soil with 75 percent or more passing the No. 200 sieve, but can include fine clayey soil mixtures with up to 64 percent retained on the No. 200 sieve. Materials of this group usually have high volume change between wet and dry states. A-7 A-7 soils are similar to A-6 but have higher liquid limits. Subgroup A-7-5 materials have moderate plasticity indexes in relation to liquid limit, and which may be highly elastic as well as subject to considerable volume change on wetting or drying. Subgroup A-7-6 materials have high plasticity indexes in relation to liquid limit, and are subject to very high volume changes. A-8 A-8 soil is peat or muck soil in obviously unstable, swampy areas. A-8 soil is characterized by low density, high compressibility, high water content, and high organic matter content. Attention is directed to the fact that the classification of soils in this group is based largely upon the character and environment of their field occurrence, rather than upon laboratory tests of the material. As a matter of fact, A-8 soils usually show laboratory-determined properties of an A-7 soil, but are properly classified as group A-8 because of the manner of their occurrence. 12.12.5 Group Index The group index gives a means for further rating a soil within its group or subgroup. The index depends on the percent passing the No. 200 sieve, the liquid limit, and the plasticity index. It is computed by the following empirical formula: Group index ¼ F À 35ð Þ 0:2 þ 0:005 LL À 40ð Þ½ Š þ 0:01 F À 15ð Þ PI À 10ð Þ ð12:6Þ in which F is the percent passing the No. 200 sieve, expressed as a whole number and based only on the material passing the 75 mm (3 in.) sieve, LL is the liquid Soil Consistency and Engineering Classification 273 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 29. limit, and PI is the plasticity index. When the calculated group index is negative it is reported as zero (0). The group index is expressed to the nearest whole number and is written in parentheses after the group or subgroup designation. A group index should be given for each soil even if the numerical value is zero, in order to indicate that the classification has been determined by the AASHTO system instead of the original Public Roads system. A nomograph has been devised to solve eq. (12.6), but it now is more conveniently solved with a computer spreadsheet. 12.13 LIMITATIONS AND COMPARISONS OF SOIL CLASSIFICATION SYSTEMS The classification systems described above use disturbed soil properties and therefore do not take into account factors such as geological origin, fabric, density, or position of a groundwater table. The classifications nevertheless do provide important information relative to soil behavior so long as the limitations are recognized. Classification is no substitute for measurements of important soil properties such as compressibility, shear strength, expandability, permeability, saturation, pore water pressure, etc. Boundary lines for fine soils in the Unified and AASHTO classification systems do not precisely coincide, but the systems are close enough that there is considerable overlapping of designations, so a familiarity with one system will present at least a working acquaintance with the other. Some approximate equivalents that will include most but not all soils are as follows: A-1-a or GW Well-graded free-draining gravel suitable for road bases or foundation support. A-1-b or SW Similar to A-1-a except that it is primarily sand. A-2 or SM or SC Sand with appreciable fines content. May be moderately frost-susceptible. A-3 or SP Sand that is mainly one size. A-4 or ML Silt that combines capillarity and permeability so that it is susceptible to frost heave. Low-density eolian deposits often collapse when wet. 274 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 30. A-5 or Low-Plasticity MH Includes micaceous silts that are difficult to compact. A-6 or CL Moderately plastic clay that has a moderate susceptibility to frost heave and is likely to be moderately expansive. All A-6 is CL but not all CL is A-6. A-7-5 or Most MH Silty clay soils with a high liquid limit, often from a high mica content. A-7-6 or CH Highly plastic clay that is likely to be expansive. Low permeability reduces frost heave. All A-7-6 soils also classify as CH. A-8 and Pt Peat and muck. 12.14 OTHER DESCRIPTIVE LIMITS Other tests and descriptive terms have been devised or defined that are not as widely used or have fallen into disuse. Some are as follows: 12.14.1 Toughness Toughness is defined as the flow index from the liquid limit test, which is the change in moisture content required to change the blow count by a factor of 10, divided by the plasticity index. 12.14.2 Shrinkage Limit The shrinkage limit test was suggested by Atterberg and has been used as a criterion for identifying expansive clay soils. However, the test involves complete destruction of the soil structure and drying from a wet mud, which makes correlations less reliable. The shrinkage limit generally is lower than the plastic limit, and the transition from the intermediate semisolid state to a solid is accompanied by a noticeably lighter shade of color due to the entry of air. The shrinkage limit test also fell into disfavor because it used a mercury displacement method to measure the volume of the dried soil pat. An alternative method now coats the soil pat with wax for immersion in water (ASTM D-4943). In order to perform a shrinkage limit test a soil-water mixture is prepared as for the liquid limit but with a moisture content that is considerably above the liquid limit, and the moisture content is measured. A sample is placed in a shallow dish Soil Consistency and Engineering Classification 275 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 31. that is lightly greased on the inside and struck off even with the top of the dish, which has a known weight and volume. The soil then is oven-dried at 1108C and the weight recorded. The soil pat then is removed and suspended by a thread in melted wax, drained and allowed to cool, and re-weighed. During oven-drying the volumetric shrinkage equals the volume of water lost until the soil grains come into contact, which is defined as the shrinkage limit. Then, SL ¼ w À V À Vd ms ! Â 100 ð12:7Þ where w and V are the soil moisture content and volume prior to drying, Vd is the volume of the pat after oven-drying, and ms is the mass of the dry soil in grams. The determination assumes that the density of water that is lost during drying is 1.0 g/cm3 . A so-called ‘‘shrinkage ratio’’ equals the dry density of the soil at the shrinkage limit: SR ¼ ms=Vd ð12:8Þ where SR is the shrinkage ratio and other symbols are as indicated above. 12.14.3 COLE The ‘‘coefficient of linear extensibility’’ (COLE) test is used by soil scientists to characterize soil expandability, and has an advantage over the shrinkage limit test in that the original soil structure is retained, which as previously discussed can greatly reduce the amount of soil expandability. No external surcharge load is applied. A soil clod is coated with plastic that acts as a waterproof membrane but is permeable to water vapor. The clod then is subjected to a standardized moisture tension of 1/3 bar, and after equilibration its volume is determined by weighing when immersed in water. The volume measurement then is repeated after oven- drying, and the volume change is reduced to a linear measurement by taking the cube root: COLE ¼ 3p Vm=Vdð Þ À 1 ð12:9Þ Where Vm is the volume moist and Vd is the volume dry. Volumes are obtained from the reduction in weight when submerged in water, which equals the weight of the water displaced. For example, if the reduction in weight is 100 g (weight), the volume is 100 cm3 . A COLE of 53 percent is considered low, 3 to 6 percent moderate, and 46 percent high for residential construction (Hallberg, 1977). 12.14.4 Slaking Shale may be subjected to a slaking test that involves measuring the weight loss after wetting and tumbling in a rotating drum (ASTM D-4644). Dry clods of soil also may slake when immersed in water as the adsorptive power may be so great 276 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 32. that air in the pores is trapped and compressed by water entering the capillaries, causing the soil clod literally to explode and disintegrate. The same soil will not slake when saturated. Slaking therefore can provide an immediate clue that a soil has been compacted too dry, discussed in the next chapter. 12.15 SUMMARY This chapter describes laboratory tests relating the plastic behavior to moisture content, which form the basis for engineering classifications. Two classification methods are presented, one that is more commonly used in highway soil engineering and the other in foundation engineering. Soils may be classified by either or both methods as part of a laboratory testing program. Classification is useful for determining appropriate uses of soils for different applications, but is not a substitute for engineering behavioral tests. Results of classification tests can be influenced by air-drying, so soil samples preferably are not air-dried prior to testing. If they are air-dried, considerable mixing and aging are required to ensure complete hydration of the clay minerals prior to testing. As soils used in classification tests are remolded, the results are not directly applicable to most field situations, exceptions being soils that are being remolded in the base of active landslides, in mudflows, and soils that have been liquefied by vibrations such as earthquakes. Classification therefore is more commonly a diagnostic than a performance tool. Problems 12.1. Define liquid limit, plastic limit, plasticity index, and activity index. 12.2. Four trials in a liquid limit test give the following data. Plot a flow curve and determine the liquid limit. Number of blows Moisture content, % 45 29 31 35 21 41 14 48 12.3. If the plastic limit of the soil in Problem 12.2 is 13%, what is the plasticity index? 12.4. If the soil in Problem 12.2 contains 30% 2 mm clay, what is the activity index? 12.5. The liquid limit of a soil is 59%, the plastic limit is 23, and the natural moisture content is 46%. What is the liquidity index? What is its significance? Soil Consistency and Engineering Classification 277 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 33. 12.6. The liquid limit of a soil is 69% and its natural moisture content is 73%. Is this soil stable, metastable, or unstable? What is the dictionary definition of metastable? 12.7. Define shrinkage limit and shrinkage ratio. 12.8. The volume of the dish used in a shrinkage limit test is measured and found to be 20.0 cm3 , and the volume of the oven-dry soil pat is 14.4 cm3 . The weights of the wet and dry soil are 41.0 and 30.5 g, respectively. Calculate the shrinkage limit and shrinkage ratio. 12.9. Describe a nonplastic soil and explain how this characteristic is determined in the laboratory. 12.10. Distinguish clearly between a nonplastic soil and one that has a PI equal to zero. 12.11. Can you think of a reason why a fine-grained binder soil should be close to or below the plastic limit when it is added to a coarse-grained soil to form a stabilized soil mixture? 12.12. If the PI of a stabilized soil pavement is too high, what adverse characteristics are likely to develop under service conditions? What may happen if the PI is too low? 12.13. Is soil containing water in excess of the liquid limit necessarily a liquid? Explain. 12.14. A soil clod coated with a semipermeable plastic membrane and equilibrated at 1/3 bar moisture tension weighs 210 g in air and 48 g submerged in water. After oven-drying, the corresponding weights are 178 g and 47 g. (a) What is the COLE? (b) Rate the expansive potential of this soil. (c) If you have no choice but to put a light slab-in-grade structure on the soil, what precautions might be taken to prevent damage? 12.15. What is the significance of the group index in connection with the AASHTO system of classification? 12.16. State the broad general character of soils included in groups A-1, A-2, and A-3 of the AASHTO system and give approximate equiv- alents in the Unified Classification system. What are the specific differences? 12.17. What are the principal differences between two soils classified as A-4 and A-5 in the AASHTO system? 12.18. What are the approximate Unified Classification equivalents of AASHTO groups A-4, A-5, A-6, A-7, and A-8? Which pairs are most nearly identical? 12.19. Give the major characteristics of soils included in (a) the GW, GC, GP, and GF groups of the Unified Classification system; (b) the SW, SC, SP, and SF groups; (c) the ML, CL, and OL groups; (d) the MN, CH, and OH groups. 278 Geotechnical Engineering Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.
  • 34. 12.20. Give four examples of borderline classifications in the Unified Classifica- tion system and explain what each means. The following problems are with reference to data in Table 7.5 of Chapter 7. 12.21. Classify soils No. 1, 2, and 3 in Table 7.5 according to the AASHTO and Unified systems. 12.22. Classify soils No. 4, 5, and 6 in Table 7.5 according to the AASHTO and Unified systems. 12.23. Classify soils No. 7, 8, and 9 in Table 7.5 according to the AASHTO and Unified systems. 12.24. Classify soils No. 10, 11, and 12 in Table 7.5 according to the AASHTO and Unified systems. 12.25. Which soil in each system is most susceptible to frost heave? What characteristics contribute to this susceptibility? 12.26. Which soil in each system is most expansive? Which is moderately expansive? 12.27. A loess soil changes from A-4 to A-6 to A-7-6 depending on distance from the source. Predict the volume change properties including expansion and collapsibility. 12.28. State the Denisov criterion for loess collapsibity. Does it take into account the increase in density with depth? 12.29. Seasonal changes in moisture content of an expansive clay deposit extend to a depth of 4 m (13 ft). Does that depth coincide with the thickness of the active layer? Why (not)? 12.30. Why classify soils? References and Further Reading American Society for Testing and Materials. Annual Book of Standards. ASTM, Philadelphia. Chen, F. K. (1988). Foundations on Expansive Soils, 2nd ed. Elsievier, Amsterdam. Grim, R. E. (1968). Clay Mineralogy. McGraw-Hill, New York. Hallberg, G. (1977). ‘‘The Use of COLE Values for Soil Engineering Evaluation.’’ J. Soil Sci. Soc. Amer. 41(4), 775–777. Handy, R. L. (1973). ‘‘Collapsible Loess in Iowa.’’ Soil Sci. Soc. Amer. Proc. 37(2), 281–284. Handy, R. L. (2002). ‘‘Geology, Soil Science, and the Other Expansive Clays.’’ Geotechnical News 20(1), 40–45. Katti, R. K., Katti, D. R., and Katti, A. R. (2005). Primer on Construction in Expansive Black Cotton Soil Deposits with C.N.S.L. (1970 to 2005). Oxford & IBH Publishing Co., New Delhi. Skempton, A. W. (1953). ‘‘The Colloidal Activity of Clays.’’ Proc. 3rd Int. Conf. on Soil Mech. and Fd. Engg. 1, 57. U.S. Department of Interior Bureau of Reclamation (1974). Earth Manual, 2nd ed. U.S. Government Printing Office, Washington, D.C. Soil Consistency and Engineering Classification 279 Soil Consistency and Engineering Classification Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2007 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.