A Review of Engineering Soil Classification Systems THOMAS K. LIU, Associate Professor of Civil Engineering, University of Illinois, Urbana This paper discusses, with particular reference to transporta­ tion engineering, the nature and necessity of soil classification and the distinction between soil identification and soil classi­ fication. The soil properties suitable for classification pur­ poses are considered. The requirements for a satisfactory soil classificati<,m system are listed as (a) distinct properties as the basis for grouping, (b) logical, simple and concise scheme, (c) meaningful grouping, (d) desirable ter_minology, (e) appro­ priate symbols, (f) sufficient flexibility, and (g) ease of appli­ cation. The AASHO, Unified and FAA systems are compared in light of these requirements. Finally, the equivalent soil groups in each system are correlated on the basis of classifi­ cation procedures as well as pavement design values. •EVERY SOIL body consists of an assortment of solid constituents of various com­ positions and many sizes, a mixture of dilute solutions, and a collection of gases. It is thus a complex system, one which is open rather than closed. The complexity of soil makes it necessary to devise some systematic means so that it can be studied most effectively. Thus, the purpose of soil classification is to group the individual soil units found in nature so that their properties can be easily remembered, and facts about them will be most useful. Another important purpose of a soil classification system is to provide a language by which one engineer's knowledge of the general characteristics of a particular soil can be conveyed to fellow engineers in a brief and concise manner, without the necessity of entering into lengthy descriptions and detailed analyses. In soil investigations for transportation facilities soil classification plays a very important role. Because of the nature of loading conditions, the properties of surficial soils are of greater interest to transportation engineers. In general, the properties of natural soil deposits vary considerably in the horizontal as well as the vertical direc­ tion. Because transportation facilities usually cover very large areas, numerous soil samples must be obtained in order to characterize the soil conditions along the pro­ posed route. It would be impossible to evaluate such a large amount of highly variable samples without some systematic arrangement. Soil classification can fulfill this role admirably. The properties of surficial soils are subject to seasonal variations. In addition, soils are frequently used as material for the construction of transportation facilities. In such case, the in situ properties of the soils are likely to be drastically changed. Thus, classification systems leaning heavily on the composition of the soil are required. Great care should be exercised not to read into various classifications more infor­ mation than they are intended to convey. It is also not good practice to rely too heavily on soil classification as a basis for design, since many of the factors which enter into design cannot possibly be incorporated into an engineering soil classification. For example, the susceptibility of a road or runway to frost damage is a question that Paper sponsored by Committee on Exploration and Classification of Earth Materials and presented at the 45th Annual Meeting. 2 involves not only the properties and characteristics of the soil itself, but climatic and geophysical conditions ·as well. A clear understanding of the relation of soil identification to soil classification is necessary to prevent confusion about many factors involved in soil work. To identify is to distinguish, to classify is to group. It is logical that things must be identified before they can be grouped, or classified, and it is also logical that proper classifica­ tion depends on proper identification. Identification is factual information, whereas classification is interpretative information. Identifications, soil test data and observed or measured soil behavior are factual information because they are the result of ob­ servation or experiment. Factual information does not change with time, but forms a growing body of permanent knowledge. On the other hand, classification is essentially an inference of expected behavior deduced from interpretation of factual information. In the very nature of things, interpretative information must be constantly checked against actual experience and actual soil behavior. It must also be brought up to date with increasing knowledge and understanding. REQUIREMENTS FOR A SATISFACTORY ENGINEERING SOIL CLASSIFICATION SYSTEM Distinct Properties as Basis for Grouping Soils are heterogeneous mixtures of various solids, liquids and gases. Because of the uncontrolled natural conditions under which they Rre formed, there is an infinite variety of natural soils, each with slightly different properties. In order to classify such highly variable objects accurately and precisely, numerous attributes must be used as the basis for grouping. A soil classification system based on a large number of attributes is neither practical nor economical. Thus, a good soil classification should include only a few very distinct attributes for grouping into classes. Each class should include those soils having similar properties and similar general behavior char­ acteristics. Furthermore, the tests required to distinguish the selected attributes should be simple and inexpensive. Logical, Simple and Concise Scheme The classification scheme should be based on a logical sequence that can easily be remembered and will not require constant referencP. to tahlP.R, handhooks or other aids. It should also be simple and concise. Moreover, the number of classes or groups should be held to a minimum consistent wit}_l fue__ c!ifL~!'.enJ_i_~tiq!!_r~_g\!~r~c!_by _~!lg!n~e!i _ng _ problems. It can thus be easily learned and applied not only by experts but also by novices after a short period of training. Since soil investigation is often performed by personnel relatively inexperienced in soil mechanics, the simplest classification sys­ tem consistent with adequate utility appears to be most desirable. Meaningful Grouping Tho <T"1"1"'\HY\inl"I' nf anflc ahn111N fn-rnich <:ln inrlil"i:lHnn nf th~iT" O'AniPl"~l n,-.nnPrtiPA -••- i:,- '-"'.&._,...,..,_t) ..._.,. ,._..'-'.,..,..., ._.. .... ..,; __ --- ....... ...,. .... w•• -••-------•• -- ----- c,-••-- _________hl"n~n J,,. -J, .. _ •-- _ and behavior. It should also permit a crude estimate of permeability, compressibility and shear strength-llie three primary properties of a soil-as well as a rough ap- ._..,.,,..,.,., .... , r..f ,..,,...,;i -nn,.fn,."""'"'"'"o IC'Onnh r,c:, nnn"l't"V'lnf-~hHi+n ~nN T-rnof onc:!f'ICnf-ihHH·u l\Tr'\'t· 'Jill J:'.I.Q,,1.~U.A. V.1. OV.&.... J:,'._,.I...LV.L&&.LU.l&V"' 1-1''4'-'.I.& '4-U VV&.&..a.1-'11,1,"'-'......,""'4'"".&."J -&.&.- ...,.,._,._.., ._._._,...,...,t' .. ...,...,..,...,.., ... J • ... ~....,- .....,.,_ engineers engaged in design and construction can be acquainted in detail with all phases of soils engineering, nor is it necessary that they be. On most jobs a knowledge of the general principles of soil behavior combined with such special information as may be provided by the soil specialists will be sufficient. A usable grouping of soils should be of value in transmitting to the design and construction engineers this information. Moreover, a meaningful grouping should also provide a common language for engineers in various parts of the world in order that experience and knowledge gained in one locality can be applied elsewhere. 3 Desirable Terminology Terminology should consist-of descriptive, easily understood and commonly used terms which convey an idea of the general properties and behavior of the soils. As a rule, soil mechanics specialists do not develop information for their own use. The soil investigation is generally conducted by drillers and technicians. Then the design information and recommendations are supplied to other engineers. Consequently, the terminology used should express the intended meaning and be familiar to all interested parties, from drillers to designers to contractors. Appropriate Symbols Symbols should not replace the descriptive names of soils. However, symbols are useful for graphic abstracts of boring logs and engineering drawings. Where symbols are used as engineering shorthand, they should have a specific meaning and should be descriptive and easy to associate with actual soils. Thus, they can be learned quickly and readily translated into the commonly used descriptive terminology. Coded sym-· bols, which require the use of numbers or letters that are meaningful only to soil specialists, are generally not desirable. Sufficient Flexibility The classification, because of its interpretative character, should be made flexible and adaptable, to permit being checked against experience and being brought up to date frequently with increased knowledge and understanding of soil phenomena. It should also be capable of further subdivision without affecting the basic structure and should make allowance for regional use of certain terms which are particularly meaningful in that region. Ease of Application A classification system should be applicable from visual examination for both field and laboratory purposes. Usually, time will not permit tests to be performed on all soils encountered in the sampling program. Therefore, it becomes necessary to visually classify