INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING
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The Use of the Unified Soil Classification System by the Bureau of Réclamation L’emploi du Système Uniforme de Classification des Sols par le Bureau de Réclamation
by A. A. W agner , Assistant Chief, Earth Laboratory Branch, Bureau of Réclamation, Denver Fédéral Centre, Denver, Colorado, U.S.A.
Sununary Sommaire The Unified Soil Classification System was adopted jointly by the L’emploi du système uniforme de classification des sols a été adopté Bureau of Réclamation and Corps of Engineers in 1952. conjointment par le Bureau de Réclamation et le Corps des Ingénieurs This paper présents a brief discussion of the development, the basic de l’Armée en 1952. principles and procédures for classifying soils by this system, and the Cette communication présente une briève discussion de la méthode, application and use by the Bureau of Réclamation. des principes de base et des méthodes pour la classification des sols, ainsi que son emploi par le Bureau de Réclamation.
Introduction Modem soil mechanics considers soil as a material of con (1) The system is related to the physical properties inhérent struction having engineering properties which can be used in in the soil and not a particular use. Thus, it may be used for design. Soils are a heterogeneous accumulation of minerai ail varieties of engineering problems involving soils. grains and occasionally organic material uncemented or (2) The system is based on soil behaviour which in turn cemented together, and because of the uncontrolled natural reflects the physical properties. conditions under which they are formed, there is an infinité (3) The system recognizes that soil behaviour is a function variety of natural soils, each with slightly différent physical of the amounts and the distribution of the basic constituents properties. Other materials such as concrete, Steel, or iron are common to ail soils; these are: closely controlled during fabrication to provide a limited range (a) Soil components. This is a term given to the solid in the magnitude of their properties. minerai grains which range in size from bulky equidimensional The engineer engaged in the design and construction of grains at least 12 in. in diameter to colloidal size fiake or plate- foundations and earthworks has recognized this peculiarity of like particles. Because of this broad size range, intermediate soils. To reach a practical and economical solution to this size ranges have been established using well-known, common problem, it became apparent that some method of identifying language terms to define each range; i.e. boulders, cobbles, soils and grouping them into classes, which have similar gravel, sand, silt and clay. These are called components physical properties and which exhibit similar général behaviour (Report of Committee VII on Foundation and Soil Mechanics, characteristics, was necessary. Numerous systems have been 1947), see Table 2. The maximum size, distribution of the developed for classifying soils for engineering purposes. Some intermediate sizes, shape and minerai composition (particularly are based on modifications of existing systems such as those of the fine-grained component) affect the behaviour of the soil. used in agriculture, geology or pedology, which were developed (b) Moisture. This includes the amount of free water in the to describe and catalogue soils according to a particular, non- voids and combined moisture present in the soil grains or other engineering property of their mode of déposition and occur substances. rence on the earth. New systems were set up for specialized (c) Other substances. This includes ail other substances, fields of engineering stressing a particular use which in some such as organic material, gases, and minerais which coat the cases applied only to certain limited fractions of the soil. Some grains or act as cementing agents. systems used uncommon, unrelated, locally-coined words and (4) The system establishes 15 soil groups. These groups, phrases to describe soil properties. each of which have distinct engineering properties, are typical In 1946 the Bureau of Réclamation adopted with some of ail soils found in nature. modifications the Airfield Classification which was developed It is recognized that many soils have property characteristics by Professor A. Casagrande for the Corps of Engineers and of two groups if they are close to the border line between the published by them in 1942. Acting on a recommendation by groups. Therefore, for this substantial number of soils, Casagrande, the Bureau of Réclamation in 1952 initiated dis boundary classifications between the typical soil groups are cussions with the Corps of Engineers for the purpose of adopting provided. a system satisfactory to both organizations. With A. Casa This provides a broad scheme of classification, in keeping grande as consultant, they reached agreement on a modification with this variable type of material, rather than a précisé one of the Airfield Classification which they named the Unified Soil which attempts to define the properties of a soil by différences Classification System (1953a, b). of one or two percentage points as determined from a particular test. The Unified Soil Classification System (5) The system provides two methods for determining the The features of the Unified Soil Classification System which amount and type of the basic components in the soil: a simple make it desirable and advantageous for categorizing soils visual and manual method or a laboratory method which uses according to their engineering properties are discussed below : well-known, widely used tests (Book of ASTM Standards, 1955), 125 126
Table 1 Unified soil classification (including identification and description) Tableau de classification unifiée des sols S&.§ £g U O O U < 8 - O p ü ■s ■9 s-s <355 S* »? =/- ij 2 * « « fc,
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T1 w M îï » » > p s o “ ü h g „ t!£ cE - <0ij .S <« « S-S M«û.5?‘5'S — 1„Mrt Ocj> ■•*3— »£«. u “■«'S w.S2*n 2 fcD.d'—fl^c ï-5 y î1 2 ^ s> 9 «eu ! s - t “ «lï-poù di!!^ ®° — «ô 55 o S? m o oVI d •- .52 si M«—1 ^bo C2 OO.Æ ïl ^m S «— ü*S Me c S - ’S 127 Table 2 Soil components and fractions Composantes et fractions du sol j Soil | Grain size range and description | Significant properties Soil component Symbol \ Boulder None Rounded to angular, bulky, hard, rock Boulders and cobbles are very stable components, used for fills, particle, average diameter more than ballast, and to stabilize slopes (riprap). Because of size and 12 in. weight, their occurrence in natural deposits tends to improve the stability of foundations. Angularity of particles increases stability Cobble None Rounded to angular, bulky, hard, rock particle, average diameter smaller £ than 12 in. but larger than 6 in. s: Û Gravel G Rounded to angular bulky, hard, rock Gravel and sand have essentially same engineering properties differ- particle, passing 3-in. sieve (76-2 mm) ing mainly in degree. The No. 4 sieve is arbitrary division, and o retained on No. 4 sieve, (4-76 mm) does not correspond to significant change in properties. They are easy to compact, little affected by moisture, not subject to •5 Coarse 3- to J-in. frost action. Gravels are generally more perviously stable, c résistant to érosion and piping than are sands. The well-graded Fine ^-in. to No. 4 sands and gravels are generally less pervious and more stable than & those which are poorly graded and uniform gradation. Ir- Sand S Rounded to angular, bulky, hard, rock regularity of particles increases the stability slightly. Finer, a particle, passing No. 4 sieve (4-76 uniform sand approaches the characteristics of silt; i.e., decrease mm) retained on No. 200 sieve (0-74 in permeability and réduction in stability with increase in moisture mm) Coarse No. 4 to 10 sieves Medium No. 10 to 40 sieves Fine No. 40 to 200 sieves Silt M Particles smaller than No. 200 sieve Silt is inherently unstable, particularly when moisture is increased, (0-74 mm) identified by behaviour; with a tendency to become quick when saturated. It is rela- that is, slightly or non-plastic regard- tively impervious, difficult to compact, highly susceptible to frost less of moisture and exhibits little or heave, easily erodible and subject to piping and boiling. Bulky no strength when air dried grains reduce compressibility; flaky grains, i.e., mica, diatoms, increase compressibility, produce an ‘elastic’ silt 1 Clay C Particles smaller than No. 200 sieve The distinguishing characteristic of clay is cohésion or cohesive o (0-74 mm) identified by behaviour; strength, which increases with decrease in moisture. The per that is, it can be made to exhibit meability of clay is very low, it is difficult to compact when wet "S plastic properties within a certain and impossible to drain by ordinary means, when compacted is 5 range of moisture and exhibits con résistant to érosion and piping, is not susceptible to frost heave, sidérable strength when air dried is subject to expansion and shrinkage with changes in moisture. 5 The properties are influenced not only by the size and shape, fiat, £ plate-like particles, but also by their minerai composition; i.e., the type of clay-mineral, and chemical environment or base 1 exchange capacity. In général, the montmorillonite clay minerai has greatest illite and kaolinite the least adverse effect on the properties Organic O Organic matter in various sizes and Organic matter present even in moderate amounts increases the matter stages of décomposition compressibility and reduces the stability of the fine-grained com ponents. It may decay causing voids or by chemical altération change the properties of a soil, hence organic soils are not desirable for engineering uses N o te.—Names and définitions of soil components have been suggested by others (1953a). The names with the exception of clay, which is called clay soil, and the size range given in the table are being considered jointly by the American Society for Testing Materials and the American Society of Civil Hngineers The symbols and fractions were developed for the Unified Classification System. For field identification, i in. is assured équivalent to the No. 4, and the N o. 200 is defined as, ‘about the smallest particle visible to the unaided eye\ The sand fractions are not equal divisions on a logarithmic plot; the No. 10 wasselected because of the significance attached to that size by some investigators. The No. 40 was chosen because the 'Atterberg limits’ tests are performed on the fraction o f soil finer than the N o. 40 see (‘Field Identification Procédures’ and ‘Laboratory Classifi Both methods are based on gradation and plasticity. They cation Criteria’) Classification Chart, Table 1. The maximum vary only in degree of accuracy. Thus, the objective of field size, amount, distribution, and shape of the particles in the classification is primarily to assign to a soil one of the 15 soil coarse-grained soil components and the amount of the fine- group names and, in addition, to distinguish that soil from grained soil components are estimated by visual examination others in the same group by descriptive words and phrases. or accurately determined by a gradation test (Book of ASTM This is well within the accuracy of the method, and a high Standards, 1955); and the type of fine-grained component is degree of proficiency can be attained in placing a soil in the determined by estimating the plasticity by the ‘Field Identifi proper group and describing it by the visual and manual cation Procédures for Fine-grained Soils or Fractions’ (see methods. The laboratory method places the soil in the proper Dilatancy, Dry Strength and Toughness Tests), Table 1, or by group and distinguishes the soil from others in the same group performing Soil Consistency Tests, Book of ASTM Standards, by test data. 1955. (6) The system uses names and symbols to distinguish 128 Table 3 Description of soils Description des sols Borrow Foundation Items o f descriptive data Coarse-grained Fine-grained Coarse-grained Fine-grained soils soils soils soils Typical name (examples are shown in classifi cation (chart) XX XX XXXX Approximate percentages of gravel and sand X X Maximum size of particles (including cobbles and boulders) XX X Shape of the coarse grains—angularity X X Surface condition of the coarse grains— coatings X Hardness of the coarse grains—possible- breakdown into smaller sizes X X Colour (in moist condition for fine-grained soils) XX XX Moisture and drainage conditions (dry, moist, wet, saturated) XX XXXXXX Organic content X XXX Plasticity (of fine fraction in coarse-grained soils; degree and character for fine-grained soils) XXX X XX Amount and maximum size of coarse grains X X Structure (stratification, etc., give dip and strike; honey comb, flocculent, root holes) XXXX Cementation—type XXXX Degree of compactness—loose or dense (ex- cepting clays) XXXX Consistency in undisturbed and remoulded states (clays only) XX Local or géologie name XX X X Group symbol XXXXXXXX between the typical and boundary soil groups. These symbols Locally coined words, geological, or pedological terms and are simple and derived from the ternis descriptive of the soil phrases are often helpful in localized areas and may be given components, gradation, and liquid limit. These are as follows : in addition to but not as a substitute for the required description. (8) The system provides for the classification and description Components of soils both for construction and for use in-place as a Boulders—none Silt—M Cobbles—none Clay—C foundation. Gravel—G Organic—O (9) The system is not restrictive. It is recommended and Sand—S Peat—Pt advisable to use ail available information, géologie, pedological or other, to establish the aerial extent, occurrence, and in-place Gradation Liquid limit conditions of the différent types or strata of soil in a foundation Well graded—W High liquid limit—H Poorly graded—P Low liquid limit—L or proposed material source. However, classification by this system is necessary to define the potential engineering properties. These are combined to form the group symbols which corre The addition of detailed descriptive information or précisé test spond to the names of typical soils, Table 1. The same symbols data to define the properties of a particular soil in greater détail are used in combination to indicate boundary soils. will not adversely affect the classification, but will add to it and (7) Adequate description of a soil is of extreme importance. make it more précisé. In addition to assigning to a soil one of the group names and Field classification (visual classification)—This procédure does Symbol of the typical soils, the soil should be distinguished from not require spécial equipment; however, a rubber syringe or other soils in the same group by the use of descriptive words small oil can, a supply of clear water, a small bottle of dilute and phrases. This description should be accurate and précisé hydrochloric acid, a classification chart, and familiarity with but expressed in simple engineering and technical terms. the descriptive information required, Table 3, will facilitate the 129 Table 4 Data forms of field classification by visual and manual methods » Tableau ou fiche de classification par méthodes visuelles et manuelles (a) Sack samples Gradation Description and soil classification Identification {estimated) 1 J ) J. Descriptive classification 2. Particle size, shape and gradation (uniformly, Colour well, poorly graded, etc.) (wet (wet state) Sand Gravel sample sample No. 3. Consistency, elasticity, etc. Laboratory plus plus No. 4) 4. Reaction to shaking test, dry strength, etc. minus minus No. 200) Silt and Silt and clay Depth Depth feet in Group Group symbol Maximum Maximum size (% Excavation Excavation No. Field Field sample No. y0 No. y0 4 to No. 200 (% 16Y- 1 3 0-3-5 i in. 5 70 25 Brown Sand, very fine, silty; non-plastic; contains a few SM angular gravel particles 2 3 35-6-5 50 0 40 60 Tan Clay, lean, sandy; sand particles coarse, CL rounded ; moderately tough at plastic limit 3 5 0-6-5 l i in. 15 80 5 Tan Sand, coarse to fine, well graded, clean; sand is SfV sub-rounded, gravel is angular 4 5 6-5-10 1 in. 10 50 40 Grey Sand, clayey; medium to fine sand; contains s c gravel-size shale fragments; clay portion moderately plastic 5 14 0-9-5 100 0 5 95 Tan Silt, non-plastic; contains slight amount of very ML fine sand 6 14 9-5-14-5 100 0 15 85 Tan Clay, lean, silty; slightly sandy; slight to CL moderate plasticity when wet (Loess) 7 14 14-5-19-0 8 in. 30 60 10 Tan Sand, well graded; silty fines SfV (Approximately 10 per cent oversize 3 to 8 in. estimâtes made in field) (b) Undisturbed 12-in. cube samples Gradation Identification Description— undisturbed Soil classification (iestimated) state 1. Consistency— very soft, 1. Descriptive classi soft, firm, hard, very hard, fication sticky, brittle, friable, 2. Particle size spongy shape, gradation Colour 2. Structure — stratified, (uniformly, well, (wet (wet state) Sand Gravel sample sample No. Laboratory varved, single grain, etc. poorly graded, plus plus No. 4) 3. Cementation and moisture etc.) Depth Depth feet in Silt and clay Group Group symbol Maximum Maximum size (% (% Excavation Excavation No. 4. General géologie descrip 3. Consistency, elas Field Field sample No. yQ No. 4 to No. yQ 200) '“/ minus No. 200) tion ticity, etc. 16Y- 8 207 1 35-7-36-7 No. 50 0 30 70 Blue and Topôin.: soft very moist; Top 6 in.: sand, SP tan bluish-grey sand with slightly clayey, minor fat clay stringers; fine poorly porous structure; odorous graded Bottom 6 in.; firm, moist; Bottom 6 in.: silt, ML tan non-plastic 9 207 2 44-3-45-3 No. 200 0 0 100 Brown Soft, very moist, homogene- Clay, fat ; very CH ous ; butterlike consist tough at plastic ency limit; sticky when wet ,0 207 3 46-1-47-0 2 in. 30 60 10 Brown Firm, dense, fairly well Sand, fairly well s w cemented ; appears per- graded, rounded; vious; calcareous; (Ogal- gravelly lalla) 130 work. Data sheets, Table 4, are recommended for training should be noted so that their effect on the physical properties and may be used as a guide. and possible construction problems can be evaluated. A person can be taught to classify soils by following the The rest of the procédure is, in effect, a process of simple simple rules and procédures without a technical background of élimination, beginning at the left side of the Classification Chart, soils. However, a knowledge of the factors and conditions Table 2, and working to the right until the proper group name is which influence soil behaviour and experience in performing obtained. While performing these steps, pertinent descriptive the consistency and gradation tests combined with practical information should be noted. Table 3 shows items which training, including comparisons of visual estimâtes with should always be reported (XX) and those which may be laboratory test results, are desirable pre-requisites to accurate, reported depending upon use (X); see examples, Tables 1, 3, 4 adequate field classification. and 5. A représentative disturbed or undisturbed sample (to describe (1) Spread the sample on a fiat surface or in the palm of the in-place conditions for foundations) is required. hand to aid in observing the various components. Classify The size of the largest particle is estimated. Then the the soil as coarse-grained or fine-grained by criteria in left- boulders and cobbles, particles larger than 3 in., are removed hand column on chart. and the amount, percentage by weight, in the total sample is (2) If fine-grained, see (6) below. If coarse-grained, classify estimated. (See Sample No. 7, Table 4a and 5b.) The am ount as gravel or sand by the criteria shown on the chart. of oversized material may be of importance in the selection of (3) If gravel or sand, classify as ‘clean’ or ‘with appréciable sources for em bankment material. The occurrence and estimate fines’. Fines are defined as the fraction smaller than the 200 of the percentage of boulders and cobbles in a foundation mesh sieve size. Table 5 Examples of field classification by visual and manual methods Exemples de classification par méthodes visuelles et manuelles LOG OF TEST PIT OR AUGER HOLE for borrow and foundation investigations Feature: Cachuma Dam Project: Cachuma Hole No.: TP-101 Area Désignation: Spillway Coordinates: N17, 210, E2, 600 Cround Elévation: 779-4 Depth To Ground Method o f Approximate Dimensions Dates o f Water Level *: Not reached Excavation: Hand-dug pit o f Hole: 4 x 5 ft. Excavation: 5-2 to 6-17-49 Hole Logged by: L. E. Garst Percentage o f cobbles and bouldersf Size and Classification and description o f material symbol Depth type o f (See chart—unified soil classification Volume o f Weight o f Percentage Weight o f Percentage ft. sample give géologie and in-place description hole 3 to 5-in. by volume plus 5-in. by volume taken for foundation investigations) sampled sampled o f 3 to sampled o f plus Letter Graphie (eu. ft.) (lb.) 5-in.{ (lb.) 5-in.î SM 150-lb. Silty Sand: gravelly; about 25 per cent 3 ~ sack gravel and cobbles to 12-in. maximum, — 3 in. coarse grains are medium hard to friable, rounded sandstone; about .5 20 per cent non-plastic fines; dry; alluvial fan material 3-1 8 1-6 73 150 1 GW-GC 300-lb. Gravel with sand-clay binders: well-grated ; sack about 50 per cent gravel; 30-in. maxi- .g — 3 in. mum-size boulder; sub-rounded sand £ stone gravel, cobbles and boulders, Q somewhat weathered; about 10 per c cent of plastic fines; dry; alluvial fan material 15-2 246 10 3 256 10-8 1 '5 o-JU 20— 5 ï; 300-lb. Well-graded gravel: clean; about 30 per GW sack cent sand; maximum size 30 in.; hard — 3 in. sub-rounded gravel, cobbles and 25— boulders; dry; alluvial fan material 6-2 82 8-4 137 14-1 Reraarks : Average specific gravity of cobbles and boulders 2-51 by displacement. Samples obtained from sampling trench. Notes:—Record water test and density lest data, if applicable, under remarks. . ______(Lb. of rock sampled) 100______* Record after water has reached its natural level; give date of reading adjacent to graphie symbol or (Bulk specific gravity of rock) 62-4 (Cubic feet o f hole sampled) in remarks. t Applicable only to borrow pits and to foundations which are potential sources of construction Record bulk specific gravity in remarks, stating how obtained, materials. measured or estimated) (a) Log of Test pit—Materials Investigation. 131 T able 5— continued LOG OF TEST PIT OR AUGER HOLE for borrow and foundation investigations Feature: Pumping Plant Project: Example Hole No.: TP-2 Area Désignation: P. E. 4 Coordinates: Sta. 39 + 45 Ground Elévation: 911 Depth To Ground Method of Excavation: Hand Approximate Dimensions Dates o f Water Level * : None of Hole: 4 x 4 ft. Excavation: 10-1 to 15-54 Hole Logged by: M. N. i Percentage o f cobbles and bouldersf Classification Classification and description o f material symbol j Depth type o f (see chart—‘ unified soil classification ’ Volume o f Weight o f Percentage Weight o f Percentage ! (ft.) sample give géologie and in-place description hole 3 to 5 in. by volume plus J-in. by volume taken for foundation investigations) sampled sampled o f 3 to sampled o f plus Letter jGraphie (eu. ft.) (lb.) 5-in.t (lb.) 5-in.î Y - ✓ '-V Clay lean: Dark brown, very moist, ' ' y, greater than (PL) medium plasticity, ’/ ' ' maximum size No. 40, 50% fines and ; CL ' 50% smaller than No. 200 Clayey Gravel: 40% gravel, sub-rounded and Hat 5 — hard particles, maximum size 6 in., 30% sand, 4 « * 30% clayey fines, moderate plasticity, wet, com / 1 l ' Und’d. 25 J cube from 33 ft. See foolnoies on preuious page Remarks: Two undisturbed cubes and disturbed sample sent to the Denver Laboratory. (b) Log of Test pit—Foundation Investigation (4) If the gravel or sand is clean, décidé if it is well graded classifications which are common are: For the coarse-grained ( W) or poorly graded (P) and assign an appropriate group name gravelly soils—GW-GC, GW-GM, GW-GP, GM-GC— and symbol GW, GP, S fV or SP. similarly for sands; for fine-grained soils—ML-MH, CL-CH, (5) If the gravel or sand contains appréciable fines, it is OL-OH, CL-ML, ML-OL, CL-OL, MH-CH, MH-OH and classified as GM, GC, S M or SC depending upon the type of CH-OH soils ; and for boundary classifications between coarse- fines—silty (M) or clayey (C)—which are identified by the pro and fine-grained soils—SM-ML and SC-CL. cédure for fine-grained soils; see (6) below. (9) Miscellaneous tests and criteria may be developed to (6) For fine-grained soils or the fine-grained fraction of a identify the occurrence of other substances and constituents, coarse-grained soil the ‘dilatancy’, ‘dry strength’, and ‘tough- such as Acid test, réaction when dilute hydrochloric acid is ness’ tests are performed in accordance with the instructions applied indicates presence of calcium carbonate, and Shine test, given at the bottom of the chart. The group name and shiny surface when dry or moist soil eut with a knife indicates symbol are arrived at by selection of that group the character- plastic clay. Heating sometimes intensifies organic odours. istics of which most nearly compare to that of the sample. Laboratory classification—The same descriptive information Depending upon the organic content, the fine-grained soil or as required for field classification is also necessary. Instead of the fine-grained fraction of the coarse-grained soil is classified estimating gradation and plasticity, laboratory tests, gradation, as low to medium plasticity silt (ML), clay (CL), or organic silt liquid limit and plastic limit (1955) are performed. The criteria or clay (OL); i.e. the liquid limit is estimated to be less than for classifying coarse-grained soils is given in the right-hand 50 per cent; or elastic sût (MH) plastic clay (CH), or organic column of the chart, ‘Laboratory Classification Criteria’. The silt or clay (OH), i.e. liquid limit is estimated to be more than plasticity chart is used to classify the fine-grained soils or the 50 per cent. fine-grained fraction of coarse-grained soils. Différentiation (7) Highly organic soils are classified as peat (Pt). These between coarse- and fine-grained soils and between gravels and are identified by colour, odour, spongy feel and fibrous mixture. sands is as given for the visual method; i.e. the 50 per cent (8) Soils which have characteristics of two groups, either criterion. because of percentage of the coarse-grained components or Application and use—The value of this system lies in the basic plasticity characteristics, are given boundary classification using purpose for which it was devised, namely to classify soils a name most nearly describing the soil, and the two group according to their potential physical or engineering properties. symbols connected by a hyphen, such as GW-GC. Boundary It is not intended to delineate precisely a soil for a spécifié 132 Table 6 Engineering Use Chart Propriétés des sols à l’usage des ingénieurs 1 S- 0 -§ * “*3 *s <5 <3 3 S ! « * > « S! •SU P 3 3 ' t | l 1:1? “<3 S fc &* P ^o-e 2 5-a 2 j §'^ g e f s ? 8 5 . t i 5 i.§^S i.§^S 5 i l è ü *o .ü ûi c5 * fc-5 g ^ § ; *•*s; 5 a * O^ O*§^ 0 Ci, 00 S 5 | >S § j o > ; 5 * c tj 5r Çj ' <3 ^ S C S’ S? ’S 'S è I é, 3-g** 3-g** é, O «4, CO « l p 3 $ S.è? C §p l I « S § 8 t S > S Ct v «O ■v* § i i i 1 § U " o . § s .§ £ Ô tsi 3 t? 3 ? g Srg Srg g .5 § S § § * w O 6\ § §, s c *5 o. -s en^- vo n -v ^ n ^e —< n ^ | en ’5 _D û en ’Sb S *o TJ '>*>*> o o o o bo o o D 2 9 2 CO CO 1/5 O bü O f) u o u § > t & a a a E-. I_l t-l P 3 P ) _ c a) T3.S o ü •*-* Ot2 O bo o > îîa vo ’âb [3) .o à CO 8 O O X c u G d cd «d fc) â ^*S ^*S •OS •OS T u u *T3 PL, a« -a w o § g ^ .ti^ > Q> G rt o fl *G T3 53 0 É) cd 60 cd O O bo co O in 2 o .2 a >< -a •a U M U W F - • ; *; § « « § O2 H G w ^ >,73 cdT3 c g,s H 9 CO O U 4-t c i* f O. a 3 r, O O P 3 o 3 c ‘"'«H co cd 3 ’O G D g ^3 o û i cd c U 4>(U o - r o o ‘S M T3 l ~ O m bo o bo OJ o O *o ■• J2 w co S O G t i O O «i g *o *o bQ o O in Û v 3-g 3 o cd u > d Cd rS>> d y s i o c ùDG c ü cdç-P, p ..a TJ bû O O y t-H O CU VO co \S m >>*w O »» T3 5 ü m - • c 2 - •a p< O O a o o e o. a a 3 'c 2 . >. O u. bO cd h 133 purpose. This is the function of the soils engineer who than 8 per cent fines. The sandy soils SM and SW-SC require evaluates a particular soil for a particular purpose in terms of spécial considération, and their suitability depends on the its past history, the present conditions, and anticipated future gradation of the sand and the plasticity of the fines. Some SM conditions imposed by the construction and opération of an soils with fines as high as 16 per cent have been found to be engineering structure. suitable. Other sandy soils SW-SM, SP-SM and SP-SC (5 The laboratory method as such is seldom if ever used to to 12 per cent fines, see classification chart) are suitable. classify soils. If subséquent laboratory testing of a soil, which The adoption of this system has eliminated misunderstand- has been previously classified by the visual and manual method, ings between the field and office, which has resulted in savings includes gradation and consistency tests, the classification may of time and costs of design and construction. The soils in be checked and adjustments made if it appears to be in error. formation on logs of exploration holes in which the soils have The laboratory method is used to check and help ‘calibrate’ been properly classified are usually adequate for final designs students while training to classify soils by the visual and manual of minor structures and for preliminary designs and estimâtes method. of major structures. For major structures field classification Table 6, called an Engineering Use Chart, shows four im must be supplemented by laboratory tests to determine the portant properties of the soil groups. Based on these properties spécifié properties of the soil under the conditions imposed by and experience, the relative desirability of the typical soils for the construction and opération of the structure. use in rolled-earth dams, canal sections, foundations, and Accurate logging and adequate soil classification reduce in roadways is also shown. The numerical ratings, No. 1 is best, vestigation costs. The results of detailed laboratory tests or are intended as a guide in comparing soils for various purposes. field tests can be applied to the same material in other parts of The chart as presented serves as a guide for those who are not a borrow or foundation area, or testing can be limited to the specialists but who need to know the potential properties of most critical, représentative, or competent soil strata as soils to form opinions during preliminary investigations for required. feasibility estimâtes, and to convey to the soils engineer pre A knowledge of the typical soils and their properties is par liminary soils information. For those who are technically ticularly helpful during the initial or reconnaissance stage of qualified, the chart is quite flexible; that is, it can be extended investigation when prospecting for earth materials for a par to specialized uses and can be expanded by the use of boundary ticular use or selecting possible sites for structures. classifications. Ail samples submitted to the laboratory are classified by the The relative desirability of materials for rolled-earth dams, visual field method before testing. This provides descriptive both the homogenous embankment and for core, would be a information not obtained from laboratory tests, and in those boundary soil, well-graded gravel with clay fines, GW-GC, to cases when test results do not agree with the classification incorporate the shear strength of the gravel with sufficient clayey further investigations are made to determine if the soil contains fines to make the soil impervious. The relative desirability of unusual constituents, or if the testing procédures have produced materials for homogenous embankment construction would be certain irréversible effects, or finally if the soil was incorrectly GW-GC, GC, GM-GC, GM, SW-SC, SC— etc. classified. Visual classification helps to catalogue soils for For compacted earth canal lining, permeability is the first discussions with geologists and designers when developing considération, with stability and érosion résistance also im detailed laboratory testing programmes. It also aids in select portant; therefore, the relative desirability of materials would ing specimens which are most représentative of the samples be GW-GC, GC, SW-SC, SC, SC-CL, CL, GM. Other less submitted for laboratory testing. Classification serves as a desirable materials, such as sandy silts, SM, could be satis- guide for initial estimâtes of quantifies of materials to be factory for lining if covered to prevent érosion. Obviously, the combined to form a blended material for a canal lining or depth requirement of slope stability and workability applies for ail to be excavated in a borrow area to produce a desired material. types of lining materials. The adoption of this system has simplified the préparation of The relative desirability of soils for use as backfill around spécification drawings. The simplicity of the system and the structures depends upon backfill requirements. Résistance to use of common language terms to define the soil components consolidation is usually the primary considération. F or im and the basic 15 typical soil groups make it easily understood pervious backfill, permeability would also be considered ; thus, by those engaged in the design and construction of earthworks in addition to the impervious gravelly soiJs, finer soils are also and foundations. satisfactory, such as SW-SC, SC, CL, SC-CL, SM-SC, SC- ML —etc. If pîfeVious backfill is required, the materials are Référencés limited to the coarser grained soils, such as GP, GW, SP and Book of ASTM Standards, Part 3: (1955). SW soils. Grain Sized Analysis o f Soil, ASTM Désignation: D 422-54T, Where high shear strength and résistance to consolidation are pp. 1756 required, pervious materials, densified by wetting and vibration, Liquid Limit o f Soils, ASTM Désignation: D 423-54T, pp. 1769 are excellent for backfilling, particularly when in restricted or Plastic Limit and Plasticity Index of Soils, ASTM Désignation: in places otherwise difficult to compact; for example, backfill D 424-54T, pp. 1774 Report of Committee VII on Foundation and Soil Mechanics (1947). to form bedding and backfill for a pipe. Small percentages of Civil Engineering Bulletin, Vol. 12, No. 2 fines, particularly if the material is well graded, reduces the Unified Soil Classification System (1953a). Technical Mémorandum permeability and adversely affects densification by this method. No. 3-357, Office Chief of Engineers, Waterways Experiment It is obvious that the coarse-grained soils GW, GP, S W and SP Station, Vicksburg, Mississippi, Vol 1; Appendix A, Vol. 2; and (less than 5 per cent fines, see classification chart) are suitable. Appendix B, Vol. 3 Unified Soil Classification System (1953b). A supplemental to the Other soils such as borderline gravelly soils GW-GM, GW-GC, Earth Manual, U.S. Department of the Interior, Bureau of Ré GP-GM and GP-GC are suitable providing they contain less clamation, Denver, Colorado 134