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no. 3-35J App. (^) A App. B

TECHNICAL MEMORANDUM NO. 3-

THE UNIFIED SOIL CLASSIFICATION SYSTEM

APPENDIX A

CHARACTERISTICS OF SOIL GROUPS PERTAINING TO

EMBANKMENTS AND FOUNDATIONS

APPENDIX B

CHARACTERISTICS OF SOIL GROUPS PERTAINING TO

ROADS AND AIRFIELDS

April 1960 (Reprinted May 1967)

Sponsored (^) by

Office, Chief of Engineers

U. S. Army

Conducted by

U. S. Army Engineer Waterways Experiment Station

CORPS OF ENGINEERS

Vicksburg, (^) Mississippi ARMY-MRC VICKSBURG. MISS.

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED

iii

Contents

Preface. (^)........... Introduction........ The Classification System (^).... Discussion of Coarse-grained (^) Soils Discussion of Fine-grained (^) Soils Discussion of Highly (^) Organic Soils Identification (^) of Soil Groups... General Identification (^)..... Laboratory (^) Identification.. (^).. Expansion of (^) Classification... (^). Descriptive Soil (^) Classification.. Tables 1- Plates 1-

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UNIFIED SOIL CLASSIFICATION SYSTEM

Introduction

Need for a classification system

1. The adoption of the principles of soil mechanics by the engi- neering profession has inspired numerous attempts to devise a simple classification system that will tell the engineer the properties of a given soil. As a consequence, many classifications have^ come^ into^ exist- ence based on certain properties of soils such as texture, plasticity, strength, and other characteristics. A few classification systems have gained fairly wide acceptance, but it is seldom that any particular sys- tem has provided the complete information on a soil that the engineer needs. Nearly every engineer who practices soil mechanics will add judgment and personal experience as modifiers to whatever soil classifi- cation system he uses, so that it may be said that there are as many classification systems as there (^) are engineers using them. Obviously,

within a given agcncy, where designs and plans are reviewed by persons entirely removed frcm a project, a common basis of soil classification is necessary so that when an engineer classifies a soil as a certain type, this classification (^) will convey to another engineer not familiar with the region the proper (^) characteristics and behavior of the material. Further than this, the classification should reflect (^) those behavior characteris- tics of the soil that are pertinent to the (^) project under consideration. Basis of the unified soil classification system

2. The unified soil classification system is based on the

embankments, foundations, roads, and airfields, is treated separately in appendices hereto which will be issued as the need arises. It is rec- ognized that the unified classification system in its present form may not prove entirely adequate in all cases. However, it is intended that the classification of soils in accordance with this system have^ some^ de- gree of elasticity, and that the system not be followed blindly nor re- garded as completely rigid. Definitions of soil components

4. Before soils can be classified properly in any^ system,^ includ- ing the one presented in this manual, it is necessary to establish a basic terminology for the various soil components and to define the terms used. In the unified soil classification the names "cobbles," "gravel," "sand," and "fines (silt or clay)" are used to designate the size ranges of soil particles. The gravel and^ sand^ ranges^ are^ further^ subdivided into the groups presented below. The limiting boundaries between the various size ranges have been arbitrarily set at certain U. S. Standard sieve sizes in accordance with the following tabulation: Component Size Range Cobbles Above 3 in. Gravel 3 in. to No. 4 (4.76 mm) Coarse gravel 3 in. to 3/4 in. Fine gravel 3/4 in. to No. 4 (4.76 mm) Sand No. 4 (4.76 mm) to No. 200 (0.074 mm) Coarse sand No. 4 (4.76 mm) to No. 10 (2.0 mm)

Medium Fine sand sand No. 10 (2.0 mm) to No. 40 (0.42 mm)

No. 40 (0.42 mm) to No. 200 (0.074 mm)

Fines (silt or clay) Below No. 200 (0.074 mm)

These ranges are shown graphically on the grain-size sheet, plate 1. In

the finest (^) soil component (below No. 200 sieve) the terms "silt" and "clay" are used respectively to distinguish materials exhibiting lower plasticity from those with higher plasticity. The minus No. 200 sieve material is "silt" if the liquid limit and plasticity index plot below the "A" line on the plasticity chart (plate 2), and is "clay" if the liquid limit and plasticity index plot above (^) the "A" line on the chart (all Atterberg limits tests based on minus No. 40 sieve fraction of a soil). The foregoing definition holds for inorganic silts and clays and for organic silts, but is not valid for organic clays since these latter soils plot (^) below the "A" line. The names of the basic soil com- ponents can be used as nouns or adjectives in the name (^) of a soil, as explained later.

The Classification System

5. A short discussion of the unified soil classification sheet, table 1, (^) is presented in order that the succeeding detailed description may be more easily understood. (^) This sheet is designed to apply gener- ally to the identification of soils regardless of the intended (^) engineer- ing uses. The first three columns of the classification sheet show the major divisions of the classification and the group symbols that distin- guish the individual (^) soil types. Names of typical and representative soil types found in each group (^) are shown in column 4. The field proce- dures for identifying soils by general characteristics (^) and from perti-

nent tests and visual observations are shown in column 5. The desired

descriptive information for a complete identification of a soil is pre-

sented in column 6. In column 7 are presented the laboratory

sieve. Borderline cases may be classified as belonging to both groups. The gravel (G) and sand (S) groups are each divided into four secondary groups as follows: a. Well-gradedGroups GW and material SW. with little or no fines. Symbol W.

b. Poorly-graded material with little or no fines. Symbol P. Groups GP and SP. c. Coarse material with nonplastic fines or fines with low plasticity. Symbol M. Groups GM and SM. d. Coarse material with^ plastic^ fines.^ Symbol^ C.^ Groups^ GC and SC.

8. Subdivisions, fine-grained soils. The fine-grained soils are subdivided into groups based on whether they have a relatively low (L) or high (H) liquid limit. These two groups are further subdivided as follows: a. Inorganic silts and very fine sandy soils; silty or clayey fine sands; micaceous and diatomaceous soils; elastic silts. Symbol M. Groups ML and MH. b. Inorganic clays. Symbol C. Groups CL and CH. c. Organic silts and clays. Symbol O. Croups OL and OH.

Discussion of Coarse-grained Soils

GW and SW groups

9. These groups comprise well-graded gravelly and sandy soils having little or no nonplastic fines (less than 5 per cent passing the No. 200 sieve). The presence of the fines must not noticeably change

the strength characteristics of the coarse-grained fraction and must not

interfere with its free-draining characteristics. If the material con-

tains less than 5 per cent fines that exhibit plasticity, this

7 information should be evaluated and the soil classified (^) as discussed sub- sequently under "Laboratory Identification." In areas subject to (^) frost action, the material should not contain more than about 3 per cent of soil grains smaller than 0.02 mm in size. Typical examples of GW and SW soils are shown on plate (^) 3. GP and SP groups

10. Poorly-graded gravels and sands containing little or no non- plastic fines (less than 5 per cent passing the No. 200 sieve) are classed in the GP and SP groups. The materials may be classed as uniform gravels, uniform sands, or nonuniform mixtures of very coarse material and very fine sand, with intermediate sizes lacking (sometimes called skip-graded, gap-graded, or step-graded). The latter group often results from borrow excavation in which gravel and sand layers are mixed. If the fine fraction exhibits plasticity, (^) this information should be evaluated and the soil classified as discussed subsequently under (^) "Laboratory Identification." Typical examples of various types of GP and SP soils are shown on plate 4. GM and (^) SM groups
11. In general, the (^) GM and SM groups comprise gravels or sands with fines (more than 12* per cent passing the No. (^200) sieve) having low or no plasticity. The plasticity index and liquid limit (based on minus No. (^) 40 sieve fraction) of soils in the group should plot below the "A" line on

* In the preceding two paragraphs soils of the GW, GP, SW, and SP

groupssieve. wereSoils defined which haveas having between less 5 andthan 125 perper centcent passing the No. 200

sieve are classed as "borderline" passing the^ No.^^200

under that heading. and^ are^ discussed^ in^ paragraph^33

9

predominantly silty materials and micaceous or diatomaceous soils.^ The symbols L and H represent low and high liquid limits, respectively, and an arbitrary dividing line^ between^ the^ two^ is^ set^ at^ a^ liquid^ limit^ of

50. The soils in the ML and MH groups are sandy silts, clayey silts, or inorganic silts with relatively low^ plasticity.^ Also^ included^ are loess-type 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 illite clays having relatively low plas- ticity. Typical examples^ of^ soils^ in^ the^ ML^ and^ MH^ groups^ are^ shown on plate 7. CL and CH groups 14. In these groups the^ symbol^ C^ stands^ for^ clay,^ with^ L^ and^ H denoting low or high liquid limit. The soils are primarily inorganic clays. Low plasticity clays are classified as CL and are usually lean clays, sandy clays, or silty clays. The medium and high plasticity clays are classified as CH. These include the fat clays, (^) gumbo clays, certain volcanic clays, and bentonite. The glacial clays of the northern United States cover a wide band in the CL and CH groups. Typical exam- ples (^) of soils in these groups are shown on plate 8. OL and (^) OH groups
51. The soils in the OL and OH groups are characterized by the presence of organic matter, hence the symbol O. Organic silts and clays

are classified in these groups. The materials have a plasticity range

that corresponds with the ML and MHB groups. Typical examples of OL and

OH soils are presented on plate 9.

Discussion of Highly Organic Soils

Pt group

16. The highly organic soils usually are very compressible and

have undesirable construction characteristics. They are not subdivided

and are classified into one group with the symbol Pt. Peat, humus, and

swamp soils with a highly organic texture are typical soils of the

group. Particles of leaves, grass, branches, or other fibrous vegetable

matter are common components of these soils.

Identification of Soil Groups

17. The unified soil classification is so arranged that most soils

may be classified into at least the three primary groups (coarse grained,

fine grained, and highly organic) by means of visual examination and

simple field tests. Classification into the subdivisions can also be

made by visual examination with some degree of success. More positive

identification may be made by means of laboratory tests on the materials.

However, in many instances a tentative classification determined in the

field is of great benefit and may be all the identification that is

necessary, depending on the purposes for which the soils in question are

to be used. Methods of general identification of soils are discussed

in the following paragraphs, and a laboratory testing procedure is pre-

sented. It is emphasized that the two methods of identification are

never entirely separated. Certain characteristics can only be estimated

by visual examination, and in borderline cases it may be necessary to

verify the classification by laboratory tests. Conversely, the field

The particles are first thoroughly oven- or sun-dried, then submerged in water for at least 24 hours, and finally their strength is tested and compared with the original strength. Some types of shales will com- pletely disintegrate when subjected to such a slaking test.

20. Examination of fine fraction. Reference (^) to the identification sheet (table 1) shows that classification criteria of the various coarse- grained soil groups are based on the amount of (^) material passing the No. 200 sieve and the plasticity characteristics of the binder fraction (passing (^) the No. 40 sieve). Various methods may be used to estimate the percentage of material passing the No. (^200) sieve; the choice of method will (^) depend on the skill of the technician, the equipment (^) at hand, and the time available. (^) One method, decantation, consists of mixing the soil with water in a suitable container and pouring off the turbid mix- ture of water and fine soil; successive decantations will remove prac- tically (^) all of the fines and leave only the (^) sand and gravel sizes in the container. A visual comparison (^) of the residue with the original (^) material will give some idea of the amount (^) of fines present. Another useful (^) meth- od is to (^) put a mixture of soil and water in a (^) test tube, shake it thor- oughly, (^) and allow the mixture to settle. (^) The coarse particles will fall to the (^) bottom and successively finer particles (^) will be deposited with increasing time; (^) the sand sizes will fall out (^) of suspension in 20 to 30 seconds. If the assumption (^) is made that the soil weight (^) is proportional to its volume, this method (^) may be used to estimate the (^) amount of fines

present. A rough estimate of the amount of fines may be made by spread-

ing the sample out on a level surface and making a visual estimate of

the percentage of fine particles present. The presence of fine sand can

usually be detected by rubbing a sample between the fingers; silt^ or^ clay particles feel smooth and stain the fingers, whereas the sand feels gritty and does not leave a stain. The "teeth test"^ is^ sometimes^ used^ for^ this purpose, and^ consists^ of^ biting^ a^ portion^ of the^ sample^ between^ the teeth. Sand feels^ gritty^ whereas^ silt^ and^ clay^ do^ not;^ clay^ tends^ to stick to the teeth while silt does not. If there appears to be more than about 12 per cent of the material passing the No. 200 sieve, the sample should be separated as well as possible by hand,^ or^ by^ decanta- tion and^ evaporation,^ removing^ all^ of^ the^ gravel^ and^ coarse^ sand,^ and the characteristics of the fine fraction determined. The binder is mixed with water and its dry strength and plasticity characteristics are examined. Criteria for dry strength are shown (^) in column 5 of the clas- sification sheet, table 1; evaluation^ of^ soils^ according^ to^ dry^ strength and plasticity criteria is discussed in succeeding (^) paragraphs in connec- tion with fine-grained soils. Identification of active cementing agents other than clay usually is not possible by visual and manual examination, since such agents may require a curing period-of days or even weeks. In the absence of such experience the soils should be classified tentatively into their apparent groups, (^) neglecting any possible development of strength because (^) of cementation. Fine-grained (^) soils

21. The principal procedures for field identification of fine- grained soils are the test for dilatancy (reaction to shaking), the

examination of plasticity characteristics, and the determination of dry

strength. In addition, observations of color and odor are of value,

particularly for^ organic^ soils.^ Descriptions^ of^ the^ field^ identification

particularly of the rock-flour type, also for diatomaceous earth (MH). The reaction (^) becomes somewhat more sluggish with decreasing uniformity of gradation (and increase in plasticity up to a certain degree). Even a slight content of colloidal clay will impart to the (^) soil some plasticity and slow up materially the reaction to the shaking test. Soils which react in this manner are somewhat plastic inorganic and organic silts (ML, OL), very lean clays (CL), and some kaolin-type clays (ML, MH). Ex- tremely slow or no reaction to the shaking test is characteristic of all typical clays (CL, CH) as well as of highly plastic organic clays (OH).

23. Plasticity characteristics. Examination of the plasticity characteristics of fine-grained soils or of the fine fraction of coarse- grained soils is made with a small moist sample of the material. Parti- cles larger than about the No. 40 sieve size are removed (by hand) and a specimen of soil about the size of a 1/2-in. cube is molded to the con- sistency of putty. If the soil is too dry, water must be added and if it is sticky, the specimen (^) should be spread out in a thin layer and allowed to lose some moisture (^) by evaporation. The sample is rolled by hand on a smooth surface or between the palms into (^) a thread about 1/8 in. in diameter. The thread is then folded and rerolled repeatedly. (^) During this manipulation the moisture content is gradually reduced and the speci- men (^) stiffens, finally loses its plasticity, and crumbles when the plastic limit is reached. After (^) the thread crumbles, the pieces should be lumped together and a slight kneading action continued until the lump crumbles.

The higher the position of a soil above the "A" line on the plasticity

chart, plate 2 (CL, CH), the stiffer are the threads as their water con-

tent approaches the plastic limit and the tougher are the lumps as the

soil is remolded (^) after rolling. Soils slightly above the "A" line (CL, CH) form a medium tough thread (easy to roll) as the plastic limit is approached but when the threads are formed into a lump and^ kneaded^ below the plastic limit, the soil crumbles readily. Soils below the "A" line (ML, MH, OL, OH) form a weak thread and, with the exception of the OH soils, cannot be lumped together into a coherent mass below the plastic limit. Plastic soils containing organic material or much mica (well below the "A" (^) line) form threads that are very soft and spongy near the plastic limit. (^) The binder fraction of coarse-grained soils may be ex- amined in the same manner (^) as fine-grained soils. In general, the binder fraction of coarse-grained soils with (^) silty fines (GM, SM) will exhibit plasticity (^) characteristics similar to the ML soils, and that of coarse- grained soils with clayey fines (GC, (^) SC) will be similar to the CL soils.

24. Dry strength. The resistance of a piece of dried (^) soil to crushing by finger pressure (^) is an indication of the character of the colloidal (^) fraction of a soil. To initiate (^) the test, particles larger than (^) the No. 40 sieve size are removed from the (^) soil (by hand) and a specimen (^) is molded to the consistency of putty, (^) adding water if neces- sary. The moist (^) pat of soil is allowed to dry (in oven, (^) sun, or air) and is then crumbled (^) between the fingers. Soils with slight (^) dry strength crumble (^) readily with very little finger (^) pressure. All nonplastic (^) ML and MH soils have almost (^) no dry strength. Organic (^) silts and lean organic clay of low plasticity (OL), (^) as well as very fine sandy (^) soils (SM), have

slight dry strength. Soils of medium dry strength require considerable

finger pressure to powder the sample. Most clays of the CL group and

some OH soils exhibit medium dry strength. This is also true of the fine