Unified Soil Class

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Unified Soil Classification System B-1

Appendix B

The Unified Soil Classification System

The adoption of the principles of soil mechanics by the engineering

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, man y clas sifica tions ha ve come into existence

based on certain properties of soils such as texture, plasticity, strength,

a nd other chara cteristics. A few class ifica tion syst ems ha ve ga ined fairly

wide acceptance, but rarely has any system provided the complete

informa tion on a soil th a t the engineer needs. Near ly every engineer wh o

practices soil mechanics will add judgment and personal experience as

modifiers to wh a tever soil clas sifica tion syst em he uses. Obviously, wit hina given agency (where designs and plans are reviewed by persons entirely

removed from a project) a common basis of soil classification is necessary

so that when an engineer classif ies a soil as a certain type, this

classification will convey the proper characteristics and behavior of the

ma terial . Furth er tha n this, the classif icat ion should reflect those

behavior characteristics of the soil that are pertinent to the project under

consideration.

BASIS OF THE USCS

The USCS is based on identifying soils according to their textural and

plast icity qua lities a nd on their grouping with respect to beha vior. Soils

seldom exist in nature separately as sand, gravel, or any other single

component . They ar e usually found a s mixtures with va rying proport ions of

par ticles of different sizes; each component par t contribut es its char a cteristics

to the soil mixture. The US CS is based on th ose chara cteristics of the soil tha t

indica te how it will behave as an engineering construction materia l. The

following properties have been found most useful for this purpose and form

th e basis of soil identifica tion. They can be determined by simple tests an d,

w ith experience, can be estima ted w ith some a ccura cy.

Pe rcentages of g ravel , sand , and f ines (f ract ion pass ing the No. 200

sieve).

Shape of the g ra in-s ize-d ist r ibut ion curve.

P las t ici ty and compress ib il ity chara cte r is t ics . In the US CS, the soi l i sgiven a descriptive name and a letter symbol indicating its principal

characteristics.

PURPOSEAND SCOPE

I t is the purpose of th is appendix to describe the va rious soil groups in deta il

and to discuss the methods of identification so that a uniform classification

procedure may be follow ed by all wh o use the syst em. P lacement of th e soils

RETURNTO TOC
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B-2 Unified Soil Classification System

FM 5-472/NAVFAC MO 330/AFJMAN 32-1221(I)

into their respective groups is accomplished by visual examination and

labora tory tests a s a means of ba sic identifica tion. I t is recognized tha t the

USCS in i ts present form may not prove entire ly adequate in al l cases.

However, it is int ended tha t t he clas sificat ion of soils a ccording to th is system

ha ve some degree of ela sticity a nd t ha t t he system not be follow ed blindly nor

regarded a s completely r igid.

DEFINITIONSOF SOIL COMPONENTS

Before soils can be classified properly in any system, including the one

presented in t his ma nua l , it is necessa ry t o esta bl ish a basic terminology for

the var ious soi l component s an d to def ine the terms used. In th e USC S, the

terms cobbles, gravel, sand, and fines (silt or clay) are used to designate the

size ra nges of soi l part icles. The gravel a nd san d ra nges are further

subdivided into the groups as presented in Table B-1. The limit ing

boundaries between the various size ranges have been arbi trari ly se t at

certain US standard sieve sizes as l is ted in Tabl e B-1. In the f inest soil

component (below the No. 200 sieve), the terms silt and clay are used

respectively to dist inguish ma terials exhibi t ing lower plast ici ty f rom those

wit h higher pla sticity. The minus No. 200 sieve ma teria l is silt if the LL a nd

P I plot below t he A l ine on the plast ici ty cha rt a nd is c lay i f the LL a nd P I

plot a bove the A l ine on t he cha rt (al l LL a nd P L tests a re based on minus

No. 40 sieve fra ction of a soil). The foregoing definit ion holds for inorga nic

si l ts and clays and for organic si l ts but is not val id for organic clays since

th ese la tt er soils plot below th e A line. The na mes of th e basic soil

components can be used as nouns or adjectives when describing or

classifying a soil .

THE CLASSIFICATION SYSTEM

In its simplest form, Fi gure B-1i l lustrates the process of the classification

system. The following pa ra gra phs provide deta iled informa tion on the soil

properties and groups as t hey perta in to the system.

A short discussion of the USCS procedures (see Fi gure B-1, page B-3) is

presented so that the succeeding detailed description may be better

understood. The procedures ar e designed to apply generally to th e

Table B-1. Soil particle-size ranges

Component Size Range

Cobbles Above 3 inches

Gravel

Coarse

Fine

3 inches to No. 4 sieve

3 inches to 3/4 inch

3/4 inch to No. 4 sieve

Sand

Coarse

Medium

Fine

No. 4 to No. 200 sieves




Fines (clay or silt) Below No. 200 sieve (no minimum size)


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Figur

eB-1.USCSprocedures

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ident ifica tion of soils r egardless of th e intended engineering uses. Tabl e B-2,

pages B-6 and B -7, a lso assists in identifying the symbols a nd soil descriptions

within this system. Fi gure B-1shows the schematic method of classifying

soils from th e results of labora tory test s. Columns 1 through 5 of Tabl e B-2,

pages B -6 and B -7identify the three major divisions of the classification

system and the group symbols that distinguish the individual soil types.Nam es of typical a nd representa tive soil types found in ea ch group ar e show n

in column 6.

SOIL GROUPSAND GROUP SYMBOLS

Soils are primarily identified as coarse grained, f ine grained, and highly

organic. On a textura l basis, coarse-gra ined soils are those tha t ha ve 50

percent or more by w eight of th e overa ll soil sa mple reta ined on th e No. 200

sieve; f ine-grained soils are those that have more than 50 percent by weight

pa ssing th e No. 200 sieve. High ly-orga nic soils a re, in genera l, rea dily

identified by visua l examina tion. The coa rse-gra ined soils ar e subdivided into

gra vel a nd gra velly soils (G ) a nd sa nds a nd sa ndy soils (S). Fine-gra ined soils

ar e subdivided on t he basis of their LL an d plast icity properties; the symbol L

is used for soils with LLs of 50 an d less an d th e symbol H for soils with LLs in

excess of 50. Pea t a nd other highly organ ic soils are designat ed by the symbol

Pt and are not subdivided.

In genera l pra ctice there is no clear-cut boundar y betw een gra velly soils and

sa ndy soils an d, a s fa r a s behavior is concerned, the exact point of division is

relat ively unimporta nt. For identifica tion purposes, coa rse-gra ined soils are

classified as G if the greater percentage of the coarse fraction (that which is

reta ined on the No. 200 sieve) is lar ger th a n th e No. 4 sieve. They a re cla ssed

as S if the greater portion of the coarse fraction is f iner than the No. 4 sieve.

B orderline ca ses may be classified a s belonging to both groups. The G a nd S

groups ar e each divided int o four secondar y groups a s follows:

Well-graded mat erial with l i t t le or no finessymbol W, groups GWa nd S W.

Poor ly graded mat er ia l wi th li t t le or no f inessymbol P, groups GPand SP.

Coarse mate r ia l wi th nonplas t ic f ines or fines wi th low plas t ici tysymbol M, groups GM a nd SM.

C oa r s e ma te r ia l w i th p la s t i c f ines s y mb ol C, g roups G C a nd S C .

The fine-gra ined soils ar e subdivided int o groups ba sed on wh ether t hey ha ve

a relat ively low (L) or high (H) LL. These two groups a re furth er subdivided

as follows :

Inorganic s i lt s and very f ine sandy soi ls , s il ty or clayey f ine sands ,

micaceous a nd d iat omaceous soils, a nd elastic siltssymbol M, groupsML and MH.

I nor g an ic cl ay ss y mb ol C , g r oups C L and C H .

O r gan i c s il t s and cl ay ss y mb ol O, gr oups O L and OH .


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Coarse-Grained Soils

In t he following pa ra gra phs, soils of the G W, G P, SW, an d SP groups are

defined as ha ving less tha n 5 percent pa ssing th e No. 200 sieve. Soils which

have between 5 and 12 percent passing the No. 200 sieve are classed as

borderline and will be discussed lat er in this a ppendix.

GW and SW Groups

These groups comprise w ell-gra ded gra velly a nd sa ndy soils ha ving litt le or n o

nonpla st ic fines (less tha n 5 percent pa ssing t he 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

chara cteristics. I f the materia l conta ins less tha n 5 percent fines tha t exhibit

plasticity, this information should be evaluated and the soil classified and

discussed subsequently under Laboratory Identification. In areas subject to

frost a ction, the ma teria l should not conta in more tha n 3 percent of soil grains

sma ller t ha n 0.02 millimeter in s ize.

GP an d SP G r o u p s

Poorly-graded gravels and sands containing little or no nonplastic f ines (less

th an 5 percent pa ssing the No. 200 sieve) a re classed in th e GP an d SP groups.

The materials may be classed as uniform gravels, uniform sands, or

nonuniform mixtures of very coarse material and very fine sand, with

intermediat e sizes la cking (sometimes ca lled skip gra ded, gap gra ded, or st ep

gra ded). The latt er group often results from borrow excava tion in which

gra vel a nd sa nd layers a re mixed. If the fine fra ction exhibits plast icity, this

information should be evaluated and the soil classified as discussed

subsequently under Laboratory Identification.

GM an d SM G r o u p s

In general, the GM and SM groups comprise gravels or sands with fines (more

th a n 12 percent pass ing th e No. 200 sieve) ha ving low or no plasticity. The P Ia nd LL of soils in th e group should plot below th e A line on t he plast icity chart .

The gradation of the materials is not considered significant and both well- and

poorly graded ma terials ar e included. Some of the sands a nd gra vels in this

group will ha ve a binder composed of na tur a l cementing a gents, so proport ioned

th a t th e mixtur e shows negligible swelling or shrinka ge. Thus, the dry strengt h

of such ma teria ls is provided by a sm all a mount of soil binder or by cementa tion

of calcareous ma teria l or iron oxide. The fine fra ction of other ma teria ls in the

G M a nd S M groups ma y be composed of silts or r ock-flour t ypes ha ving litt le or

no plasticity, and the mixture will exhibit no dry strength.

GC and SC Groups

In general, the GC and SC groups comprise gravelly or sandy soils with fines

(more than 12 percent passing the No. 200 sieve) which have either low or

high pla sticity. The P I a nd LL of soils in th e group should plot above the A

line on the plast icity char t. The grada tion of the mat erials is not considered

significa nt a nd both w ell- a nd poorly gra ded ma teria ls are included. The

plast icity of the binder fra ction h a s more influence on the beha vior of th e soils

th an does var iat ion in gra da tion. The fine fra ction is genera lly composed of

clays.


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NOTES: 1. Values in columns 7 and 11 are for guidance only. Design should be based on actual test results.2. The equipment listed in column 9 will usually produce the desired densities with a reasonable number of passeswhen moisture conditions and thickness of lift are properly controlled.3. The range of dry unit weights listed in column 10 are for compacted soil at OMC when using the StandardProctor Test (ASTM 1557-91).

Symbols Permeabilitycm per sec

(8)

Major Divisions(1) (2)

Letter(3)

Hatching(4)

Color(5)

Name(6)

Value forEmbankments (7)

GW

GP

GM

GC

SW

SP

SM

SC

ML

CL

OL

MH

CH

OH

Pt

Red

Yellow

Yellow

Red

Green

Blue

Orange

Graveland

GravellySoils

Sandand

SandySoils

Siltsand

ClaysLL < 50

Silts

andClaysLL > 50

Highly OrganicSoils

Coarse-Grained

Soils

Fine-Grained

Soils

Well-graded gravels or gravel-sand mixtures, little or no fines

Poorly graded gravels or gravel-sand mixtures, little or no fines

Very stable, pervious shells ofdikes and dams

k > 102

k = 103

to 106Silty gravels, gravel-sand-siltmixtures

Clayey gravels, gravel-sand-clay mixtures

Well-graded sands or gravellysands, little or no fines

Poorly graded sands orgravelly sands, little or no fines

Silty sands, sand-silt mixtures

Clayey sands, sand-siltmixtures

Inorganic silts and very finesands, rock flour, silty or clayeyfine sands or clayey silts withslight plasticity

Inorganic clays of low to mediumplasticity, gravelly clays, sandyclays, silty clays, lean clays

Organic silts and organic silt-clays of low plasticity

Inorganic silts, micaceous ordiatomaceous fine sandy orsilty soils, elastic silts

Inorganic clays of highplasticity, fat clays

Organic clays of medium tohigh plasticity, organic silts

Peat and other highly organicsoils

Reasonably stable, perviousshells of dikes and dams

Reasonably stable, notparticularly suited to shells,but may be used forimpervious cores or blankets

Fairly stable, may be used forimpervious core

Very stable, pervious sections,slope protection required

Reasonably stable, may beused in dike section with flatslopes

Fairly stable, not particularlysuited to shells, but may be usedfor impervious cores or dikes

Fairly stable, use forimpervious core or flood-controlstructures

Poor stability, may be used forembankments with propercontrol

Stable, impervious cores andblankets

Not suitable for embankments

Poor stability, core of hydraulic-fill dam, not desirable in rolled-fill construction

Fair stability with flat slopes,thin cores, blankets and dikesections

Not used for construction

k > 102

Not suitable forembankments

k = 106

to 108

k > 103

k > 103

k = 106

to 108

k = 103

to 106

k = 103

to 106

k = 106

to 108

k = 104

to 106

k = 104

to 106

k = 106

to 108

k = 106

to 108

Table B-2. Characteristics of soil groups pertaining to embankments and foundations


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Max Dry Unit WeightStd Proctor (pcf)

(10)

Requirementsfor SeepageControl (12)

CompactionCharacteristics

(9)

Value forFoundations (11)

Good; tractor, rubber-tired, orsteel-wheeled roller

Good; with close control; rubber-tired or sheepsfoot roller

Fair; rubber-tired or sheepsfootroller

Good; tractor

Good with close control; rubber-

tired or sheepsfoot roller

Fair; sheepsfoot or rubber-tiredroller

Good to poor; close controlessential; rubber-tired orsheepsfoot roller

Fair to poor; sheepsfoot orrubber-tired roller

Fair to poor; sheepsfootroller

Poor to very poor; sheepsfootroller

125 -135 Good bearing value

Good to poor bear-ing value depending ondensity

Very poor, susceptibleto liquefaction

Fair to poor bearingvalue, may have ex-cessive settlements

Poor bearing value

Fair to poor bearingvalue

Very poor bearingvalue

Positive cutoff

Toe trench to none

None

Upstream blanket andtoe drainage or wells



None

Toe trench to none

None

None

None

None

None

Good bearing valuePositive cutoff


Compaction not practical Remove from foundations


Good; tractor

110 -130

100 -120

110 -125

105 -125

95 -120

95 -120

80 -100

70 - 95

75 -105

65 - 100

Good bearing value

Good to poor bearing

value depending ondensity

Good to poor bear-ing value

Good to poor bear-ing value

Fair to poor; sheepsfootroller

Poor to very poor; sheepsfootroller

115 -125Good; tractor, rubber-tired, orsteel-wheeled roller

Table B-2. Characteristics of soil groups pertaining to embankments and foundations

(continued)


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Fine-Grained Soils

The following pa ra gra phs discuss fine-gra ined soils in their su bgroupings:

ML a n d MH G r ou p s

In t hese groups, the symbol M has been used to designa te predomina nt ly silty

ma teria ls a nd mica ceous or diat omaceous soils. The symbols L and H

represent low and high LLs, respectively, and an arbitrary dividing line

between the tw o is set at a n LL of 50. The soils in the ML and MH groups are

sa ndy silts, clay ey silts, or inorgan ic silts w ith relat ively low pla sticity. Also

included a re loess-ty pe soils a nd rock flours. Micaceous a nd dia toma ceous

soils generally fall within the MH group but may extend into the ML group

wh en their LL is less tha n 50. The same is true for certa in types of kaolin

clays a nd some elite clay s ha ving relat ively low pla sticity.

CL a n d CH G r o u p s

In these groups, the symbol C stands for clay, with L and H denoting low or

high LL. These soils are primarily inorga nic clay s. Low-plast icity clay s are

classified as CL a nd a re usually lean, sa ndy, or silty clays. The medium a ndhigh plast icity clays a re classified as CH . These include th e fat clays, gumbo

clay s, certa in volcanic clays, a nd bentonite. The glacial clay s of the north ern

US cover a wide band in the CL and CH groups.

OL a n d OH G r ou p s

The soils in the OL and OH groups are characterized by the presence of

organic mat ter, hence the symbol O. Orga nic silts a nd clays a re classified in

these groups. The mat erials ha ve a plast ic ity r an ge that corresponds w ith th e

ML an d MH groups.

Highly-Organic Soils

The highly-organic soils usua lly a re very compressible and ha ve undesirable

const ruct ion cha ra cterist ics. They ar e classified into one group, designa ted bythe symbol Pt . Peat , humus, and sw a mp soi ls with a highly-organ ic texture

ar e typical soils of the group. Pa rticles of leaves, gra ss, branches, or other

fibrous vegeta ble mat ter ar e common components of these soils.

IDENTIFICATIONOF SOIL GROUPS

The USCS is arranged so that most soi ls may be classi f ied into at least the

three primary groups (coarse grained, f ine grained, and highly organic) by

mean s of visua l exam inat ion and simple field tests. Cla ssifica tion into the

subdivisions can also be made by visual examination with some degree of

success. More positive identifica tion may be made thr ough laborat ory testing.

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 t he purposes for wh ich t he soils in q uestion ar e to be used. The

general or field-identification methods as well as the individual laboratory

test methods ar e a ll explained in grea t deta il in Cha pter 2. I t is empha sized

tha t the two methods of identifica tion are never entirely separa ted. Cert a in

cha ra cteristics ca n only be estima ted by visual exam inat ion. In borderline

cases, it may be necessary to verify the classification by laboratory tests.

Conversely, th e field methods ar e entirely practical for prelimina ry la borat ory


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identification and may be used to an advantage in grouping soils in such a

ma nner tha t only a minimum number of la bora tory tests need be run.

LABORATORYIDENTIFICATION

Identifying soils in the laboratory is done by determining the gradation and

plast icity char acterist ics of the ma teria ls. The grada tion is determined by sieveanalysis, and a grain-size curve is usually plotted as percent finer (or passing)

by weight a gainst a logar ithmic scale of grain size in millimeters. DD Form

1207 is typica lly used for th is purpose. P last icity char act eristics a re evalua ted

by mea ns of the LL a nd P L tests on t he soil fraction finer th a n t he No. 40 sieve.

The laborat ory test procedures for the LL an d P L determina tion can be found in

Section IV of Ch a pter 2.

MAJ OR SOIL GROUPS

In the la borat ory-identifica tion procedures shown in Fi gur e B-1, page B-3, the

first st ep in identifying a soil is to determine w hether it is coarse gra ined, fine

gra ined, or highly orga nic. This may be done by visual examina tion in most

cases. In some borderline ca ses, as w ith very-fine sands or coar se silts, i t ma y

be necessary to screen a representa tive dry sa mple over a No. 200 sieve an d

determine the percenta ge pa ssing. Fifty percent or less passing th e No. 200

sieve identifies t he soil as coarse gra ined, an d more tha n 50 percent identifies

th e soil a s fine gra ined. The percenta ge limit of 50 ha s been selected

arbitrarily for convenience in identification, as it is obvious that a numerical

difference of 1 or 2 in th is percenta ge will ma ke no significa nt change in t he

soils beha vior. After the major group is esta blished, th e ident ificat ion

procedure is continued according to the proper headings in Fi gure B-1.

Coarse-Grained Soils

A complete sieve an a lysis must be run on coar se-gra ined soils an d a gra da tion

curve plott ed on a gra in-size chart . For some soils conta ining a subst a ntia l

amount of f ines, it may be desirable to supplement the sieve analysis with ahydrometer analysis to define the gradation curve for particle sizes smaller

tha n the No. 200 sieve size. P relimina ry identifica tion is made by

determining t he percenta ge of mat erial in t he gra vel (a bove No. 4 sieve) and

sa nd (No. 4 to No. 200 sieve) sizes. If th ere is a grea ter percenta ge of gra vel

tha n sa nd, the mat erial is c lassed as G; i f there is a great er percenta ge of sand

tha n gravel, the ma teria l is classed as S. Once a ga in, th e distinction between

th ese groups is purely ar bitra ry for convenience in following t he system. The

next step is to determine the amount of material passing the No. 200 sieve.

Since the subgroups are th e same for gravels an d sa nds, they w ill be discussed

jointly in th e followin g para gra phs.

GW, SW, GP, an d SP G r oup s

These groups comprise nonpla stic soils ha ving less th a n 5 percent passing theNo. 200 sieve and in which the fine fraction does not interfere with the soils

free-dra ining propert ies. I f the above criteria a re met, a n examina tion is

ma de of the shape of the grain-size curve. Ma teria ls tha t a re well gra ded are

cla ssified a s G W or SW; poorly graded ma teria ls ar e classified as G P or SP.

A soil s gradation curve and curve data should meet the following

qua lifications to be clas sed as w ell grad ed:


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The grain-size distr ibutions of well-graded materials general ly plot as

smooth a nd r egular concave curves w ith no sizes lacking or no excess

of material in a ny size range .

The coef fi ci en t o f uni for m ity (C u) of well-graded gravels is greater

tha n 4 a nd of well-gra ded sa nds is great er th a n 6. The Cu

is

determined by dividing the gra in-size dia meter pa ssing a t 60 percent

by th e grain-size dia meter pas sing at 10 percent.

The coef fi ci en t of cur va tur e (C c) mus t be betw een 1 a nd 3. The C c is

determined by th e follow ing formula:

where

D30= grai n d iam eter at 30 percent passing

D60= grai n d iam eter at 60 percent passing

D10

= grai n d iam eter at 10 percent passing

The C c ensures th at the gra ding curve wil l have a concave curvat ure within

relatively narrow limits for a given D 60 and D 10 combina tion. All gra da tions

not meeting t he foregoing criteria a re clas sed as poorly gra ded. Thus, poorly

graded soi ls (GP and SP) are those having nearly straight- l ine gradations,

convex gradat ions, nearly vert ical grada tions, a nd hump gra dat ions t ypical

of skip-gra ded ma teria ls.

NOTE: In the preceding paragraph, soils of the GW, GP, SW, and SPgroups were defined as having less than a 5 percent fraction passingthe No. 200 sieve. Soils having between 5 and 12 percent passing theNo. 200 sieve are classed as borderline and are discussed later.

GM, SM , GC and SC Gr oups

The soils in th ese groups a re composed of those mat erials ha ving more tha n a

12 percent fr a ction passin g the No. 200 sieve. They ma y or may n ot exhibit

plast icity. For identificat ion, the LL an d PL t ests ar e required on the fra ction

finer tha n t he No. 40 sieve. The tests sh ould be run on r epresenta tive sam ples

of moist ma teria lnot on a ir- or oven-dried soils. This precaut ion is desira ble

as drying a ffects th e limits va lues to some extent , as w ill be explained further

in the discussion of f ine-gra ined soils. Ma teria ls in which the LL a nd P I plot

below the A line on the plasticity chart (see Fi gur e 2-54, page 2-100) are

classed a s GM or SM. Gr avels an d sands in which the LL and P I plot a bove

the A line on the pla sticity cha rt a re classed a s GC or SC. I t is considered

that in the identification of materials in these groups, the plasticity

characteristics overshadow the gradation characteristics; therefore, no

distinction is made between w ell- a nd poorly gra ded mat erials.

Bo rde r l i n e Soi l s

Coarse-grained soils containing between 5 and 12 percent material passing

the No. 200 sieve are classed as borderline and carry a dual symbol (for

exam ple, GW-G M). Sim ilar ly, coar se-gra ined soils ha ving less tha n 5 percent

passing t he No. 200 sieve but w hich ar e not free draining, or wh erein the fine

D30

( )2

D60

D10

-------------------------- betw een 1 and 3=


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fra ction exhibits plasticity, ar e also classed a s borderline and a re given a dua l

symbol.

Fine-Grained Soils

Once the identity of a f ine-grained soil has been established, further

identification is accomplished principally by the LL and PL tests inconjunction wit h the pla sticity cha rt. The plast icity chart is a plot of LL

versus P I on w hich is imposed a diagona l line called the A line and a vertical

line at a LL of 50. The A line is defined by th e equa tion P I = 0.73 (LL-20).

The A line above a liquid limit of about 29 represent s a n importa nt empirica l

boundary between typical inorganic clays (CL and CH), which are generally

located above the line and plastic soils containing organic colloids (OL and

OH) or inorgan ic silty soils (ML a nd MH ). The vertica l line at a n LL of 50

separa tes silts a nd clays of low LL (L) from those of high LL (H). In t he low

part of the chart below an LL of about 29 and in the range of PI from 4 to 7,

th ere is considera ble overlapping of the properties of the clayey a nd silty soil

types. Hence, the separa tion betw een CL a nd OL or ML soil types in this

region is a ccomplished by a cross-ha tched zone on t he plast icity chart between

4 and 7 P I a nd a bove th e A line. The CL soils in th is region a re those having

a P I a bove 7 while OL or ML soils are those having a P I below 4.

Soils plotting within the cross-hatched zone should be classed as borderline.

The various soil groups are shown in their respective positions on the

plast icity cha rt . Experience ha s shown tha t compressibility is a bout

proportional to the LL a nd t hat soi ls having the sa me LL possess a bout equal

compressibility (a ssuming tha t other fa ctors a re essent ially the same). On

comparing the physical characteristics of soils having the same LL, you find

that with increasing the PI , the cohesive characterist ics increase and the

permeability decreases. From plots of the results of limits tests on a num ber

of samples from t he sa me fine-gra ined deposit, i t is found t ha t for most soils

these points l ie on a stra ight l ine or in a na rrow band t ha t is almost para l lel to

th e A line. With t his background informa tion in mind, th e ident ifica tion ofthe various groups of fine-grained soils is discussed in the following

paragraphs .

ML , CL , a n d OL G r ou p s

A soil ha ving a n LL of less th a n 50 fa lls into th e low LL (L) group. A plot of

the LL a nd P I on th e plast ici ty chart wil l show w hether the soil fa l ls above or

below t he A line a nd cross-ha tched zone. Soils plott ing a bove th e A line

and cross-hatched zone are classed as CL and are usually typical inorganic

clays. Soils plot tin g below the A line or cross-ha tched zone a re inorgan ic

silts or very fine sandy silts (ML) or organic silts or organic silt-clays of low

plas ticity (OL). Sin ce tw o groups fall below t he A line or cross-ha tched zone,

furth er ident ifica tion is necessary. The distinguishing factor betw een th e ML

a nd OL groups is the a bsence or presence of organ ic ma tt er. This is usua lly

identified by color a nd odor. How ever, a comparison ma y be made betw een t he

LL a nd P L of a m oist sa mple a nd one tha t ha s been oven-dried.

An orga nic soil will show a ra dica l drop in plast icity aft er oven- or air-drying.

An inorga nic soil will generally show a cha nge in th e limits values of only 1 or

2 percent, w hich may be either a n increase or a decrease. For t he foregoing

reasons, the classification should be based on the plot of limits values


12/28



determined before drying. Soils cont a ining orga nic ma tt er genera lly ha ve

lower specific gravities and may have decidedly higher water contents than

inorganic soils; t herefore, these propert ies may be of assista nce in identifying

organic soils. In special cases, determining the organ ic cont ent ma y be ma de

by chemica l meth ods, but th e procedures just described a re usua lly sufficient.

MH , CH , a n d OH G r o u p s

Soils wit h a n LL great er tha n 50 a re classed in group H. To identify such

soils, the LL an d PI va lues ar e plott ed on the plast icity char t. I f the points fall

above the A line, the soil classifies as CH; if they fall below the A line, a

determination is made as to whether or not organic material is present (as

described in the preceding pa ra gra ph). Inorga nic ma teria ls are classed a s MH

and orga nic mat erials a re classed as OH.

H i g h l y -O r g a n i c Soi l s

Little more can be said as to the laboratory identification of highly-organic

soils (P t) tha n ha s been identified in the field-identifica t ion procedur es. These

soils are usually identified readily on the basis of color, texture, and odor.

Moisture determinations usual ly show a natural water content of several

hundr ed percent , wh ich is fa r in excess of tha t found for most soils. Specific

gra vities of the solids in th ese soils may be quit e low. Some pea ty s oils ca n be

remolded and t ested for the LLs and P Ls. Such materials usual ly have an LL

of several hundred percent and fall well below the A line on the plasticity

char t .

Borderline Classifications

I t is inevitable in the use of the classification system that soils will be

encountered tha t fa ll close to the boundar ies esta blished between th e various

groups. In a ddition, boundar y zones for th e a mount of ma teria l passing th e

No. 200 sieve and for the lower part of the plasticity chart have been

incorporat ed as a par t of the system, a s discussed subsequently. The acceptedrule in clas sifying border line soils is to use a double symbol (for exam ple, G W-

G M). It is possible, in rare inst an ces, for a soil to fa ll into more th a n one

borderline zone and, if appropriate symbols were used for each possible

classification, the result should be a multiple designation consisting of three

or more symbols. This a pproa ch is unnecessa rily complica ted, an d it is

considered best to use only a double symbol in t hese cases, selecting th e tw o

tha t a re believed most r epresenta tive of the probable behavior of th e soil . In

cases of doubt, the symbols representing the poorer of the possible groupings

should be used.

Coa rse-Gr a i ned Soi l s

In pr evious discussions, the coarse-gra ined soils were class ified in the G W, G P,

SW, an d SP groups if they cont ained less tha n 5 percent of ma teria l passingthe No. 200 sieve. Similar ly, soils were cla ssified in th e GM, G C, SM, a nd S C

groups if they ha d more tha n 12 percent passing the No. 200 sieve. The ra nge

between 5 and 12 percent passing the No. 200 sieve is designated as

borderline. Soils falling with in it a re as signed a double symbol depending on

both the gradation characteristics of the coarse fraction and the plasticity

cha ra cteristics of the minus No. 40 sieve fra ction. For exam ple, a w ell-gra ded

sa ndy soil with 8 percent passing t he No. 200 sieve, a LL of 28, a nd a P I of 9


13/28



w ould be designat ed as SM-SC. Another type of borderline classifica tion

occurs for those soils conta ining a ppreciable a mounts of f ines (groups G M, G C,

SM, and SC) and whose LL and PL values plot in the lower portion of the

plast icity cha rt. The method of classifying t hese soils is the sa me as for fine-

grained soils plotting in the same region, as presented in the following

paragraph.

F i n e -G ra i n ed Soi l s

Discussion ha s been presented of a zone on t he plast icity cha rt below a LL of

a bout 29 a nd ra nging betw een P I values of 4 a nd 7. Several soil types

exhibiting low pla sticity plot in t his general region on the plast icity cha rt, a nd

no definite bounda ry betw een silty a nd clayey soils exists. Thus, if a f ine-

gra ined soil , groups CL a nd ML, or the m inus No. 40 sieve fraction of a coar se-

gra ined soil (groups GM, G C, SM, a nd S C) plots w ithin t he cross-ha tched zone

on the pla sticity cha rt, a double symbol (such as ML-CL) is used.

Note that in the descriptive name of the soil type as indicated on Tabl e B-2,

pages B-6 and B-7, silty a nd clayey m ay be used to describe silt or clay soils.

Since the definitions of these terms are now somewhat different from thoseused by many soils engineers, it is considered advisable to discuss their

connotat ion as used in this system. In the US CS, the terms si lt a nd clay a re

used to describe those soils with LLs a nd P Ls plotting respectively below a nd

a bove the A line and cross-ha tched zone on the plast icity cha rt. As a logica l

extension of this concept, t he terms silty a nd clayey ma y be used as a djectives

in th e soil na mes wh en the limits va lues plot close to th e A line. For

example, a clay soil wit h a n LL of 40 a nd a P I of 16 may be called a silty clay.

In general, the adjective silty is not applied to clay soils having an LL in

excess of a bout 60.

Expan si o n o f C l a ssi f i c a t i o n

In some cases, it may be necessary to expand the USCS by subdividing

existing groups to cla ssify soils for a par ticular use. The indiscrimina te use ofsubdivisions is discouraged and careful study should be given to any soil

group before adopting such a st ep. In a ll cases, subdivisions should be

designat ed preferably by a su ffix to an existing group symbol. The suffix

should be selected carefully so there will be no confusion with existing letters

tha t a lready ha ve meanings in the classi f ica t ion system. In each ca se where

an existing group is subdivided, the basis and criteria for the subdivision

should be explained so that anyone unfamil iar with i t may understand the

subdivision properly.

Descriptive Soil Classification

At many stages in the soils investigation of a projectfrom the preliminary

boring log to the final reportthe engineer finds it convenient to give the soils

he is working with a name rather than an impersonal c lassi f icat ion symbol

(such as GC ). This results prima rily from the fact tha t he is accustomed to

talking in terms of gravels, sands, silts, and clays and finds it only logical to

use these sa me nam es in presenting t he dat a . The soil na mes have been

associated with certain grain sizes in the textural classification as shown on

th e grain-size chart . Such a division is genera lly feasible for the coarse-

grained soils; however, the use of such terms as silt and clay may be entirely


14/28



misleading on a textural basis . For this reason, the terms si lt a nd clay h ave

been defined on a pla sticity ba sis, a s discussed previously. With in a given

region of th e count ry, th e use of a n a me cla ssificat ion based on t exture is often

feasible since the general behavior of simila r soils is consistent over the a rea.

However, in anoth er area , the same clas sificat ion ma y be entirely inadequa te.

The descriptive classification, if used intelligently, has a rightful place in soilmecha nics, but its us e should be car efully eva luat ed by a ll concerned.

Desc r i p t i on F r om C l a s si f i c a t i o n Sheet

Column 6 of Tabl e B-2, pages B-6and B-7, l ists typical na mes given t o the soil

types usually found with in the var ious classification groups. B y following

either the field- or laboratory-investigation procedure and determining the

proper classification group in which the soil belongs, it is usually an easy

ma tt er to select an a ppropriat e na me from the classification sheet. Some soils

ma y be readily identified and properly na med by only visual inspection. A

word of caution is considered appropriate on the use of the classification

system for certain soils (such as marls, calyces, coral, and shale) where the

gra in size can va ry w idely depending on the am ount of mecha nical breakdown

of soil part icles. For these soils, th e group symbol and t extura l nam e have

little significance and t he locally used nam e may be importa nt .

Oth er Desc r i p t i v e Ter ms

Records of field explorations in the form of boring logs can be of great benefit

to the engineer if they include a dequa te informa tion. In a ddition to th e group

symbol and the name of the soil , the general characteristics of the soils as to

plasticity, strength, moisture, and so forth provide information essential to a

proper an alys is of a pa rticular problem. Loca lly accepted soil na mes should

also be used to cla rify th e dat a to local bidders and t o protect t he government

aga inst la ter legal cla ims. For coa rse-gra ined soils, the size of par ticles,

mineralogical composition, shape of grains, and character of the binder are

relevan t feat ures. For fine-gra ined soils, strengt h, moistur e, and plas ticitychara cteristics a re importa nt . When describing undisturbed soils, such

characteristics as stratif ication, structure, consistency in the undisturbed and

remolded states, cementation, and drainage are pertinent to the descriptive

cla ssifica tion. Pert inent items to be used in describing soils are shown in

column 6 of Table B-3, pages B-16 and B-17. To a chieve uniformit y in

estima ting t he consistency of soils, i t is recommended t ha t th e Terzagh i

cla ssifica tion based on unconfined compressive strength be used a s a tent a tive

sta nda rd. This classifica tion is given in Tabl e B-4, page B-18.

Several exa mples of descriptive classifications a re shown below:

U n i for m , fi ne, cl ean s and w i th r ounded g ra i nsS P.

Well-graded gravelly s il ty sand; angula r chert gravel, 1/2 inch

maximum size; silty binder with low plasticity, well-compacted and

moistSM.

L ight brown, f ine, sandy s i lt ; very low plas t ici ty ; sa tura ted and sof t in

the un disturbed st at eML.

Dark g ray, fa t c lay ; s t if f in the und is turbed s ta te ; sof t and s t icky when

remoldedCH.


15/28



CHARACTERISTICS OF SOIL GROUPS PERTAINING TOEMBANKMENTS AND FOUNDATIONS

The ma jor properties of a soil proposed for use in a n emba nkment or foundat ion

that are of concern to the design or construction engineer are its strength,

permeability, an d consolidat ion a nd compaction char a cteristics. Oth er feat uresmay be investigated for a specific problem, but in general, some or all of the

properties mentioned are of primary importance in an earth-embankment or

foundat ion project of a ny ma gnitu de. It is common pra ctice to eva lua te the

properties of the soils in question by means of laboratory or field tests and to

use the results of such tests a s a ba sis for design and construction. The factors

tha t influence strength, consolidat ion, a nd other char a cteristics a re numerous,

a nd some of them a re not completely understood; consequently, it is impra ctical

to eva lua te these fea tur es by means of a genera l soils cla ssifica tion. How ever,

the soil groups in a given classification do have reasonably similar behavior

char a cteristics. While such informa tion is not sufficient for design purposes, it

will give the engineer an indication of the behavior of a soil when used as a

component in construct ion. This is especially true in the prelimina ry

examination for a project when neither time nor money for a detailed soils-testing progra m is a vailable.

Keep in mind that only generalized characteristics of the soil groups are

included th erein, and they should be used prima rily as a guide and not a s the

complete answ er to a problem. For example, it is possible to design a nd

construct an earth embankment of almost any type of soil and on practically

a ny founda tion. How ever, when a choice of ma teria ls is possible, certa in of the

ava ilable soils may be bett er-suited to the job tha n others. It is on this basis

that the behavior characteristics of soils are presented in the following

par a gra phs and on the classificat ion sheet. A str uctures use is often th e

principal deciding fa ctor in selecting soil types as w ell as t he ty pe of protective

measu res tha t will be used. Since each structur e is a specia l problem with in

itself, it is impossible to cover all possible considerations in the brief descriptionof pertinent soil cha ra cteristics conta ined in this a ppendix.

FEATURESONTHE SOILS-CLASSIFICATION SHEET

General characteristics of the soil groups pertinent to embankments and

foundations are presented in Table B-2, pages B-6 and B-7. Columns 1

through 5 show major soil divisions, group symbols, and the hatching and

color symbols. The na mes of soil ty pes a re given in column 6. The basic

features are the same as those presented previously in soils classification.

Columns 7 through 12 show the following: the suitability of the materials for

use in embankments (strength and permeability characteristics); the

minimum or range of permeability values to be expected for the soil groups;

general compaction char a cteristics; the suita bility of the soils for foundat ions

(strength and consolidation); and the requirements for seepage control,especially when the soils are encountered in the foundation for earth

embankment s (permeability). B rief discussions of th ese featu res a re

present ed in th e follow ing para gra phs.


16/28



NOTES:

Orange

1. Divisions of the GM and SM groups (column 3) into subdivisions of d and u are applicable to roads andairfields only. Subdivision is based on the LL and PI; suffix d (for example, GMd) will be used when the LLis 25 or less and the PI is 5 or less; the suffix u will be used otherwise.

SymbolsMajor Divisions

(1) (2)Letter

(3)Hatching

(4)Color

(5)Name

(6)

Value AsSubgrade When

not Subjectto Frost Action (7)

GW

GP

GM

GC

SW

SP

SM

SC

ML

CL

OL

MH

CH

OH

Pt

Red

Yellow

Yellow

Red

Green

B

lue

Graveland

GravellySoils

Sandand

SandySoils

Siltsand

ClaysLL < 50

Siltsand

Clays

LL > 50

Highly OrganicSoils

Coarse-Grained

Soils

Fine-Grained

Soils

Well-graded gravels or gravel-sand mixtures, little or no fines

Poorly graded gravels or gravel-sand mixtures, little or no fines

Excellent

Silty gravels, gravel-sand-siltmixtures

Clayey gravels, gravel-sand-claymixtures

Well-graded sands or gravellysands, little or no fines

Poorly graded sands or gravellysands, little or no fines

Silty sands, sand-silt mixtures

Clayey sands, sand-silt mixtures

Inorganic silts and very fine sands,rock flour, silty or clayey fine sandsor clayey silts with slight plasticity

Inorganic clays of low to mediumplasticity, gravelly clays, sandyclays, silty clays, lean clays

Organic silts and organic silt-clays of low plasticity

Inorganic silts, micaceous ordiatomaceous fine sandy or siltysoils, elastic silts

Inorganic clays of high plasticity,

fat clays

Organic clays of medium to highplasticity, organic silts

Peat and other highly-organicsoils

Good to excellent

Good to excellent

Good

d

u

Value As SubbaseWhen not Subjectto Frost Action (8)

Excellent

Good

Good

Fair

Good

Good

Fair to good

Fair to good

Fair

Poor to fair

d

u

Poor to fair

Poor to fair

Poor

Poor

Poor to fair

Poor to very poor

Not suitable

Fair

Fair to good

Fair to good

Poor to fair

Fair

Poor

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Table B-3. Characteristics of soil groups pertaining to roads and airfields


17/28



Compressibilityand Expansion

(11)

Dry UnitWeight

(pcf) (14)

CompactionEquipment (13)

Crawler-type tractor, rubber-tiredroller, steel-wheeled roller

125 -140

Value As BaseWhen not Subject

to Frost Action(9)

Good

Fair to Good

Fair to Good

Poor to notsuitable

None tovery slight

None tovery slight

Slight tomedium

Almost none

Almost none

Excellent

DrainageCharacteristics

(12)

Poor to practi-cally impervious

CBR(15)

40 -80

Subgrade Modulusk (lb per cu in)

(16)

Typical Design Values


Rubber-tired roller, sheepsfootroller; close control of moistureRubber-tired roller,sheepsfoot roller

Poor to notsuitable

Slight tomedium

Very slight

Slight

Fair to poor


Excellent 110 -140

125 -145

115 -135

30 -60

40 -6020 -30

300 - 500

300 - 500

300 - 500

200 - 500

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Not suitable

Poor

Poor to notsuitable

Poor

Not suitable

Not suitable

Slight tomedium

None tovery slight

None tovery slight

Slight tohigh

Slight tohigh

Slight tohigh

Medium tovery high

Medium tohigh

Medium tohigh

Medium tovery high

Medium

Medium

Slight

Slight

Almost none

Almost none

Very slight

Slight tomedium

Slight tomedium

Slight tomedium

Medium

Medium to high

High

High

High

Very high

Excellent

Excellent

Fair to poor


Fair to poor


Practicallyimpervious

Poor

Fair to poor

Fair to poor



Rubber-tired roller,sheepsfoot roller



Rubber-tired roller, sheepsfootroller; close control of moisture

Rubber-tired roller, sheepsfootroller

Rubber-tired roller,sheepsfoot roller; closecontrol of moisture







Compaction not practical

130 -145

110 -130

105 -135

120 -135

100 -130

100 -135

90 -130

90 -130

90 -105

80 -105

90 -115

80 -110

20 -40

20 -40

10 -40

15 -40

10 -20

5 -20

15 orless

15 orless

5 orless

10 orless

15 orless

5 orless

- - -

200 - 500

200 - 400

150 - 400

150 - 400

100 - 300

100 - 300

100 - 200

50 - 150

50 - 100

50 - 100

50 - 150

25 - 100

2. The equipment listed in column 13 will usually produce the required densities with a reasonable number of passes whenmoisture conditions and thickness lift are properly controlled. In some instances, several types of equipment are listed becausevariable soil characteristics within a given soil group may require different equipment. In some instances, a combination of twotypes may be necessary.

a. Processed base materials and other angular material. Steel-wheeled and rubber-tired rollers are recommended for hard,angular materials with limited fines or screenings. Rubber-tired equipment is recommended for softer materials subject todegradation.

b. Finishing. Rubber-tired equipment is recommended for rolling during final shaping operations for most soils and processedmaterials.

c. Equipment Size. The following sizes of equipment are necessary to assure the high densities required for airfieldconstruction: Crawler-type tractortotal weight in excess of 30,000 pounds. Rubber-tired equipmentwheel load in excess of 15,000 pounds; wheel loads as high as 40,000 pounds may be necessary

to obtain the required densities for some materials (based on contact pressure of approximately 65 to 150 psi). Sheepsfoot rollerunit pressure (on 6- to 12-square-inch foot) to be in excess of 250 psi and unit pressures as high as 650

psi may be necessary to obtain the required densities for some materials. The area of the feet should be at least 5 percent of thetotal peripheral area of the drum, using the diameter measured to the faces of the feet.3. The ran e of dr unit wei hts listed in column 14 are for com acted soil at OMC when usin the Standard Proctor Test ASTM

PotentialFrost

Action(10)

Table B-3. Characteristics of soil groups pertaining to roads and airfields

(continued)

2. The equipment listed in column 13 will usually produce the required densities with a reasonable number of passes whenmoisture conditions and thickness lift are properly controlled. In some instances, several types of equipment are listed becausevariable soil characteristics within a given soil group may require different equipment. In some instances, a combination of twotypes may be necessary.

a. Processed base materials and other angular material. Steel-wheeled and rubber-tired rollers are recommended for hard,angular materials with limited fines or screenings. Rubber-tired equipment is recommended for softer materials subject todegradation.

b. Finishing. Rubber-tired equipment is recommended for rolling during final shaping operations for most soils and processedmaterials.

c. Equipment Size. The following sizes of equipment are necessary to assure the high densities required for airfieldconstruction: Crawler-type tractortotal weight in excess of 30,000 pounds. Rubber-tired equipmentwheel load in excess of 15,000 pounds; wheel loads as high as 40,000 pounds may be necessary

to obtain the required densities for some materials (based on contact pressure of approximately 65 to 150 psi). Sheepsfoot rollerunit pressure (on 6- to 12-square-inch foot) to be in excess of 250 psi and unit pressures as high as 650

psi may be necessary to obtain the required densities for some materials. The area of the feet should be at least 5 percent of thetotal peripheral area of the drum, using the diameter measured to the faces of the feet.3. The range of dry unit weights listed in column 14 are for compacted soil at OMC when using the Standard Proctor Test (ASTM1557-91).4. The maximum CBR values (column 15) that can be used in design of airfields is, in some cases, limited by gradation andplasticity requirements.

NOTES (continued):


18/28



Suitability of Soils for Embankments

Three major factors that influence the suitability of soils for use in

embankment s are permea bility, str ength, an d eas e of compaction. The

gra velly a nd sa ndy soils wit h litt le or no fines (groups GW, G P, SW, an d SP )are stable, pervious, and able to attain good compaction with crawler-type

tra ctors a nd rubber-tired rollers. The poorly gra ded mat erials may n ot be

quite as desirable as t hose which are w ell graded, but a l l of the ma terials are

suita ble for use in the pervious sections of ea rth embankment s. Poorly graded

sands (SP) may be more difficult to use and, in general, should have flatter

embankment slopes tha n the SW soils. The gravels an d san ds with fines

(groups GM, GC, SM, and SC) have variable characteristics depending on the

na tur e of th e fine fra ction an d the grada tion of the entire sample. These

materials are often sufficiently impervious and stable to be used for

impervious sections of emba nkment s. The soils in th ese groups should be

carefully examined to ensure that they are properly zoned with relation to

other ma terials in an embankment.

Of the fine-grained soils, the CL group is best adapted for embankment

construction; the soils are impervious, fairly stable, and give fair to good

compa ction wit h sheepsfoot or rubber-tir ed rollers. The MH soils, w hile not

desirable for rolled-fill construction, may be used in the core of hydraulic-fill

str uctures. Soils of the ML group may or may n ot have good compaction

chara cteristics a nd, in general, must be closely contr olled in th e field to secure

th e desired strengt h. CH soils have fa ir sta bility when used on flat slopes but

have detrimental shrinkage chara cterist ics w hich may necessita te blanketing

th em or incorpora ting th em in th in interior cores of emba nkments. Soils

cont a ining organic ma tt er (groups OL, OH, a nd P t) are not commonly used for

embankment construction because of the detrimental effects of the organic

ma tter present . Such materials may of ten be used to advant age in blankets

an d sta bi li ty berms where strength is not importa nt .

Permeability and Seepage Control

Since the permeability (column 8) and requirements for seepage control

(column 12) a re essentially functions of the sa me property of a soil , th ey w ill

be discussed joint ly. The subject of seepage in relat ion to emba nkment s an d

founda tions ma y be roughly divided into three cat egories:

Table B-4. Terzaghi classification

Unconfined Compressive

Strength (Tons/Sq Ft)Consistency

< 0.25 Very soft

0.25 to 0.50 Soft

0.50 to 1.00 Medium

1.00 to 2.00 Stiff

2.00 to 4.00 Very stiff

> 4.00 Hard


19/28



S eepa g e t h rou gh em ba n km en t s.

S eepa g e t h r ou gh f ou n da t i on s.

C on t r ol of u pli ft p res su r es.

These are discussed in relation t o the soil groups in th e following par agr aph s.

Seepage Th r ough Emban km en t s

In t he control of seepage th rough embankm ents, it is th e rela tive permea bility

of adjacent materials rather than the actual permeabil i ty of such soi ls that

governs th eir use in a given locat ion. An ea rth emba nkment is not wa tert ight,

and the allowable quantity of seepage through it is largely governed by the

use to which th e st ruct ure is put. For exam ple, in a flood-cont rol project,

considerable seepage may be allowed and the structure will still fulfill the

stora ge requirements; w hereas for a n irriga tion project, much less seepage is

a llowa ble beca use pool levels must be ma int a ined. The more impervious soils

(GM, GC, SM, SC, CL, MH, and CH) may be used in core sections or in

homogeneous emba nkments t o reta rd th e flow of wa ter. Where it is importa nt

that seepage not emerge on the downstream slope or the possibility ofdrawdown exists on upstream slopes, more pervious materials are usually

placed on th e out er slopes. The coa rse-gra ined, free-dra ining soils (G W, G P,

SW, SP ) a re best-suited for t his purpose. Where a va riety of mat erials is

available, they are usually graded from least pervious to more pervious from

th e cent er of the emba nkment outw ar d. Ca re should be used in th e

arrangement of materials in the embankment to prevent piping within the

section. The foregoing sta tem ents do not preclude th e use of other

ar ra ngements of materials in embankments. Da ms have been constructed

successfully ent irely of sa nd (SW, SP, a nd S M) or of silt (ML) w ith th e section

ma de large enough to reduce seepage to an allowa ble va lue without t he use of

a n impervious core. Coar se-gra ined soils are often used in drains a nd toe

sections to collect seepage water in downstream sections of embankments.

The soils used will depend la rgely on th e mat erial th a t t hey dra in; in genera l,free-dra ining san ds (SW and S P ) or gravels (G W a nd G P ) a re preferred, but a

silty sand (SM) may effectively drain a clay (CL and CH) and be entirely

satisfactory.

Seepage Th r ough Found a t i o n s

As in the case of embankments, the use of the structure involved often

determines the a mount of seepage control necessar y in founda tions. Ca ses

could be cited where the flow of water through a pervious foundation would

not const itute a n excessive wa ter loss and no seepage contr ol mea sures would

be necessa ry if adequa te provisions w ere ma de aga inst piping in critica l area s.

If seepage control is desired, then the more pervious soils are the soils in

w hich necessary mea sures must be ta ken. Free-dra ining gra vels (G W a nd

G P ) a re ca pable of ca rrying considera ble qua nt ities of wa ter, and some means

of positive control (such as a cutoff tr ench) may be necessa ry. Clean sa nds

(SW and SP) may be controlled by a cutoff or by an upstream impervious

blanket. While a dra ina ge trench at t he down str eam toe or a line of relief

wells will not reduce the amount of seepage, either will serve to control

seepage and route the flow into collector systems where it can be led away

ha rmlessly. Slightly less pervious ma teria l (such as silty gr avels [G M], silty


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sa nds [SM], or silts [ML]) may require a minor a mount of seepa ge cont rol such

as that afforded by a toe trench, or if they are sufficiently impervious, no

cont rol may be necessa ry. The relat ively impervious soils (G C, SC, CL, OL,

MH, CH, and OH) usual ly pass such a small volume of water that seepage

cont rol measures a re not n ecessary.

Con t r o l o f Up l i f t P r essu r es

The problem of control of uplift pressures is directly associated with pervious

foundat ion soils. Uplift pressures may be reduced by length ening the pat h of

seepage (by a cutoff or u pstrea m bla nket) or by measur es for pressure relief in

the form of wells, drainage trenches, drainage blankets, or pervious

downst ream shells. Free-dra ining gravels (G W a nd GP ) ma y be tr eat ed by

a ny of the a forementioned procedures; however, to obta in t he desired pressure

relief , the use of a positive cutoff ma y be preferred, as blanket, w ell, or t rench

installations would probably have to be too extensive for economical

a ccomplishment of the desired results. Free-dra ining san ds (SW a nd SP ) a re

generally less permeable than the gravels and, consequently, the volume of

wa ter t ha t mu st be cont rolled for pressure relief is usua lly less. Therefore a

positive cutoff may not be required and an upstream blanket, wells, or a toe

tr ench ma y be entirely effective. In some cas es a combina tion of blanket a nd

tr ench or w ells ma y be desira ble.

Silty soils (silty gravels [GM], silty sands [SM], and silts [ML]) usually do not

require extensive treatment; a toe drainage trench or well system may be

sufficient t o reduce uplift pressures. The more impervious silty ma teria ls ma y

not be permeable enough to permit da ngerous uplift pr essures to develop, and

in such ca ses, no treat ment is indicat ed. In genera l, the more impervious soils

(G C, SC, CL, OL, MH , CH, a nd OH ) require no treat ment for contr ol of uplift

pressures. However, they do a ssume importa nce w hen they occur a s a

relat ively th in top stra tum over more pervious mat erials. In such cases, uplift

pressures in the lower la yers a cting on th e base of the impervious top str at um

can ca use heaving a nd format ion of boils; trea tment of the low er layer by someof the methods mentioned above is usually indica ted in these cases. I t is

emphasized that control of uplift pressures should not be applied

indiscrimina tely just because certa in types of soils are encount ered. Ra th er,

the use of control measures should be based on a careful evaluation of

conditions th a t do or can exist, a nd a n economical s olution should be reached

th a t w ill accomplish the desired results.

Compaction Characteristics

Column 9 of Tabl e B-2, pages B-6 and B-7, shows the general compaction

char acteristics of the va rious soil groups. The eva luat ions given an d the

equipment listed a re based on a verage field conditions w here proper moistur e

contr ol a nd th ickness of lif t a re at ta ined and a reasona ble number of passes of

t he compa ction equ ipment a re required to secure th e desired density. For lift

construction of embankments, the sheepsfoot and rubber-tired rollers are

commonly used pieces of equipment. Some adva nt a ges may be claimed for th e

sheepsfoot roller in that it leaves a rough surface that affords better bond

between lif ts and it kneads the soilaffording better moisture distribution.

Rubber-tired equipment referred to in the table is considered to be heavily

loaded compactors or earthmoving equipment with a minimum wheel load of


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15,000 pounds. If ordina ry w obble-w heel rollers ar e used for compact ion, the

th ickness of a compa cted lift is usua lly reduced t o about 2 inches.

Granular soils with little or no fines generally show good compaction

characteristics, with the well-graded materials (GW and SW) usually

furnishing bett er results th a n the poorly gra ded soils (G P a nd SP ). The sandy

soils, in most cases, are best compacted by crawler-type tractors; on the

gra velly m at erials, rubber-tired equ ipment a nd sometimes steel-w heel rollers

a re a lso effective. Coar se-gra ined soils with fines of low plast icity (groups G M

a nd S M) show good compa ction cha ra cteristics w ith either sheepsfoot rollers

or rubber-tired equipment; however, the range of moisture contents for

effective compaction may be very narrow and close moisture control is

desira ble. This is also true of th e silty soils in the ML group. Soils of the ML

group may be compacted with rubber-tired equipment or with sheepsfoot

rollers . G ravels and san ds with plast ic f ines (groups GC a nd SC) show fa ir

compaction chara cterist ics , a l though this qua l ity ma y va ry somewh at with the

cha ra cter a nd a mount of f ines.

Rubber-tir ed or sheepsfoot rollers may be used. Sh eepsfoot rollers a re

genera lly used for compa cting fine-gra ined soils. The compa ction

characterist ics of such materials are variablelean clays and sandy clays

(CL) being th e best, fa t clays a nd lean organic clays or silts (OL an d CH ) fair

to poor, a nd orga nic or micaceous soils (MH a nd OH ) usua lly poor.

For most construction projects of any magnitude, it is highly desirable to

investigate the compaction characteristics of the soil by means of a f ield test

section. Column 10 shows t he ra nges of unit dry weight for soils compacted

according to the compaction test method as described in ASTM 1557-91 and

Cha pter 2 of this manua l . I t is emphasized tha t th ese values are for guidance

only. Design or construction control should be based on laborat ory test

results.

Suitability of Soils for FoundationsSuitability of soils for foundations of embankments or structures depends

primarily on the strength and consolidation characteristics of the subsoils.

The type of structure and its use will largely govern the adaptability of a soil

as a sat isfactory foundat ion. For emban kments, large sett lements may be

allowed and compensated for by overbuilding; whereas the allowable

settlement of structures (such as control towers) may be small to prevent

overstressing the concrete or steel of which they are built or because of the

necessity for adh ering to esta blished gra des. Therefore, a soil may be entirely

sa tisfa ctory for one type of construction but m ay require specia l trea tm ent for

other ty pes.

Strength and settlement characteristics of soils depend on a number of

va riables (such a s st ructure, in-place density, moisture content, a nd cycles ofloading in their geologic history) which are not readily evaluated by a

cla ssifica tion system such as used here. For th ese reasons, only very genera l

statements can be made as to the sui tabi l i ty of the various soi l types as

founda tions. This is especially tr ue for fine-gra ined soils.

In genera l, the gra vels a nd gra velly soils (G W, G P, G M, an d G C) have good

bearing capa city an d undergo litt le consolida tion under loa d. Well-gra ded


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sa nds (SW) usua lly have a good bea ring value. Poorly gra ded sands a nd silty

sands (SP and SM) may exhibit variable bearing capacity depending on their

densit y. This is tr ue to some extent for all coar se-gra ined soils but is

especially critical for uniformly graded soils of the SP a nd SM groups. Such

soils, when saturated, may become quick and present an additional

const ruct ion problem. Soils of the ML group may be subject to liquefa ctionand may have poor bearing capaci t ies , part icularly where heavy structure

load s a re involved. Of the fine-gra ined soils, the CL gr oup is probably t he best

from a foundat ion st an dpoint, but in some ca ses, the soils ma y be soft a nd w et

and exhibit poor bearing capacity and fairly large settlements under load.

Soils of the MH groups and normally consolidated CH soils may show poor

bearing capa city a nd lar ge sett lements. Orga nic soils (OL an d OH) ha ve poor

bearing capacity and usually exhibit large settlement under load.

For most of the fine-grained soils discussed above, the type of structure

founda tion selected is governed by such fa ctors as t he bea ring capa city of th e

soil and t he magn itude of the loa d. I t is possible tha t simple spread footings

might be adequate to carry the load without excessive settlement in many

cases. I f the soils a re poor an d structure loa ds are relatively heavy, thenalt erna te meth ods a re indicat ed. P ile founda tions may be necessary in some

cases and in special instancesparticularly in the case of some CH and OH

soilsit may be desirable and economically feasible to remove such soils from

the founda tion. Highly-organic soils a re genera lly very poor foundat ion

ma teria ls. These ma y be capable of ca rrying very light loa ds but, in genera l,

a re unsuit ed for most const ruct ion purposes. If highly -orga nic soils occur in

the foundation, they may be removed (if l imited in extent), they may be

displaced (by dumping firmer soils on top), or piling may be driven through

them to a str onger layer. P roper treat ment will depend on the structure

involved.

GRAPHICAL PRESENTATIONOF SOILS DATA

I t is customary to present the results of soils explorations on drawings orplans as schematic representations of the borings or test pits with the soils

encountered using var ious symbols. Commonly used ha tching symbols are

sma ll, irregular round sy mbols for gravel; dots for sand ; vertical lines for silts;

an d diagonal lines for clays. Combina tions of these symbols represent t he

var ious combina tions of mat erials found in th e explora tions. This system ha s

been adapted to the various soi l groups in the USCS and the appropriate

symbols are shown in column 4 of Tabl e B-2, pages B-6 and B-7. As an

al terna tive to the hat ching symbols , they ma y be omitted a nd th e appropriate

group letter symbol writt en in the boring log. In a ddition to the symbols on

logs of borings, th e effective size of coa rse-gra ined soils a nd t he na tur a l wa ter

cont ent of fine-gra ined soils should be show n by th e side of th e log. Oth er

descriptive a bbrevia tions may be used a s deemed a ppropria te. In certa in

instances, the use of color to delineate soil types on maps and drawings is

desira ble. A suggest ed color scheme to show t he ma jor soil groups is described

in column 5 of Tabl e B-2.


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CHARACTERISTICS OF SOIL GROUPS PERTAINING TO ROADS ANDAIRFIELDS

The properties desired in soils for founda tions under roads an d a irfields a nd

for base courses under flexible pavements are adequate strength, good

compaction characteristics, adequate drainage, resistance to frost action inareas where frost is a factor, and acceptable compression and expansion

chara cteristics. Some of these propert ies, if ina dequa te in the soils ava ilable,

ma y be supplied by proper constr uction meth ods. For insta nce, mat erials

having good drainage characteristics are desirable, but if such materials are

not available locally, adequate drainage may be obtained by installing a

properly designed water-collecting system. Strength requirements for base-

course materials (to be used immediately under the pavement of a f lexible

pavement structure) are high and only good-quality materials are acceptable.

However, low strengths in subgrade materials may be compensated for in

ma ny cases by increasing the t hickness of overlying concrete pavement or of

base ma teria ls in flexible pavement const ruction. From the foregoing brief

discussion, it may be seen that the proper design of roads and airfield

pavements requires the evaluation of soil properties in more detail than ispossible by using the genera l soils cla ssificat ion system. How ever, the

grouping of soils in the classifica tion syst em is such t ha t a general indicat ion

of th eir beha vior in road a nd a irfield construction may be obta ined.

FEATURESONTHE SOILS-CLASSIFICATION SHEET

General characteristics of the soil groups pertinent to roads and airfields are

presented in Tabl e B-3, pages B-16 and B -17. Columns 1 thr ough 5 show

major soil divisions, group symbols, hatching and color symbols; column 6

gives names of soil types; column 7 evaluates the performance (strength) of

the soil groups when used as subgrade materials that will not be subject to

frost action; columns 8 and 9 make a similar evaluation for the soils when

used as subbase and base materials; column 10 shows potential frost action;

column 11 shows compressibility and expansion characteristics; column 12presents drainage characteristics; column 13 shows types of compaction

equipment that perform satisfactorily on the various soil groups; column 14

shows r a nges of unit dr y w eight for compa cted soils; column 15 gives ra nges of

typical CBR values; and column 16 gives ranges of modulus of subgrade

rea ction (k). The various feat ures presented ar e discussed in th e following

paragraphs .

Subdivision of Coarse-Grained Soil Groups

Note that in column 3 the basic soil groups (GM and SM) have each been

subdivided into two groups designated by the suffixes d and u which have

been chosen to represent desirable and less desirable (undesirable) base

ma teria ls, respectively. This subdivision a pplies to roa ds an d a irfields only

and is based on field observation and laboratory tests on the behavior of the

soils in these groups. B a sis for the subdivision is the LL an d P I of th e fra ction

of the soil passin g th e No. 40 sieve. The suffix d is used w hen t he LL is 25 or

less and the P I is 5 or less; otherwise, the suffix u is used. Typical sy mbols for

soils in these groups are GMd a nd SMu.


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Values of Soils as Subgrade, Subbase, or Base Materials

The descriptions in columns 7 through 9 give a general indication of the

suita bility of the soil groups for use as subgra des, subbase, or base ma terials,

provided they ar e not subject to frost action. In a reas w here frost heaving is a

problem, the value of materials as subgrades or subbases will be reduced,

depending on th e potent ial frost a ction of the ma teria l as sh own in column 10.

P roper design procedures should be used in situa tions w here this is a problem.

The coa rse-gra ined soils, in genera l, ar e the best s ubgra de, subbase, and ba se

ma teria ls. The GW group ha s excellent qua lities a s a subgra de a nd subbase,

an d is good a s base mat erial. Note tha t th e adjective excellent is not used

for an y of th e soils for ba se cours es; excellent sh ould be used in reference to a

high-qua lity processed crushed stone. Poorly gra ded gravels and some silty

gravels (groups GP and GMd) are usually only slightly less desirable as

subgrade or subbase materials and, under favorable conditions, may be used

as ba se ma teria ls for certa in conditions. However, poor gra da tion and other

factors sometimes reduce the va lue of such soils to the extent tha t t hey offer

only modera te strength, and t heir value as a base mat erial is less . The GMu,

GC, and SW groups are reasonably good subgrade materials but are generallypoor to not suita ble a s bases. The SP a nd SMd soils a re usually considered

fair to good subgrade and subbase materials but, in general, are poor to not

suita ble for base ma teria ls. The SMu an d SC soils a re fa ir to poor subgrad e

and subbase mat erials a nd ar e not sui ta ble for base materials . The fine-

gra ined soils range from fair to very poor subgrade ma teria ls as follows:

S i lt s and lean c lay s (M L and C L)f a ir t o poor.

Organic s il t s, lean organic clays , and micaceous or d ia t omaceous soi ls

(OL and MH)poor.

Fa t clays and f a t organic clays (CH a nd OH)poor to very poor.

These qua lities a re compensat ed for in flexible pavement design by increa sing

the thickness of overlying base material and in rigid pavement design byincrea sing th e pavement th ickness or by a dding a base-course layer. None of

the fine-gra ined soils are suita ble a s subbase or base mat erials. The fibrous

organic soils (group Pt) are very poor subgrade materials and should be

removed wherever possible; otherwise, special construction measures should

be a dopted. They are not suitable a s subbas e a nd base ma teria ls. The CB R

values shown in column 15 give a relative indication of the strength of the

var ious soil groups a s used in flexible pavement design. Similar ly, values of

subgra de modulus (k) in column 16 ar e relative indicat ions of str engths from

plat e-bearing test s as used in rigid pa vement design. As these tests a re used

for t he design of pavements, a ctua l test va lues should be used for t his purpose

instead of the approximat e values shown in the ta bulat ion.

For w earing sur faces on un surfa ced roads, sa nd-clay -gra vel mixtur es (G C) aregenerally considered the most sa tisfa ctory. How ever, they should not conta in

too la rge a percent age of f ines a nd t he P I should be in t he ra nge of 5 to about

15.

Potential Frost Action

The relative effects of frost action on the various soil groups are shown in

column 10. Rega rdless of th e frost susceptibility of the var ious soil groups,


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Unified Soil Class

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