Elementary Soil Engineering

7/28/2019 Elementary Soil Engineering

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United StatesDepartment ofAgricultureSoilConservationSewice

Engineering FieldManualChapter 4.Elementarv Soil

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Contents

Page

Basic Concepts .......................................................... 4-2Soil ...............................................................................-2Soil Solids ....................................................................-2Soil Voids .....................................................................-2Origin ...........................................................................-2Structure .................................................................... 4-2Soil Water ....................................................................-3Volume-Weight Relationships .....................................-3

.................................................................oid ratio 4-4Porosity...................................................................-4...............................................egree of saturation 4-4

Water Content..............................................................-4........................................................pecific gravity 4-4Unit weight and density ...........................................4-4

Atterberg Limits........................................................4-6.................................................Shrinkage limit (SLI1

4-6......................................................lastic limit (PL) 4-6Liquid limit (LL) ........................................................4-6Plasticity index (PI) ................................................. 4-6Nonplastic (NP) .................... ..................4-6

me unified soil classification system ......................... 4-8Classification Using Laboratory Data...........................4-8NamesandSymbols.................................................... 4-8

Coarse-grained soils .........................................4 - 8Fine-grained soils ................................................4-8Highly organic soils ................................................4-8

Sieve Sizes ............................................................. - 8The Plasticity Chart ................... ...........................4-8Classifying Fine-Grained Soils...................................4-10

Clay or silt ........................................................... 4-10High or low liquid limit .........................................4-10Fine-grained soils with sand andlor gravel ...........4-10Organic soils .................................................... 4-10Summary of fine-grained soils .............................. 4-10

PageClassifying Coarse-Grained Soils ..............................-10

Soils with less than 5 percent fines ....................... -10...........................oils with 12 to 50 percent fines 4-11Soils with 5 to 12 percent fines ............................. -11

...................................................................ummary 4-13..............Field Identification and Description of Soils 4-19

................................ual and Borderline Classes 4-19.............................pparatus needed for the tests 4-19..........................................................ample size 4-19

Field classification.....................................................4-19Highly organic soil ................................................ 4-19Percentages of sand. gravel. and fines .................4-19.....................................lassifying fine-grained soils 4-20Dilatancy............................................................... -20Toughness ............................................................4-20Dry strength ........................... ............ ................4-20

............................................stimating liquid limit 4-21Modifiers for sand and gravel................................ -21

Classifying coarse-grained soils ................................ -21Soil description ..........................................................4-23

Numbering test holes and samples .......................4-25Maximum particle size ..........................................4-25Percentages of cobbles and boulders...................4-25Particle-size range of coarse materials .................4-25Particle angularity .................................................4-25Particleshape ....................................................4-25Plasticity of fines ................................................... -25Color .................................................................... 4-27Natural water content ............................................4-27Reaction with HCI ............................................... 4-27Consistency .......................................................... 4-27

Structure...........................................................4-27Cementation.................................................4-27

Site investigation...................................................4-27Preliminary data .............................................. 4-28Boring program ...............................................4-28

Ground water ........................................................4-28


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PageEstimation of Soil Performanceand PerformanceRequirements...............................................................-29

Strength .....................................................................-29Permeability ...............................................................-29. .Compressib~lrty.........................................................-30Bearing capacity ........................................................-30Compaction................................................................-30Erosion resistance .....................................................-31Earth pressures .........................................................-32Cracking.....................................................................-33...............................................................hrink-swell 4-33Frost...........................................................................-34.................................................ispersive Clay Soils 4-34

Behavior characteristics of soils ............................4-34Design for EarthDams.....4-38

Scope......................................................................... -38General ...................................................................... -38Embankment stability.................................................4-38.................................................equired design date 4-42Selection of embankment type .................................. -42

Page1 Types of soil structure.............................................4-3

Soil volume and weight relationships .....................4-53 Atterberg limits ....................................................... -6

Unified Soil Classification System plasticitychart........................................................................-9Grain-size distribution .......................................... -12Classification of coarse-grained soils by...............................................................radation 4-13General relationships between systems used. o classify soils-USDA textural soil classes.the Unified Soil Classification System. and......................................he AASHTO soil classes 4-15

Page4-8 Relationship between particle size and the

USDA textural soil classes. the Unified SoilClassification System.and the AASHTO ........................................................oil classes 4-18

4-9 Dilatancy, strength, and toughness tests..............4-22...................... ........................-10 Particle shapes .. 4-264-1 1 Flat and elongated particles.................................. -264-12 Earth pressures against walls............................... -33...........................-13 Minimum sand or gravel section 4-33.........-14 Unified classification and properties of soils 4-35............................................-15 Embankment types 4-39Tables

Page4-1 Volume-weight relationships................................... 4-74-2 Sieve designation and site of openings .................4-84-3 Soil classification chart ................................4-144-4 Minimum sample size .......................... ...........4-194-5 Classification of inorganic fine-grained soil...........4-21 .-6 Classification of organic fine-grained soil..............4-214-7 Field identification by visual-manual methods ......4-24.................................-8 Test hole numbering system 4-25........................................-9 Sand and gravel sizes 4.25

Field determinationof consistency of finegrainedsoil ....................................................................... 4-27

4-1 1 Maximum lift thickness for.compactionequipment .............................................................4-32

4-12 Comparative erosion resistance of soils ...............4-324-13 Volume change potential ...................................... 4-344-14 Working classification of soils for use as fill

.........................aterials for rolled earth damage 4-43...........-15 Characteristics of compacted fill materials 4-44.................-16 Characteristics of foundation materials 4-44........-17 Evaluation of site and foundation conditions 4-45..............................-18 Selection of basic design type 4-46
http://efm4b.pdf/http://efm4b.pdf/


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Introduction

All structural measures and many land treatment measuresfor which SCSprovides technical assistance involve the useof soil as a building material or a supporting medium. Soilengineering is the application of physical, chemical, andmechanical properties of soil to its use as a constructionmaterial and as a foundation for structures.This chapter is about soil engineering. It includes thefollowing major sections: (I)an explanation of basic soilconcepts that relate to engineering; (2) an engineeringclassifiqtion and description system for soil; and (3) guidesfor estimating soil strength, permeability, erosion resistance,and other performance characteristics. It also discusses siteinvestigation and presents a procedure for the preliminaryembankment design of earth dams.


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ask Concepts

sediments or other unconsolidated accu-ced by the physical disinte-l decomposition of rocks. It may or may

olid so il matter and void space ' called

Solids ineral particles resultinghysical disintegration of the parent rock or the mineralsing up the parent rock. For example, granite oftenks down into individual particles of its m inerals: feldspar,

ical action on rocks causes decomposition of the rocks., when associatedminerals under favorable conditions, combine toical compounds not present in the parent

osition usually follows disintegration. The mineralparent rock and the extent of disintegrationposition determine the kind of soil and thesoil.with which the parent rock is disintegrated andlength of time these p rocesses haveine the size of the granular soil particles. Insand and g ravel particles are described assubangular, subrounded, or rounded. Detailedions for these terms are given later in this chapter.

il particles may be spherical, flat, elongated, or

shape of clay particles are determined by themolecular structure of each

Voids pied byIn a natural state, the voids may be filled by liquid,

s in the voids is usually air. In soil engineering, air intreated as being weightless. Liquid in the voids isered incompressible.

location may have been formed in place from rockrted to that site by gravity, water, wind, ice, orof these. Soil formed in place is called

residual. Transported soils are described by their m ethod oftransportation.Colluvium s soil that has been transported by gravity. Thistransportation may have been so slow that it is difficult tomeasure, or it can be nearly instantaneous. Examples areloosened particles that roll or slide down a steep slope, largemasses of soil that slow ly 'creep' downhill, and landslides.Alluvium is soil that has been transported to its presentlocation by water. Aeolian or eolian soil has been trans-ported to its present location by wind. This term is derivedfrom Aeolus, Roman god of the wind. The transportation ofsoil by wind and water is characterized by three processes:(1) soil particles are transported by be ing rolled or slid alongthe ground surface or by being picked up and carried insuspension; (2) particles tend to separate and depositaccord ing to their weight (size); and (3) certain particle sizesmay be in a loose arrangement and subject to compaction asthey settle.Glacial till is soil that has been gouged out of the earth'scrust by ice (glaciers) at one location and dropped atanother. In this process, there is usually little or no separa-tion of particles by weight.Transportation processes are important agen p of disintegra-tion, particularly in produc ing very fine particles by wearingaway larger particles. An area's soil and its engineeringproperties are partly the result of whether and how that so ilwas transported and deposited.StructureSoil structure is defined as the arrangement and grouping ofsoil particles in a soil mass. :Flocculent structure" describesa structure of elongated particles (usually clay particles) heldtogether in groups or clusters of individua l soil particlescalled "flocs," (fig. 4-1).When not flocculated, soil particles are sa id to be dispersed.Such soil has a "single-g rain" structure, charac teristic ofcoarse -grained or granular soils. Most clay particles areattracted to each other by e lectro-chemical bonding, whichcauses them to flocculate and adhere or cling together.Some soils contain salts or other compounds in the porewater, creating a condition in which the soil particles are notattracted to each other. Clays in which soil particles tend toseparate or repel each other are called dispersive clays.When clays have defloccu lated in water they are calleddispersed clays."Honeycomb structure" describes an arching arrangement ofsoil pa rticles somewhat resembling a honeycomb in appear-ance. Particles are usually silt-sized; that is, relatively loose


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Figure 4-1.-Types of soil structure.,- - .

(a) Single-grainstructure (b) Honeycombstructure (c) Flocculentstructure

(open) but stable. Soils with this type of structure are usuallyhighly compressible and may collapse when the applied loadbecomes large enough to destroy the structural bonding orarching at the contact between particles.SoilWaterMost soils contain water. Water can exist in soil in severaldifferent forms. The water in soil voids may be influenced byexternal or internal pressures and can have positive, neutral,or negative pressure. Some definitions pertaining to water insoil are as follows:Hygroscopic water-The water adsorbed on the surface ofsoil particles as a thin film that (1) has properties substan-tially different from ordinary water, and (2) is removed byoven drying but not by air drying.Capillary water-Water that is under tension in a soil due tostresses produced by menisci forming in the soil pores aswater recedes into the voids from evaporation or is lost byother means. It is also the water which has been movedfrom one point to another through interconnected voidsbecause of a change in capillary stress or tension.Gravitational water-Water that is free to move through asaturated soil mass under the influence of gravity.

Water content (w) is the percentage of the weight of water tothe weight of the dry solids. The term "moisture content" issometimes used instead of "water content."A saturated soil has its voids completely filled with water, andits water content is denoted by w Soil below the watertable is usually considered to be saturated.Dry soil contains only air in the voids. Drying a soil to aconstant weight in an oven at a temperature of 110"L 5"C isthe standard commonly used to determine the "dry weight" or"dry mass" of a soil.

The optimum water or moisture content is the percentage ofwater in a soil, based on its dry weight, at which the maxi-mum unit weight or density is obtained under a givencompactive effort and is denoted by wo. The commonprocedure for determining water or moisture content is to drythe soil in an oven or by other means. Most clay soils thathave drained to field capacity after wetting have a watercontent near optimum. Dry soils require the addition ofconsiderable water to reach optimum water content. Soilswith water content between dry and saturated are termed'wet." Saturated and dry conditions represent definite wateicontents, whereas wet makes up the range between thesetwo limits.

VolurneWeight RelationshipsAll soils are composed of solids, air, and water. In thischapter, the term "soil" includes all three components.Figure 4-2 (sketcha) represents a definite volume of soilcomposed of solids and voids. Figure 4-2 (sketch b)represents the same soil with the solids and void volumesseparated into their respective proportions. The total volumeis always equal to the sum of the volumes of the solids andthe voids.

Figure 4-2 (sketch c) represents the same soil volume withsome water added but not enough to fill the voids completely.The soil is now composed of solids, water, and air. In thisstate, the volume of the voids is equal to the sum of thevolumes of water and air.

The total volume is equal to the sum of the volumes of thesolids, water, and air.


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ume relationships have been found to be usefulinengineering.

Void ratio "9" s the ratio of the volume of the

ve =-& (dimensionless)Vsless than Ia decimal.

Porosity 'n" is the percentage of the total volume

Vvn =7 100 (dimensionless)

The volume used in determining specificgravityof solidsdoes not include any voids. This type of specific gravity, des-ignatedG is commonly reported for sands and fines.Values usually fall between2.5 and 2.8, and most fall near2.65. Therefore, 2.65 is often used as an average value inqualitative evaluations. A high organic content will lead to alower value, whereas some of the heavy minerals will givelarger values.Unitweight and density. Unit weight, y (gamma), is definedas weight per unit volume. The total unit weight includes theweight of soil solids and water divided by the total volume, V.The equation is:

Units of weight and volume must be consistent.0 percent.

of saturation. Degree of saturation, 'S," is the ratiothe volume of water in the voids to the volume of the voids,a percentage.VwS =- 100 (dimensionless)V"

re completely filled with water (V 51 V,), theof saturation equals 100 percent.

4-2 (sketch C) points out that the total weight is equal

or W = Wg+WWsince the weight of the air is con-be zero.

ratio ofin the soil to the weight of the solids.

multiplied by 100 is the percentageof water contentWw /w =- 100 percentage of water contentwsgravlty. The specificgravity (G,) of soil solids is theof the weight of a given volume of the soil solids to theof pure water. To be precise, the

temperature of the water. Morerefinement is ignored in soil engineering,a standard set of conditions is used in deterrnin-

W (Ibs)Ft-lb units, y = ( ft) = P C ~

Although not technically correct, density and unit weight areoften used interchangeably in soil engineering. Unit weight isrepresented by y and density by p.The unit weight of Icubic centimeter of distilled water at 4"Cis 10 gram. Therefore, the unit weight of distilled water at 4"C in the SI system of units is:

Since 1.0 gm = ,002205 Ibs and

:w = 62.4 Ibslcu ft in foot-pound unitsAt each of four most frequently usedwater contents, the unitweight of soil has a standard subscript. The four may bewritten as follows:

WDry unit weight, yd = $,where Vw.;0Wet unit weight, y = WsfWw+Wa ,whereWa=O


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Figure 4-2.-Soil volume andweight relationships.

Bulk soil

Bulk Soil = Solids + Water + AirVolumes Wejahts

v = v , + v w + v , w=w,+w.+w,vv= v,+ vw w,=o

Void ratio, e Moisture contentPorosity, n - v w =% x 100 %)vPercent saturation, S = -% 100 (%)vv


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w + w wSaturated weight, y = ,where V, = 0Submerged unit weight, yb = y - y

Submerged unit weight is sometimes referred to as thebuoyant unit weight, yThe 4 u m -we/&? rddiarrships are summarized rr table4 4 .Afierberg UmStsAs water is added to a dry plastic soil, the remolded mixturewill eventually have the characteristics of a liquid. In chang-ing from a solid to a liquid, the material first becomesasemisolid and then plastic. A Swedish scientist namedAtterberg developed tests to determine the water content atwhich these changes take place. The points at which thechanges occur are knawn as the Atterberg limits. Thesystem uses standardized testing procedures to establish*.-...- -+.-.+.-.- -6 .-..-.--;-+.-.--, -. -. 1: .4 -.-.-:-.-.l:A -+ .-....AlU Ul SLULP5 U I b U I 515LWI GJ-5UIIU, OPI I115UIIU, pIUSLIG, UI1Uliqu id -on the basis of water content.Soils increase in volume as their water content increasesabove the shrinkage limit. This is illustrated in figure 4-3.The Atterberg limits are very useful in judging the behavior offine-grained sail ar f ie fine-grained camparrent af coarse-grained soils with fines. Although fines are soil particles thatpass the No.200sieve,Atkcberg h i t s are determined onmaterials that pass the No. 40 sieve. This apparent inconsis-tency occurs because Atterberg limits were established andmeasured on materials passing the No. 40 sieve many yearsbefore the No. 200 sieve was established as the largest sizefor fines.Refer to figure 4-3. In the determination of Atterberg limits,the water content of minus 40 material is measured atvarious levels determined by prescribed test procedures.The plastic limit (PL) is the water content, by percent, atwhich the soil changes from a semisolid to a plastic state.The liquid limit (LL) is the water content, by percent, at whichthe soil water mixture changes from a plastic state to theliquid state. The difference between these two values is therange in water content at which the soil is plastic and iscalled the plasticity index (PI). The test procedures for liquidlimit, plastic limit, and plasticity index of soils are given inASTM designation D4318. The test procedure for shrink-age limit is given in ASTM D-427. ,In practice, the percent sign is dropped when referring tonumerical values of the Atterberg limits. A material with aliquid limit of 50 percent is referred to as having a LL of 50.

Figure 4-3.-Atterberg limits.Consistency diagram relating water content and

volume change.

Liquid state

II Plastic- state+8L95 Semi-solidstateI /Shrinkage limit (SL)Solidstate

Volume change-at which a further reduction in water does not cause adecrease in the volume of the soil mass. This defines thelimit between the solid and semisolid states of consistency.Plastic limi t (PL). The plastic limit is the water contentcorresponding to an arbitrary limit between the plastic andsemisolid states of a soil's consistency. This is the watercontent at which a soil will just begin to crumble when rolledinto a thread approximately 3mm (118 in) in diameter.Liquid limit (LL). The liquid limit is the water contentcorresponding to the arbitrary limit between the liquid anddastic states of a soil's consistencv. This is the waterkontent at which a pat of soil, cut b;a groove 2mm wide (5164 in), will flow together for a distance of 13mm (112 in) underthe impact of25 blows in a standard liquid limit apparatus.Plasticity index (PI). The plasticity index is the numericaldifference between the liquid limit and plastic limit.PI = LL- PLNonplastic (NP). When the liquid limit or plastic limit cannotbe determined, or if the plastic limit is equal to or greater thanthe liquid limit, the soil is termed "nonplastic."

The Atterberg limits are defined as:Shrinkage limit. (SL)The shrinkage limit is the water content


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4-1 .-Volume-weight relationshipsSaturated UnsaturatedProperly sample Other useful relationships

2 3 4 5 6 7 8 9Volume W s - V v" 5 - V-(V4+Vw) ~ ( 1 - n ) -f solids Gsrw I+e eVolume

w w"w - eS V- eSV,of water 7 w v ~ - v a S v ~ I+e

Volume Zero v- (vS+ Vw) ( l-S)eV ( 1 - s lev( I-S)Vvva of air ' ' v - v ~ I+eVolume W w V - v nV, eVv v -I +e e Vsof voids TW I-n

WTotal volume " S VV( +ev v S + v w vS+vw+vI fd( I+') - Vs( I+e) -f sample I n e" v 7d e- Vn Porosity - I +eV I-GZ

e Void ratio v 55 1 - 'ciwsat2 Y * IV s 7 d I -n f~-adwsr t '5Weight W-W, - W ~ G ~* s of solids I +w G , v ~ ( I - n ) eS- V %eWsSu~ Weight W -W W W ~ S * ~ V v -f water c. 5 V w fw

W Total weightof sample Ws+Ww Us( I+w) VTd( +w)Saturated /

Wsnt weight of ws+vvawsample A s(1+wsat) v ' ~ ( ~ + w ~ ~ ~ )Submerged

W s u b weightof ws-v,awsaturated samplew 7, 3 Gsa,Dry unit s -l+cl) I +W u s a t I+e'8 weight V s + V v V s + V w + V a caw G s a ~ G~ aGsfw(1-n) w G(I+e)ws,, I+-? 1 + G 5 w s a t

a m Moist unit L(weight v 7d(' Saturated ] rW ( w n ( G , + e ) Iunit weight swsat+l I + eSubmerged

r s , unit weight 7 ~ ~ t - f ~Moisture W

W- 1 -S nScontent 5 6s

Saturated n TW- e a ~ I 7 ~ ~ t - r dW s a t moisture content W s Ti- (1+e)ad .7,

Degree of V VS -"=(,O 2 ~ 1 . 0 W WG, * G S V .saturation v v v * s a t e v zSpecific 5 r d ( l + e ) fd A sG s gravity Vsaw 7 rn

Notes: 1. Weight of air is assumed to be zero.2. Values of w, w S, and n are used as decimals.3. w is the moisture content which corresponds to the particular W or y being used.


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The Unified Soil Classification System

Classification Using Laboratory DataSCS uses the Unified Soil Classification System to classifysoils for engineering purposes. This system is based on theidentification of soils according to their particle-size, grada-tion, plasticity index, liquid limit, and organic matter content.ASTM D-2487 describes the Unified Soil ClassificationSystem. Gradation and particle-size are determined bysieve analyses. Plastic and liquid limits are determined bystandard laboratory tests.This system is for use on naturally occurring soiis. The groupnames and symbols may be used to describe shale, claystone, shells, crushed rock, and the like.Names and S y m b ~ l sThe Unified Soil Classification System uses both names orsoil descriptions and letter symbols to describe a soil'sproperties. The symbol is based on the minus 75mm (3 in)fraction of the material. The soil name description maydescribe larger material.Soils having similar engineering properties are placed ingroups. Each group is designated by a name and atwdet ter symbol. The most important engineering charac-teristic of the group is described by the first letter in thesymbol, and the second most important engineering charac-teristic is indicated by the second letter. Some soils aregiven dual symbols.The letters used as symbols and the soil properties theyrepresent are listed below:

G -gravel C- lay 0 organicS- sand M- ilt PT-peatW -well graded H -high liquid limitP-poorly graded L- ow liquid limit

The names are mostly two-or three-word names that havemodifiers when needed. The names, with their two-lettersymbols are:Coarse-grained soilsGW -well graded gravel SW -well graded sand

GP- poorly graded gravel SP - poorly graded sandGM-silty gravel SM- ilty sandGC -clayey gravel SC -clayey sand

These names may be modified by adding "with silt," "withclay," 'with organic fines," "with sand," and/or "with gravel."Fine-grained soils ML-silt

CL- ean clay MH-elastic siltCH- at clay OL or OH -organic siltCL-ML silty clay OLorOH-organic clay

These names may be modified by adding: sandy, gravelly,with sand, with gravel.Highly organic soilsPT - peatSle w SizesU.S. sieve sizes are used in describing soil classes. Thesieves used in the descriptions and the sizes of their open-ings are shown in table 4-2.Table 4-2.-Sieve designation and size of openings

U.S. standard Size of opening Size of openingsieve sizes in mm in inches

3" 75.0 3"314" 19.0 314#4 4.75 3116"

#I 0 2.00 -#40 0.425 -#200 0.075 -

The plasticity ChartThe plasticity chart, figure 4-4, is used to classify soil fines.It is constructed with the liquid limit (LL) as abscissa and theplasticity index (PI) as ordinate. Two lines are plotted on thechart, the A-line and the U-line. The A-line has beenplotted so that it lies approximately parallel to the plot ofmany basic geologic materials and separates clay from silt.To find the formula for the A-line, look horizontal at PI = 4 toand along the line PI = 0.73 (LL-20). The U-line is theapproximate "upper limit" for the plot of natural soils. It isused to check for erroneous data. Any test data plotting tothe left or above the U-line is probably in error and should beretested for verification. The formula for the U-line is verticalfrom LL = 16 to PI = 7, then along the line PI = 0.9 (LL-8).The plasticity chart also has a crosshatched area above theA-line between Pl's of 4 to 7 that defines an area of siltyclays (CL-ML). The A-line and U-line can be extended forplotting soils with LL > I0 0 and PI> 60.


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4-4.-Unified Soil Classification System plastlclty chart.


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Classifying Fine-Grained Soils. If soil that will pass througha 3-in sieve is passed through a No. 200 sieve, it will bedivided into two portions based on particle size. Theparticles retained on the No. 200 sieve are sand and gravelsize and are called coarse-grained. The particles passingthe No. 200 sieve are termed fines.Soils are classified as fine-grained or coarse-grained by thepercentage of soil that passes the No. 200 sieve. If morethan 50 percent of the soil, by dry weight, is retained on theNo. 200 sieve, it is a coarse-grained soil. If 50 percent ormore passes the No. 200 sieve, it is a fine-grained soil. Thefine-grained soils are classified by referring to the plasticitychart. The name and verbal description include reference tothe coarse fragments in the soil when applicable.

Clay or silt. Plasticity is one of the most important indexproperties of a fine-grained soil. Therefore, a fine-grainedmaterial is classified first according to its plasticity. Thename associated with the more plastic soils is clay (C), andthe name for the less plastic or nonplastic soils is silt (M).A soil whose plot of the liquid limit and plasticity index on theplasticity chart (figure4-4) falls on or above the A-line andhas a PI of 4 or greater is classed as a finesrained clayeysoil (C). If it falls below the A-line or has a PI less than 4,the material is classified as a fine-grained silty soil (M). Siltyclays (CL-ML) have Pl's of 4 through 7 and fall above theA-line. This is in the crosshatched area on the plasticitychart.Under the Unified Soil Classification System, "clay" or"clayey" and "silt" or "silty" is based on the PI and LL.High or low liquid limit The second most important indexproperty in classifying a fine-grained soil is its liquid limit.Soil that has a liquid limit of 50 or greater has a high (H)liquid limit and may be either a clay or silt. Clays with a highliquid limit are called fat clays and have the symbol CH. Siltswith a high liquid limit are called elastic silts and have thesymbol MH. Soil that has a liquid limit less than 50 has a low(L) liquid limit and may be eithera clay or a silt. Clays with alow liquid limit are called lean clays and have the symbol CL.Silts having a low liquid limit are called silts and have thesymbol ML.Fine-grained soils with sand andlor gravel. If the soil has 15percent or more but less than 30 percent sand and/or gravelthen the words 'with sand" or "with gravel" are added to thegroup name. Use "with sand" if the coarse-grained portion isone-half or more sand. Use "with gravel" if thecoarsegrained portion is more than one-half gravel.Examples: silt with sand, ML; fat clay with gravel, CH.

If the soil has 30 percent or more sand or gravel, add thewords "sandy" or "gravelly" to the group name. Examples:sandy elastic silt, MH; gravelly lean clay, CL.Organic soils. Silts and clays that contain enough organicmaterial to affect their engineering behavior significantly areclassified as organic soils. They can be identified n thelaboratory as being organic by performing iquid limit tests onboth air dried and oven dried samples. If the liquid limit ofthe oven dried sample is less than 75 percent of the liquidlimit of the air dried sample, classify the soil as organic (0).It is either a low liquid limit organic silt or clay (OL) or a highliquid limit organic silt or clay (OH) based on its air dry liquidlimit and plotted position on the plasticity chart. Soils that fallon the crosshatched area of the chart are classified asorganic clay (OL).A soil composed primarily of plant tissue in various stages ofdecomposition is a highly organic soil and should be classi-fied as peat (PT).Summary of fine-grained soils.There are eight major break-downs of fine-grained soils: clayey soil with low liquid limit, orlean clay (CL); clayey soil with high liquid limit, or fat clay(CH); silty soil with low liquid limit, or silt (ML); silty soil withhigh liquid limit, or elastic silt (MH); organic soil with low liquidlimit, or organic clay (OL) and silt (OL); organic soil with highliquid limit, or organic clay and silt (OH);silty clay (CL-ML);and highly organic soil peat (PT).ClassifyingCoarse-Grained Soils. Coarse-grained soilparticles are divided into sand and gravel by passing themthrough a No. 4 sieve. Particles retained on the No. 4 sieveare gravel-size. Particles passing the No. 4 and retained onthe No. 200 are sand-size. If more than 50 percent by dryweight of the coarse portion of a coarse-grained soil isgravel-size, the soil is classified as gravel. If 50 percent ormore of the coarse fraction of a coarse-grained soil is sand-size, it is classified as sand.Soils with less than 5percent fines. If less than 5 percent ofthe total sample by dry weight is fines, the fines do not affectthe soil's engineering properties for most uses. These soilsare sometimes referred to as "clean sand or gravel." Onlythe characteristics of the coarse portion are important. Theclassification as sand or gravel is the material's primarycharacteristic.The second most important characteristic of soil'having lessthan 5 percent fines is its gradation (range of particle sizes).The soil may consist predominantly of one size; a mixture ofcoarse and fine materials with the intermediate sizes missing;or a mixture of relatively equal portions of all particle sizes.Soils in the first two groups are classified as poorly gradedand those in the last group as well graded. The range of


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particle sizes may be obtained by making a sieve analysis.The results are plotted on a grain-size distribution graph(figure4-5) and analyzed to determine if they meet thefollowing criteria.Gravels are well graded under the criteria of the Unified SoilClassification System when:

Dso .Cu =- IS greater than 4, andDl0Cc = in at least 1 but not more than 3D l ~60

Sands are well graded when:

Cu =& is greater than 6, andD l0Cc -*s at least 1 but not more than 3D l ~D60

Where:Cu = Coefficient of uniformityCc = Coefficient of curvature

D = Diameter of particles determined by a sieve orhydrometer analysis

D6,, D D = Diameter of particles such that 60 percent,10 percent, and 30 percent of the sample issmaller than that diameter. The D sizes canbe determined by reading from a grain-sizedistribution graph opposite 60, 10, and 30percent fines by dry weight.

Both conditions (Cu and Cc) must be met in order to have awell graded soil. If one or both of the conditions is not met,the soil is poorly graded.Some soils can be classified by visual inspection of the grain-size distribution graph. When their plot is smooth and isconcave upwards between the 10 percent and 60 percentlines they are well graded. The gravel plotted on figure 4-5is well graded. Soils that are not well graded are poorlygraded.There are four classifications of coarse-grained soils wherethe fines content is less than 5 percent. They are wellgraded gravel (GW), poorly graded gravel (GP), well graded

sand (SW), and poorly graded sand (SP). If a gravelcontains more than 15 percent sand, add "with sand" to thesoil name. If a sand contains more than 15 percent gravel,add "with gravel" to the soil name.Classifications of the coarse soils described above are basedon sieve analysis only.Soils with 12 to 50percent fines. If a soil is coarse-grainedand has a fines content of more than 12 percent but lessthan 50 percent, the primary behavior characteristic is thesame as for a clean coarse-grained soil, whether the soil is asand or gravel. The seconci behavior characteristic is basedon that portion of the material passing the No. 40 sieve. Ifthis portion of the material is clayey (has a PI > 7 and plotson or above the A-line), the material is a coarse-grained soilwith clayey fines (SC or GC). If it has a PI c 4 or plots belowthe A-line, it is a coarse-grained soil with silty fines (SM orGM). If the PI is at least 4 and not greater than 7 and plotson or above the A-line, the soil is clayey sand (SC-SM) orclayey gravel (GC-GM). If a gravel contains 15 percent ormore sand, add "with sand" to the group name. If a sandcontains 15 percent or more gravel, add "with gravel" to thegroup name.There are four major and two minor breakdowns of coarse-grained soils where the content of fines is greater than 12percent. They are clayey gravel (GC) or (GC-GM), siltygravel (GM), clayey sand (SC) or (SC-SM), and silty sand(SM).Soils with 5 o 12percent fines. Materials containing at least5 percent but not more than 12 percent fines are given a dualclassification, which consists of the symbol for the soil withless than 5 percent fines first and the symbol for soils withmore than 12 percent fines second. The dual symbol meansthat the soil has significant engineering characteristicsrepresented by both symbols. The classifications are:SW-SM, SW-SC, SP-SM, SP-SC, GW-GM, GW-GC,GP-GM, and GP-GC. The first symbol is obtained byclassifying the soil as though it had fewer than 5 percentfines. The second symbol is obtained by classifying the soilas though it had more than 12 percent fines. The groupname corresponds to the name associated with the firstsymbol plus "with clay" or "with silt." If a gravel contains 15percent or more sand, add "and sand" to the group name. Ifa sand contains 15 percent or more gravel, add "and gravel"to the group name. Examples: well graded sand with silt(SW-SM), poorly graded gravel with clay (GP-GC). Figure4-6 graphically represents the classification of coarse-grained soils.


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Grain size distributionD l 0 D15 D30 D50 D60 4 3 6 D~~~ cu cc

Fines I Sands I Gravels 1 Cobbless g $-cg g : - - -Sieve opening, (mm) 2 7 z g g 5 q~ 2 2 s- = 3% - - % 2% m s b Ln -


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Figure 4-6.-~lassificatlon of coarse-grained soils by gradation.I

0 10 20 30 40 50 60 70 80 90 100Percent passing No. 4 sieve

50QI>.g 40u)8No 30-zF-V) 20-21CE 12L 5

0

Dual classesGravelsGW-GMGP-GMGW-GCGP-GC-

GM or GC

Dual class sandsI I I I

SandsSW-SMSP-SMSW-SCSP-SC

SUMMARY. Table 4-3 gives the basic outline of the UnifiedSoil Classification System. The Unified Soil ClassificationSystem has several outstanding features:1. It is simple. Technicians are normally concerned with 12soils-four coarse-grained soils, four fine-gra ined soils, andfour combined soils. In addition, there are four organic soils,one h ighly organic soil, and the dual symbol soils.2. It provides information on important physical eharacteris-ties, such as sire , gradation, plasticity, strength, brittleness,consolidation, and potential.Interpretationsbasedon classification must be made withcare. A single classification represents a range of materialsand may approach the classification of materials next to it.An example is an ML with-a LL = 42 and PI = 15. This plotsclose to the A-line on the plasticity chart. It has characteris-tics closer to a CL than to a nonplastic ML (PI = 0). An MLwith 49 percent sand and gravel approaches an SM or GMclassification, A well graded nonplastic SM with 15percentfines may have a h igh resistance to piping, whereas a poorlygraded SM with 45 percent nonplastic fines and fine sandswould have very low resistance to piping.Detailed discussion and problems on the Unified SoilClassification System are g iven in the SCS Soil MechanicsTraining Module No. 1.

Figure 4-7 shows the general relationsh ip between theUSDA textural soil classes, the U nified Soil Classifications,and the AASHTO so il classes. The clsssifications in thedifferent systems are based on differing soil characteristics,and any attempt to translate between systems will beapproximate at best. Design decisions should not be basedon translations between systems.This chart was prepared by using the textural classificationas the basis and estimating the Unified and AASHTOclassifications rom the texture. Texture is based on thematerial passing the No. 10 sieve. The larger material,between the No. I 0 an d 3" sieves, is called gravel in thetextural classification. The amount of gravel is indicated byadding a modifier to the texture name. When the soilcontains less than 15 percent gravel, the texture is notmodified. 'Gravellyn indicates a gravel content of 15 percentto 35 percent. 'Very gravelly" describes soil that has 35percent to 60 percent gravel, and "extremely gravelly" is usedwhen the gravel content is over 60 percent. Some soilsurvey reports have used other definitions of the modifiers.Definitions should be checked to see if they correspond tothose used in this chart.In the chart, 'f" stands for fines; that is, the material thatpasses the No. 200 sieve.


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Soil Cl~s sif ica ~Criteriafor Assigning Gmup end Group NamesUsing Laboratory TestsA Group . ~ r o u ~ rns m

FinesdasslfyssCLorCH SC Ctayey sendqH'-PI> 7 and plots on or above 'A' lineJ CL LeandavKL-'PI


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Figure 4-7.-General relationships between systems used to classify soils--USDA Textural Classification,USCS, AASHTO.


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Figure 4-7.-Continued.

Loam(sil)ILow or NP f I fr36%, ~ ~ 5 4 0 ,I ~ I OMica, Iron oxide 1 f236%. LL241. ~ l r l l~oalinitic lay

f250%, LLc50 I f236%, LL5.40. ~ 1 2 1


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Figure 4-7.-Continued,Modiifier

YoGravelUSDATextureLoam(1).

SandyLoam

LoamySand(1s)

-. .USCS AASHTO USCS AASHTO

'JewGravelly35 % to 60%USCS I AASHTO

;M A-4;M A-7jM A-2-45M A-2-7jC A-6;C A-2-6; A-2-4;M A-2-7>C A-2-6

=F

Extremely SOILGravelly PROPERTIES60% to 90% RELATEDTOCLASSIFICATION

IUSCS 1 AASHTO 11 USCS 1 AASHTO I- Low or NP fMica, Iron oxide f236%, LL241, PI211


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Figure 4-7.-Continued.

RELATED TO

Figure 4-8.-Relationship between particle size and the USDA textural soil classes, the Unified SoilClassification System, and the AASHTO soil classes.I USDA Clay I Silt I Sand I Gravel I

0.002mm 0.05mm No. 10 3 inI I II I I II No. 200 No. 4

AASHTO Silt-clay I Sand 1 Gravel I


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ield Identification and Descriptionof Soils

by visual-anual methods. The methods are similar to those in ASTMB2488. The classification is basically the same as

section. It does contain someto adapt it to the less precise visual-manualascompared to laboratory methods. It must be

report that classification is based onmethods.

and Borderline Classesonfuse the dual classifications

eding section and the borderlinesometimes used in the field procedure. Dual

s may be used in field classification where soils fallual categories described in the preceding section.

fine-grained portion ofsoils falling in the crosshatched area on theML), and coarse-grained soils that have

to 12 percent fines (SPSM, SW-SC, G P q M , GW-GC,

used when classifying with laboratory data.a soil has properties that do not distinctly place it in abe given a borderline symbol indicatingA

group symbolsated by a slash, for example, CUCH, GMISM. The

for "or" (CUCH means CL or CH). The firstthe most likely classification, or it is

ification of similar samples from the adjacent area.name for the first symbol except for

CUCH - ean to fat clayMUCL- layey siltCUML - silty clay

ymbols should not be used indiscriminately.y effort should be made to place the soil in a single

entages of sand, gravel, and fines are estimated to5 percent (5%, lo%, 15% etc). If some sand,sent but are estimated to be less than

percent, the quantity is listed as a trace. Estimating inles of 5 percent results in a small change in the

f clean sands and gravels (SW, SP, GW , GP),coarse-grained soil as

5 percent or less fines, rather than less than 5as listed in laboratory classifications.

Apparatus needed orthe ests1.A small supply of water.2. Pocket knife or small spatula.Useful auxiliary apparatus:1. A jar with a lid or a test tube with stopper.2. Small bottle of diluted hydrochloric acid (one part ION HCL

to 3 parts of water).3. Hand lens.Sample size. In order to classify a soil accurately, thesample must be large enough to contain a representativepercentage of each particle size. Recommended minimumsample size based on maximum particle size is given in table4-4.Table 4-4.-Minimum sample size

Maximum Minimumparticle-size sample size

5mm (3116 in) 100g (0.25 Ib)9.5mm (318 in) 200g (0.5 Ib)19mm (314 in) 1kg (2.2 Ib)

38mm (1-112 in) 8kg (18 Ib)75mm (3 in) 60kq (132 Ib)

Field classificationSoil should be classified using several different field testsrather than a single test. Screening and weighing of samplesis not intended. Estimates of percentages of materials by dryweight may be made using visual finger techniques. With theexception of grain-size and gradation, these tests areperformed on that portion of the sample smaller than the No.40 sieve. When coarse materials that can be separated byhand are removed, the remaining material is roughly thatportion passing the No.40 sieve. Practice with materials ofknown percentages will be helpful in perfecting thesetechniques.Highly organic soil. First determine if a soil classifies ashighly organic peat (PT). Peat is composed primarily ofvegetable tissue at various stages of decomposition. It hasan organic odor, a dark brown to black color, a spongyconsistency and a texture ranging from fibrous to formless.A soil classified as peat needs no further classificationprocedure. The significant items in the description still needto be completed.Percentages of sand, gravel, and fines. If the soil is notpeat, the next step in classification is to estimate the percent-ages of sand, gravel, and fines in the sample. The percent-age is by dry weight, which differs from volume. A sample


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that is one-half grave l and one-half fines by volume would beabout 60 percent gravel and 40 percent fines by weightbecause the fines have more voids and thus lower densitythan the gravel.Estimates should be to the nearest 5 percent, and thepercentages must total 100 percent. If some component ispresent but constitutes less than 5 percent of the sam ple,then list it as a trace. A trace does no t constitute part ~f the100percent. For example: 65 percent fines, 35 percent sandwith a trace of grave l.Some suggested procedures for estimating percentages ofsand, gravel, and fines are:

Jar Method -Thoroughly shake a mixture of soil and waterin a straight-sided ar or test tube, then a llow the mixture tosettle. Sand sizes will fall out first, in 20 to 30 seconds, andsuccessively finer particles will follow. The proportions ofsand and fines can then be estimated from their relativevolumes.Mental Sacking - Mentally visualize the gravel-size pa'rticlesplaced in a sack or other container and the sand and finesin a differen t sack or sacks, then mentally compare thenumber of sacks or containers with gravel, sand, and fines.Inspection- Spread the sample on a flat surface andexamine the particles to determine the approximate grainsize. If more than 50 percent of the sample by weight hasindividual grains that are visible to the naked eye, thematerial is coarse-grained. If less than 50 percent, it is afine-grained material.

y pa rticles may appear to be sand -site grains.e the sam ple and break these aggregates down byn the thumb and forefinger.e grains can be detected, as they w ill feel rough and

(less than 5 percent fines) will notwhen handled. Practicing withles that have known percentages of sand, gravel, anding to estimate the percentages.

soilssoil is fine-grained if it contains 50 percent or more fines.il may be given a borderline classification f theortion of fines is 45 to 55 percent. Select aand remove the pa rticles40 sieve. This is about the smallest sizethat can be removed by hand. About a handful ofterial will be needed. Use this material to perform thetoughness, and strength tests. These tests are4-8.

Dllatancy.Select enough material to mold nto a ball about15mm (112 in) in diameter. Add water, if needed, un til it hasa soft but not sticky consistency. Smooth the soil in the palmof one hand with the blade of a knife or spatula. Shakehorizontally, striking the side of the hand against theotherseveral times. Note the appearance of water on the surface.Squeeze the sample and note t b isappearanceof water.Describe the reactionas:None-No visible change.Slow -Water appears slowly on the surface during shakingand does not disappear or disappears slowly whensqueezed.Rapid -Wa ter appears quickly on the surface during shakingand disappears quickly when squeezed.Toughness. Take the specimen from the dilatancy test,shape it into an elongated pat and roll it on a hard surfacg orbetween your hands into a thread about 3rnm (1/8n) indiameter. If it is too wet to roll, spread it out and let it dry.Fold the thread and reroll repeatedly un til the threadcrumbles at a diameter of 3mm (118 in). The soil has thenreached its plastic limit. Note the pressure required to rollthe thread and the strength of the thread, Circum ferentialbreaks in the thread indicate a CH or CL material. Longitudi-nal cracks and diagona l breaks indicate a MH material. Afterthe thread crumbles, lump the pieces together and kneaduntil the lump crumbles. Note the toughness of the materialduring kneading. Describe the toughness of the thread as:Low -Only slight pressure is required to ro ll the thread nearthe plastic limit. The thread and lump are weak an d sol?.Medium -Medium pressure is required to roll the thread nearthe plastic limit. The thread and the lump have mediumstiffness.High -Considerable pressure is required to roll the threadnear the p lastic limit. The thread and lump are very stiff.If a thread cannot be rolled, the soil is nonplastic.Dry strength. Take enough m aterial to m old into a ballabout 15mm (112 in) in diameter. Add water, if necessary,and m old the material until it has the ccinsistency of putty.From this m aterialmake at least three test specimens about5mm (114 in) in diameter and allow them to air dry. Naturaldry lumps of about the same size may be used. The naturallumps will usually have a lower strength than moldedmaterial. Do notuse natural lumps that contain medium orcoarse sand.


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Crush the dry lumps and describe their strength as:None- The specimen crumbles into powder with thepressure of handling.Low -The specimen crumbles into powder with fingerpressure.Medium -T he specimen breaks into pieces or crumbles intopowder with considerable finger pressure.High -The specimen cannot be broken with finger pressure,but can be broken between the thumb and a hard surface.Very High -Th e specimen cannot be broken between thethumb and a hard surface.Determ ine if the soil is organic or inorganic. An organic soilcan be identified by its odor and its dark brown or black color.Classify inorganic soils using the criteria in table 4-5,Classify organic soils using the criteria in table 4-6.Additional tests that may be useful in classifying fine-g raine dsoils are as follows:

Ribbon Test - Use a sample that has a moisture content ator slightly below the plastic limit. Form a ribbon by squeez-ing and working the sample between the thumb andforefinger.A weak ribbon that breaks easily indicates an ML soil. Ahard ribbon which breaks fairly readily indicates an MH soil.A flexible ribbon with medium strength indicates a CL soil.A strong, flexible ribbon indicates a CH soil.Adhesion Test-Saturate some soil and let it dry on yourhands. An ML soil will brush off with little effort. A CL orMH soil rubs off w ith moderate effort. A CH soil requiresrewetting to remove it completely.

* Shine Test- n making the shine test, be sure the soil is notmicaceous. Rub a small clod of moist soil with a knifeblade. A reflective and shiny surface indicates highplasticity. You are seeing the shine on the surface of theclay fines.Estimating liquid limit. Take a pa t of moist soil with avolume of about 8cc (112 cu in) and add enough water tomake the soil soft but not sticky. Rapidly add enough waterto cover the outer surface. Break the pat open immediately.A positive reaction has occurred when the water haspenetrated through the surface layer. If the water haspenetrated, the LL is low. If the water has not penetrated,the LL is high. Visual observation of this phenomenon ismuch easier in direct sunlight.

Table 4-5.-4lasslfication of inorganic finegra inedsoilDry Group GroupDllatancy Toughness strength name symbol

Slow to NP or low None to low Silt MLr w i dNone to Low to med. Low to medium Elastic silt MHSlowNone to Medium Medium to high Lean clay CLslowNone High High to very Fat clay CHhigh

Table 4-6.--Clas sifica tion of organic fine-grainedsoil - -DY Group GroupDilatancy Touqhness strength name svmbolSlow to NP None to low Organic silt OL

--None to Low Low to medium Organic clay OLNone to NP to low None to Organic silt OHslow mediumNone Low to med. M edium to high Organic clay OH

Modifiersfor sand and gravel. Mod ifiers will be added tothe soil group name of fine-grained soils to indicate thepresence of sand and/or gravel.If the so il has 15 to 25 percent sand andlor g ravel, the words'with sand" or "with gravel" will be added. If sand predomi-nates, use "with sand." If gravel predominates, use 'withgravel." Examples are organic silt with sand, OL; and leanclay with gravel, CL. Notice that where percentages areestimated to the nearest 5 percent, the division between soilsmay be stated differently than when using laboratory data forclassifying. In the preced ing paragraph, instead of saying'15 percent but less than 30 percent," it says " I 5 to 25percent." (25 percent is the first percentage less than 30percent.)If 30 percent or more of the soil is sand or gravel, the words'sandy" or 'gravelly" shall be added. Add "sandy" if there isas much or more sand than gravel. Add 'gravelly" if there ismore gravel than sand. Examples are gravelly fat clay, CH;and sandy organic clay, OH.Classifying coarse-grainedsoilsIf a soil has less than 50 percen t fines, it is classed as acoarse-grained soil.The soil is a gravel if the percentage of gravel is greater thanthe percentage of sand.


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4-9.-Dilatancy, strength, and toughness tests.

Method of shakingDilatancy test

Strength test

Toughness test

A. Method of rolling thread.8. hread of soil above plastic limit.C. Crumbling thread as plastic limit is reached.

After Dept. of the Army


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The soil is a sand if the percentage of sand is equal to orgreater than the percentage of gravel.The so il is well graded if it has a wide range of particle sizesand a substantial quantity of all intermediate sizes.A soil is poorly graded if it has a narrow range of pa rticlesites or if it has a wide range of sizes with some intermediatesizes missing (gap graded).If the soil has 5 percent or less fines, it is a clean coarse-grained soil, of which there are four kinds:

Poorly graded sand, SPWell graded sand, SWPoorly graded gravel, GPWell graded gravel, GW

If a coarse-grained soil has 15 percent or more fines, then itis a sand with fines or a gravel with fines. Class ify the finesas clay or silt as described under fine-grained soils. The so ilwill be class ified as one of the four coarse-grained soils withfines:

Silty sand, SMClayey sand, SCSilty gravel, GMClayey gravel, GC

If a coarse-grained soil has 10 percent fines, it is given aborderline classification. The first symbol is to correspond tothe symbol for clean sand or gravel (SW, SP, GW, GP ) andthe second to the symbol for sand or gravel with fines (SM,SC, GM, GC). The group nam e will correspond to the groupname for the first symbol plus "with silt" or "with clay" toindicate the plasticity of the fines. Examples are: wellgraded sand with silt, SW-SM; and poorly graded gravel withclay, GP-GC.If the coarse-grained soil is predominantly sand or grave l butcontains 15 percent or more of the other, then the words"with sand" or "w ith gravel" will be added to the group name.For example: well graded sand with gravel, SW; and poorlygraded gravel with sand and silt, G P G M .

The visual-manual classifications are summ arized in table4-7.Soil descriptionWhen a soil boring is made, a test pit dug, or a natural soilexposure examined, a log should be kept of the obse rva-tions. Table 4-8 is a checklist of recomm ended informationto be included in the log. Definitions of most of the items onthe list are given in this chapter.Table 4-8.-Soil description checklist1. Project and locationa. Test hole num ber and loca tion (table 4-8)b. Sample number and depth of samp ling2. Group name and group symbol (table 4-7)3. Maximum particle size (page 4-25)4. Percentage of cobbles and/or boulders (page 4-25)5. Percentage gravel, sand, fines (page 4-19)6.Particle-size range of coarse mate rial (table 4-1 0)a. Gravel-fine, coarseb. Sand-fine, medium, coarse7. Particle angu larity and shape (page 4-26)-angular,subangular, subrounded, roundedParticle s h a p e fl a t, elongated, flat and elongated8. Dilatancy (page 4-20)None, slow, rapid9. Toughness (page 4-20)Low, medium, high10. Dry strength of fines (page 4-20)None, low, medium, high, very high11. Plasticity of fines (page 4-25)Nonplastic, low, medium, high12. Odor - only if organic or unusual13. Color (page 4-27)14. Natural moisture content (page 4-27)Dry, moist, wet15. Reaction with HCI (page 4-27)none, weak, strong16.Consistency (page 4-27)

Very soft, soft, firm, hard, very hard17.Structure (page 4-27)Stratified, laminated, fissured, slickensided, lensed,homogeneous18. Cementation (page 4-27)Weak, moderate, strong19.Geologic name, soil series, or local name20.Additional comm ents


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4-7.-Field identificationby visual-manual methods

cm5- LLiJtiUrn>b4-L C w z$0- 8

I 0. trace or 5% I well graded

0, race or 5%fines

s -

- I fines-. I poorly graded

well gradedpoorly graded

I

10% fines

h

GW 1 well graded gravel 1

15% to 45%fines

clayeyfines

siltyfinesclayey fines

silty fines

GP-GM poorly graded gravel with s ilt- x + z z r +

GPGW-GCGP-GCGW-GM

well gradedpoorly gradedwell graded-oorly graded GM silty g a v e l I

poorly graded gravelwell graded gravel with clay

1

poorly graded gravel with claywell graded gravel with silt

SW I well graded sand ISP I poorly graded sand I

fines10%fines

finespoorly gradedwell gradedpoorly graded

Non or low plastlc fines with significant organics OLL organic siltplastic fines CH fat clay

" i5

V) .-Gj 3%d2 .Ecogs< ?g-

nonplastic & low plastic finesI

SW-SMSP-SM

plastic fines wlth significant organics

well graded sand with siltpoorly graded sand with silt-

15% to 45%fines

I nan or low plastic flnes with significant ~roanics I OH I organic silt I

-clayey fines

silty fines

for table 4-7 with organic fines GM, clayey sand with organic fines.Soils that.have 45%, 50%, or 55% fines may be classified f. Soils that hav e nearly the same percentage of sand anda borderline symbol, such a s silty sand GMIML, silt M U gravel may b e given a borderline classification, such as wellgraded gravel with sand GWISW.

. If the soil contains 15%or more sand, ad d "with sand" to g. If the soil has 15% o 25% sand or gravel, add "with sand"

plastic fineslow plastic fines with very low liquid limitnonplastlc & low plastic finesplastic fines with significant organics

ORGANIC SolL

he a r o u ~ame. or "with gravel" to the group name, such as lean clay with

SCSM

e soil contains 15% or m ore gravel, add "with gravel" to sand CL; silt with gravel ML.h. If the so il has 30% to 45% sand or gravel, add "sandy" orhave liquid limits nea r 50may be classified with "gravelly" to the group nam e, such as gravelly organic clayas lean to fat clay CUCH; silt M U OL; sandy fat clay CH.

H. i. Soils that fall close to the border between silt and clay mayIf the fines have sign ificant organic m atter, add 'with be given a borderline classification, such as silty clay CUML;nes" to the g roup nam e; for example, silty gravel clayey silt MUCL.

clayey sandsilty sand

CLCL-ML

MLOL

primarily organic matterdark in color, organlc odor

lean claysilty claysiltorganic clay

PT peat


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umbering test holes and samples. Use the followingstandard system of numbering test holes.Table 4-8.-Test holenumberingsysteml&x&!mCenterline of damBorrow areaEmergency spillwayCenterline of principal spillwayStream channelRelief wellsOtherOther

Hole Numbers1- 99101-199201 299301-399401 499501-59960 -699701-799, etc.

Principal spillway, channel, and emergency spillway holesthat are on the centerline of the dam should be givenprincipal spillway, channel, and emergency spillway numbersrather than centerline of dam numbers. Number foundationholes in the area of the base of the dam, but not in theimmediate vicinity of the centerline of the dam or appurte-nances, as 'other."Samples are numbered by using the test hole numberfollowed by a period and the sample number. Samples fromtest hole number 104 would be numbered 104.1, 104.2,O4l.3 from the top down:Maximum particle size. If the largest size particles aresand-size, then describe them as fine, medium, or coarsesand. If the largest particles are gravel-size, list the smallestsize sieve that will pass all particles. If the largest particlesare cobbles or boulders, list the maximum dimension of thelargest cobble or boulder.Percentages of cobblesand boulders. Cobbles andboulders are definedas:

Cobbles- Particles that will pass a 12-inch square openingand will be retained ona 3-inch sieve.Boulders-Particles that will not pass a 12-inch squareopening.

Estimate the percentage of cobbles and boulders by volume.The report should state "estimated by volume," since otherpercentages are by weight. Example: 'estimated 10 percentcobbles (by volume), maximum size 7 inches."Particl~sizeange of coarse materials. Table 4-10 givesparticle-size ranges and guidelines for estimating particlesires.

Table 4-10.-Sand and gravel sizesTerm Particle size ExampleGravel 75 to 4.75mm (3 to 3116 in) Orange to peaI. oarse 75 to 19mm (3 to 314 in) Orange to grape2. Fine 19 to 4.75mm (314 to 3116 in) Grape to peaSand 4.7t to 0.075mm (#4 to 6200) Pea to powde red sugar1. Coarse 4.76 to 2.00mm (#4 to # lo ) Pea to rock salt2. Medium 2.00 to 0.425mm (# I 0 to #40) Rock salt to table salt3. Fine 0.425 to 0.075mm (#40 to Table salt to powdered#zoo) sugarFines Less than 0.075mm (#200)

Particle angularityAngular-Particles have sharp edges and relatively planesides with unpolished surfaces.Subangular- Particles are similar to angular but have

rounded edges.Subrounded-Particles have nearly plane sides but have

well-rounded corners and edges.Rounded - Particles have smoothly curved sides and no

edges.Particle shape. In particle shape, the length, width, andthickness refer to the greatest, intermediate, and leastdimension.

Flat: W/T 2 3Elongated: U W > 3

Flat and elongated particles meet criteria for both flat andelongated.Plasticity of Fines. The plasticity of fines can be estimatedwith the same test as the toughness test, according to thefollowing criteria.

Nonplastic-A 3mm thread cannot be rolled at any watercontent.Low-The thread can barely be rolled, and the lump cannotbe formed when drier than the plastic limit.Medium -The thread is easy to roll, and not much time isrequired to reach the plastic limit. The thread cannot bererolled after reaching the plastic limit. The lump crumbleswhen drier than the plastic limit.High-Considerable time rolling and kneading is required toreach the plastic limit. The thread can be rerolled severaltimes after it reaches the plastic limit. The lump can beformed without crumbling when drier than the plastic limit.


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Figure 4-10.-Particle shapes.

Figure 4-1 1.- Flat and elongatedparticles.


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Color. Soil colors should be determined from moist samples.If the soil contains layers or patches of varying colors, theyshould be recorded along with their colors.Natural water contentDry-Absence of water, dusty, dry to the touch.

Moist-Damp but no visible water.Wet-Visible free water; usually soil is from below thewater table.

Reaction with HCINone- No visible reaction.Weak-Some reaction with bubbles forming slowly.Strong -Violent reaction with bubbles forming immediately.

Consistency. Consistency is defined as the relative easewith which a soil can be deformed, either in the undisturbedor molded state. Degrees of consistency for fine-grainedsoils are described by the terms very soft, soft, firm, hard,and very hard. Table 4-1 0 gives a field identification methodand estimated strength for soil consistency classes.StructureStratified -Alternating layers of varying material or color;

note thickness.Laminated-Alternating layers of varying material or colorwith the layers less than 6mm thick; note thickness.Fissured-Breaks along definite planes of fracture with littleresistance.Slickensided- Fracture planes appear polished or glossy,sometimes striated.Blocky-Cohesive soil that can be broken down into smallangular lumps which resist further breakdown.Lensed- nclusion of small pockets of different soils suchas small lenses of sand scattered through a mass of clay;note thickness.Homogeneous-Same color and appearance throughout.

CementationWeak-Crumbles or breaks with handling or little fingerpressure.

Table 4-1 0.-Field determination of consistency offine-grained soil-Shear strengthIdentification ~ ~ l c r n ~ low count

Consistency procedure (tonsltt 2, (blowsl~)Very soft Thumb will penetrate soil Less than 0.25 0 - 4more than 25mm (1 in)

soft Thumb will penetrate soil 0.25 to 0.5 5 - 8about 25mm (1 in)

Firm Thumb will indent soil 0.5 to 1.O 9 - 1 5about 6mm (114 in)Hard Thumb will not indent 1.O to 2.0 15-30

soil but soil isreadily indented withthumbnail

Very Hard Thumbnail will not indent Over 2.0 Over 30soil

Moderate -Crumbles or breaks with considerable fingerpressure.Strong -Will not crumble or break with finger pressure.

Site InvestigationAll construction sites need a geologic investigation. Theintensity of investigation depends on design requirements,complexity of the site, and class of the structure. Detailedinvestigations may not be necessary for small, low-hazardstructures such as farm ponds, drop structures, or chutesbuilt in areas of generally homogeneous soil materials. Forsuch structures, the relevant engineering characteristics ofsite materials need only be recognized and evaluated on thebasis of experience in the area and appropriately docu-mented.Sites for terraces, diversions, and waterways should bechecked for soil classification, thickness of topsoil, suitabilityof the subsoil for the intended crops, stoniness of soil, depthto bedrock, permeability, elevation and slope of the watertable, and artesian pressure. In open ditch design, erosionresistance and stability of the side slopes depend on soil typeand ground water conditions. Construction costs areaffected by soil type, stoniness, depth to rock, and depth towater table. Foundations for dams must be checked forstrength, permeability, compressibility, dispersive clays(piping), water table elevation, and depth to bedrock. Theerosion resistance of the emergency spillway materials mustbe estimated. An adequate quantity of suitable borrow mustbe located. Waste storage structure sites must be investi-gated to ensure the integrity of the structure and to avoidpollution of ground water. A site should be checked for soilclassification, permeability, depth to bedrock, and elevationof the water table. All existing and proposed wells andsprings that may be affected by the structure should belocated on a map.


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Preliminary data. First gather and record all useful soil andgeologic information that is available from soils and geologicmaps. This information should be recorded on a site map.Next, walk over the site, observe the surface geologicfeatures and record them on the site map. Features thatmay be visible on the surface are springs, seeps, rockoutcrops, boulders, slides, sinkholes, and manmade open-ings. Excavations, road cuts, and ditch and stream banksgive opportunities to see soil and geologic profiles.Boring program.After this work is completed, take a closelook at the information that has been collected and decidewhat geologic questions have been answered and whatquestions remain. Then ask yourself what additionalinformation is needed to answer these questions. A fewborings with a hand auger may suffice, backhoe pits may berequired, or a full boring and testing program with powerequipment may be required to supply the needed informa-tion. The investigation is continued until all questions areanswered with reasonable assurance. One should nothesitate to ask for assistance from an experienced geologistor engineer.Geologic observations should continue throughout construc-tion and the geologic records updated as new information isdisclosed. Occasionally, new geologic information discov-ered during construction may require design changes.

Detailed requirements or geological investigations, are givenin NEH-8, Chapter 5 .Generally, borings at dam sites should tie made on thecenterline of the embankment and on the centerline of theprincipal spillway. Usually, borings are carried through allcompressible and permeable strata to a relatively incom-pressible, impermeable base, but it is usually not necessaryto go deeper than the dam is high. The number of boringsrequired will depend on the length of the dam and theuniformity or variability of the foundation. The minimumnumber of borings on the principal spillway centerline usually

consists of one on the intersection of the centerline of fill andthe centerline of the spillway plus one each at the inlet andoutlet. Other borings may be needed at a prdpossd drainlocation or to identify unusual foundation conditions.Enough borings need to be made at the emergency spillwaylocation to identify the materials to be excavated and todetermine the erosion resistance of the soils at and belowgrade.A recommended numbering system for soil borings is givenin the section on soil description (table 4-9).Ground waterSoil borings and test pits should be kept open until theground water level has stabilized and the ground watersurface elevation is measured and recorded. If a pit needs tobe backfilled or a drill hole is caving, a perforated pipe shouldbe installed and the ground water elevation measured in thepipe. It often requires24 hours for water elevations tostabilize, and it may take weeks to months to stabilize in soilswith low permeability. Where artesian pressures areencountered, a piezometer may be needed to measure thehead.It is SCS policy to backfill and tightly seal all drill holes andpits completely to prevent ground water contamination bysurface water or to prevent loss of artesian pressure. Filledholes also remove hazards to livestock and equipment.Seasonal variation in the water table should-be determinedby direct observation and measurement at various timesduring the year, or it should be estimated. Sometimes in asoil profile, the upper and lower limits of mottling are'goodindications of the seasonal water table fluctuation. The depthbelow which the soil is entirely grey usually indicates theelevation of the seasonal low water table. The depth abovewhich the soil is primarily red, orange, or yellow usuallyindicates the seasonal high water table.


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Estimation of Soil Performance andPerformance R equirements

Estimates of soil behavior, such as its shear strength,compressibility, or permeability, can bemade from the soildesc ription and classification. These estimates are adequateto design most low hazard, low cost structures. The rea -sonableness of soil test results should be eva luated bycomparison with estimated soil parameters.StrengthShear strength of soil is made up of two elements, frictionand cohesion.In its simplest form, friction is the resistance o sliding of oneblock of nonplastic soil against another. It is similar to andcan be assumed to have the same action as the resistanceof a block of wood sliding across awooden floor. Thegrea ter the weight p laced on the sliding block, the greater theforce necessary to slide the block. Likewise, as load isapplied to the soil or the intergranular pressure is increased,the frictional resistance or frictional shear strength increases.The friction in so il is different from simple friction because ofsuch factors as the irregu larity of the planes, interlocking ofparticles, and size and shape of the soil grains. The shearstrength of clean sands and gravels and nonplastic silts isdue to friction.Cohesion is the result of the ma gn eti dik e attraction ofparticles that is noted in one form as the plasticity andstickinessof soils. It resists the shearing of a b lack of soil.Cohesive strength does not increase with increased oad orintergranular pressure in the soil. Highly plastic clays havemostly cohesive shear strength. Silty and sandy clays canhave a combination of both cohesive and frictional strength.Friction and cohesion increase as soil density increases.Friction increases as particle size increases and as the soilgoes from poorly graded to w ell graded.Over a long time period, clay soil that has mostly cohesivestrength may creep like a highly viscous liquid. This is why aretaining wall for clay m ay tip over after 20 years eventhough the soil supported itself vertically while the w all wasbeing bu ilt.Silt-size particles are larger than clay particles and generallydo not have significant cohesive attraction, as clay does. Siltparticlesact more like sand, having frictional strength; butbeing smaller, have less strength than sand.Most soils are a mixture of sand, gravel, silt, and clay andexhib it characteristics of both cohesive and noncohesive soil.Soil usually needs more than 35 percent sand and gravelbefore the coarse materials significantly affect the strengthcharacteristics.

Stable slopes for moist soils of medium or greater densitycan be estimated as:Rock 1 o 1-1 I2Gravel 1-112 to 2:1Sand 2:l to 2-112:lClay 2-112 to 4:lSilt 3 to 4:1

Silts and sands with water seeping from them and soft clayspresent difficult stability problems and should be analyzedusing refined techniques on a case-by-case basis.PermeabilityPermeability is the property of a porous medium that allowswater to flow through it. The porous mediums of interest aresoil and rock.Water moves through the voids between soil particles and,with few exceptions, through joints, fractures, and solutioncavities in rock. In some rock, such as certain sandstones,water moves through the rock mass. The symbol forpermeability is K, and it is given as a velocity through thetotal soil mass at a gradient of 1. The most commondimensions used for K in SCS are cmlsec and ftlday. (2830cmlsec = 1 Wday.)Permeability of coarse-grained soils and fine-g rained soilsmust be considered separately. In order to treat a soil as acoarse-grained soil when estimating permeability, it musthave less than 5 percent fines, and the fines must benonplastic. Sometimes, even 5 percent fines w ill clog the soilpores. The permeability of clean sand and g ravel is con-trolled by the finest 10 to 20 percent of the soil.Permeability of concrete sand will range from about 3.5 x10-3 cmlsec to 3.5 x 10-2cm/sec (10 fi per day to 100 t perday). Gravel of the smallest particle size, about 5mm (3116inch), will have a permeability around 15cmlsec (40,000 f tper day). A gravel of 5mm (318 inch) minimum size has apermeability around 25 cmlsec (70,000 ft per day).The permeability of soils that have a significant content offines depends on the characteristics of the fines. Generally,a proportion of 15percent or more fines is enough forpermeability to be controlled by the fines.Naturally occurring soils usually form aggregates or peds.Water flows between the aggregates and through channelsin the soil created by roots and burrow ing animals and


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A naturally occurringCL may range from beinghaving as much permeability as.5 x lo4 cm/sec (1 ft per day), depending on the soilure and number and size of channels.

significantoil gradation and plasticity,When the soil is compacted dry, the aggregatesbetween them in a mannerto the way it flows in sand. When the soil is com-, the aggregates are flattened and moldeda solid mass of much lower permeability.om the standard Proctored as the estimated divisiondry and wet compaction. Soils that have a signifi-

2 percent dry ofbe expected to have a permeability of 10 totimes that of the same soil compacted2 percent wet of

ity. However, the greatest influence onf soils having a significant content of fines ismoisture content during compaction.

s the property of a soil that pertains to itsto decrease in volume when subjected to load.t of changes in pore volume, soilncompressible. Organic matter in

y used terms related to compressibility aredation, settlement, collapse, and compaction.s the gradual reduction in soil volume resultingstress. It consists of initialwhich is a comparatively sudden reduction inting from the expulsion and compression of gas;consolidation, which results principally from a

ng out of water and is accompanied by a transfer ofsecondaryresulting principally from the adjustment of thef the soil mass after initial consolidation.ent of a structure due to thetion of the underlying soil.

ation of a soil by means ofcal manipulation.soils are low density soils that have considerableh when dry or moist. They lose strength and undergo

sudden compression when they are saturated. Some willcollapse under their own weight when saturated; others, onlywhen loaded.Collapsible soil foundations cause problems when they aresaturated after being loaded by a structure. Examples aresettlement of a dam foundation resulting in cracking of thedam when stored water saturates the foundation; settlementand cracking of an irrigation ditch dike or lining whenseepage from the ditch saturates the foundation; or settle-ment of a building when the foundation soil is saturated withwater from a downspout or leaks in water or sewer pipes.Collapsible soils usually occur as alluvial fans in subhumid toarid areas. Low density soils above the water table in thissituation should be suspected of being collapsible. They canbe identified by consolidation testing of undisturbed samples.In testing, samples are loaded in a moist condition, thensaturated. Collapsible soils will show sudden settlementupon wetting.Bearing capacityThe allowable bearing capacity of a soil is the maximumaverage load per unit area of a footing that will not producefailure by rupture of the supporting soil or produce excessivesettlement. Bearing capacity depends not only on the soil,but on the footing size, shape, and depth and on the maxi-mum settlement the structure can take without excessivedamage. Bearing capacity is most often controlled bysettlement.Bearing capacity of the soil is seldom a problem for light farmstructures except on obviously soft, low density or lowstrength soils. When settlement problems occur, they areusually the result of buildingon two soils with differentsettlement characteristics, such as undisturbed soil andloose backfill.Relatively heavy, rigid structures, such as concrete manuretanks, must be set on strong uniform foundations to avoiddifferential settlements that can cause cracking.Many local building.codescontain values of maximumallowable soil pressure for foundation design. They arebased on experience. Values are also found in engineeringand building construction handbooks. These maximumallowable values are called presumptive bearing pressures.They should be used only for small, low cost structures.CompactionOne method of treating soil to improve its characteristics isby compaction. Compacted soil should be thought of as adifferent material from the soil in its natural state. Increasing


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a soil's density by compaction increases soil strength anddecreases compressibility. Other things being equal,increasing soil density by compaction will decrease permea-bility. However, trying to achieve low permeability in a fine-grained soil by compacting it to a high density can be amistake. High densities require soil moisture near or belowoptimum. Low permeability requires compaction moistureabove optimum. The moisture content at the time of com-paction is more important than density in determiningpermeability. This is discussed further in the section onpermeability.Some adverse effects of compacting soil are as follows:compaction creates a potential for increased swelling insome fine-grained soils; compacting backfill against aretaining wall increases soil pressures against that wall;compaction usually makes a soil less suitable for plantgrowth; and compaction increases the potential for soilcracking as a result of differential movement.When samples of a dry soil are moistened and then com-pacted and subsequent samples are moistened and com-pacted, so that each sample has a higher moisture contentthan the last, the soil will increase in density with increasingmoisture until a "maximum density" at an "optimum moisture"is reached. After that, increases in moisture will result in adecrease in dry density. There is an optimum moisturecontent for each soil and a compaction method at whichmaximum density will be obtained.The type of test described above is known as a Proctordensity test or a moisture-density est. The SCS usuallyuses the moisture-density est described in ASTM D-698.The test sets reference soil densities and moisture contentsfor soil testing and compaction control.The most important items to control when compacting soilare soil moisture, lift thickness, compaction equipment typeand weight, and number of equipment passes.Proctor density tests are the best way to determine the bestplacement moisture content for fine-grained soils. Soils thatmust flex without cracking should be placed at moisturecontents above optimum. At locations where cracking is notimportant, soil can be placed from about 2 percent below to 2percent above optimum. Embankments in arid or semi-aridareas that may crack from deep drying need special consid-eration.Some guidelines that can be used in the field to estimatecorrect moisture where testing is not feasible are:1. If the embankment becomes firm and hard, the moisturecontent is probably too low. Equipment should sink into

the soil a few inches.

2. The soil should have enough moisture so that, whenformed into a ball, it will not break if struck sharply with apencil.3. For most soils, optimum moisture is a little below theplastic limit.The moisture content of clean gravel and coarse sand havevery little effect on their compaction characteristics. Moistfine sand has a very low density when dumped into place. Amoisture content near saturation will make compactioneasier. Dirty sands act much like fine-grained soils withsimilar plasticity.Maximum lift thickness in order to obtain satisfactorycompaction depends primarily on the equipment used forcompaction, assuming the equipment is suitable for the soil.Table 4-1 1 can be used as a guide for lift thickness.Rock si te should be limited to 213 of the loose lift thicknessso that the rock will fit within the compacted litt thickness.For efficient compaction, the equipment must be suitable forthe soil type. Clean sands and gravels need vibratorycompactors. Small, hand-guided plate vibrators are availablefor confined areas. Track tractors are also effective ncompacting clean coarse-grained soils. They should beoperated as fast as practical so they will produce vibrations.Plastic soils need sheepsfoot or tamping rollers. Rubber-tired rollers are good for wet plastic soils, low plastic, andnonplastic fine soil.Erosion resistanceResistance of soil to sheet erosion has been evaluated in thedevelopment of the Universal Soil Loss Equation. The soilerodibility factor was given the symbol K. The erodibilityfactor K should notbe used to evaluate a soil's erosionresistance to flow in a channel (diversions, waterways,drainage or irrigation channels, floodways, emergencyspillways).Procedures to evaluate a soil's resistance to water flow aregiven in Technical Release No. 25, Design of Open Chan-nels, and Technical Release No. 52, A Guide for Design andLayout of Earth Emergency Spillways.Soils resist erosion during channel flow in two ways:(I) Coarse-grained soils, such as sand and gravel, resistmovement by the weight of individual particles. Other thingsbeing equal, the heavier a particle, the greater its resistanceto movement. Platy, elongated, and lowdensity particlesare moved more easily than equidimensional and denser


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Table 4-1 1 -Maximum lift thickness for compac- Table 4-1 2.-Comparative erosion resistance oftion equipment soilsMaximum loose lift thicknessEquipment inches cmHand compactor (mechanical) 4 10Track tractor 6 15Small plate vibrator 8 20Sheepsfoot roller 9 25Rubber-tired roller 12 30Rubber-tired equipment 18 45Vibratory roller 24 60

particles. In TR-25, figure 6-2,a clean coarse-grained soil'sresistance to e rosion is evaluated on its 075 size, the sire atwhich 75 percent by weight of the so il particles are smaller.(2) Fine-grained soils resist erosion as a mass held togetherby the cohesion between particles. TR-25, figure 6-2, rateserosion resistance on the basis of a soil's Plasticity Index (PI)and Unified classification. The higher the PI, up to 20, andthe more clayey the soil, the greater its erosion resistance. ACL. with PI

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Elementary Soil Engineering