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Soil MechanicsBasics
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SoilMechanicsICE225CE 225
Originofsoilandgrainsize
1
2WhySoilproblemsareUNIQUE?
Soildoesnotpossessalinearoruniquestressstrainrelationshipp
Soilbehaviordependsonpressure,time,andenvironmentThesoilatessentiallyeverylocationisdifferentI l ll h f il i l d i d dInnearlyallcasesthemassofsoilinvolvedisundergroundandcannotbeseeninitsentiretybutmustbeevaluatedonthebasisofsmallsamplesobtainedfromisolatedlocations
Mostsoilsareverysensitivetodisturbancefromsampling,and thus the behavior measured by a laboratory test may beandthusthebehaviormeasuredbyalaboratorytestmaybeunlikethatoftheinsitusoil
Soilparticlesize 3
Soilsaregenerallycalledgravel,sand,siltorclay.
Gravelsarepiecesofrocks.
Sandparticlesaremadeofmostlyquartzandfeldspar.
Siltsaremicroscopicsoilfractionsthatcontainveryfinep yquartzgrainsandsomeflakeshapedparticles.
Claysaremostlyflakeshapedmicroscopic/submicroscopicparticlesofmica,claymineralsetc.
No.200 No.4 3in.
cobblessandfines gravel
.075mm 4.75mm 75mm
Clays 4
Whatmakesclayssuchinterestinganddangeroussoilsforfoundations?
Thissimpleexperimentshows the range ofshowstherangeofstrengthforasingleclaysample
Clays 5
Thetermclayreferstoanumberofearthymaterialsthatarecomposedofmineralsrichinalumina,silicaandwater.Clayisnotasinglemineral,butanumberofminerals.
Whenmostclaysarewet,theybecomeplastic meaningthatcanbeformedandmoldedintoshapes.
Whenclayisfired,thewaterisdrivenoffandtheybecomey , yashardasstone.
Clay is easily found all over the world. As a result, nearly allClayiseasilyfoundallovertheworld.Asaresult,nearlyallcivilizationshaveusedsomeformofclayforeverythingfrombrickstopotterytotabletsforrecordingbusinesstransactionstransactions.
Soilcomposition 6
Thecompositionofsoilandrockisquitedifferentfromthatofothercivilengineeringmaterials.g g
Engineeringpropertiesofsoilsvarysignificantlyfromplace to place and even across a single building siteplacetoplace,andevenacrossasinglebuildingsite.
Soilisaparticulatematerialthatconsistsofindividualparticles.
Soil can contain all three phases of matter (solidSoilcancontainallthreephasesofmatter(solid,liquid,andgas)simultaneously,andthesethreephasescanbepresentinvaryingproportions.p p y g p p
Soilasaparticulatematerial 7
Mostcivilengineeringmaterialsconsistofacontinuousmassheldtogetherwithmolecularbonds.
Incontrast,soilisaparticulatematerialthatconsistsofindividualparticlesassembledtogether.g
Itsengineeringpropertiesdependlargelyontheinteractionbetween these particles, only secondarily on their internalbetweentheseparticles,onlysecondarilyontheirinternalproperties.
Thethreephases 8
Soilisdifferentfrommostcivilengineeringmaterialsinthatitcansimultaneouslycontainsolid,liquid,andy , q ,gasphases.
The liquid and gas phases are contained in the voidsTheliquidandgasphasesarecontainedinthevoidsorporesbetweenthesolidparticles.
The three phases often interact and these Thethreephasesofteninteract,andtheseinteractionshaveimportanteffectsonsoilsbehavior.
Soilcomposition 9
Thesoiliseithercoarseorfinegrained.
Coarse grained are visible to the naked eye: G (gravels) + SCoarsegrainedarevisibletothenakedeye:G (gravels) S(sands)
Finegrainedareinvisibletothenakedeye:M (silts)+C (Clays)
Coarsegrainedsoilsarealsocalledgranularandfinegrainedsoilsarecommonlyreferredtoascohesive.
ThenamegravelcomesfromFrenchwordgreve.ItssymbolisG.
ThesymbolSforsandcomesfromFrenchwordsable.
ThesymbolMforSiltcomesfromSwedishwordmjala.
Mechanicalanalysis 10
Mechanicalanalysisisthedeterminationofthesizerangeofparticlespresentinasoil,expressedasapercentage of the total drypercentageofthetotaldryweight.
Two methods used:Twomethodsused: Sieveanalysis:forparticlesizeslargerthan0.075mmingdiameter.
Hydrometeranalysis:forparticlesizessmallerthan0.075mmindiameter.
Sieveanalysis 11
Sievedesignation larger 12
SieveslargerthantheNo.4sievearedesignatedbythesizeoftheopeningsinthesieve.
Commonlyusedlargersievesizes: 3in. 2in. 1.5 in.1.5in. 1in. 3/4in. 1/2in. 3/8in.
Sievedesignation smaller 13
Smallersievesarenumberedaccordingtoth b f i
10openingsthenumberofopeningsperinch.
Commonly used smaller
perinch
Commonlyusedsmallersizesieves: No.4
1 inch
No.10 No.20 No 40No.40 No.60 No.140
N 200No. 10 sieve
No.200
USstandardsievesizes 14
SieveNo. Opening(mm) SieveNo. Opening(mm)
3inch 76.200 20 0.8502inch 50.800 25 0.710c 50 800 5 0 01.5inch 38.100 30 0.6001inch 25.400 35 0.5003/4 inch 19.000 40 0.4253/4inch 19.000 40 0.4253/8inch 9.520 50 0.355
4 4.750 60 0.2505 4 000 70 0 2125 4.000 70 0.2126 3.350 80 0.1807 2.800 100 0.1508 2 360 120 0 1258 2.360 120 0.12510 2.000 140 0.10612 1.700 170 0.09014 1 400 200 0 07514 1.400 200 0.07516 1.180 270 0.05318 1.000
15
Book:GeotechnicalEngineering(CODUTO)Page115
16
Coarsegravel19 0 75 0
Finegravel4 75 19 0 mm 19.0 75.0mm4.75 19.0mm
Coarse SandM di S dFi S d CoarseSand2.00 4.75mm
MediumSand0.425 2.00mm
FineSand0.075 0.425mm
Sieveanalysis 17
Soilusedinsieveanalysisisovendriedandalllumpsarebroken.
Thesoilisthenshakenthroughastackofsieveswithopeningofdecreasingsizefromtoptobottom.p g g p
Apanisplacedbelowthestack. Breaking lumps in clayey soils may be difficult Breakinglumpsinclayeysoilsmaybedifficult.
Inthiscase,thesoilmaybemixedwithwatertomakeaslurry and then washed through sievesslurryandthenwashedthroughsieves.
Portionsretainedoneachsievearecollectedseparatelyandovendried.
Massretainedoneachsieveisdetermined.
Sieveanalysis calculations 18
Determinethemassofsoilretainedoneachsieve(i.e.M1,M2,...,Mn)andinthepan(i.e.Mp).
Determinethetotalmassofthesoil:M=M1 +M2 +...+Mn+Mp.
Determinethecumulativesoilmassretainedaboveeachsieve.Fortheith sieve,itis(Mi)retained =M1 +M2 +...+Mi.
Themassofsoilpassingtheith sieveis(Mi)passing=M (M1 +M2 +...+Mi).
Thepercentofsoilpassingith sieve(orpercentfiner)isF=(Mi)passing/M100.
D10,D30,andD6019
DeterminingD10,D30,andD60
p
a
s
s
i
n
g
60
P
e
r
c
e
n
t
30
10D60D30D10
Grain Diameter
Cu andCc20
C ffi i f if iD Coefficient of UniformityHigh Values Indicate Well-Graded Soil10
60
DDCu =
Coefficient of CurvatureValues Between 1-3 ( )
230
DDDCc = Indicate Well-Graded Soil( )1060 DD
Alsocalledcoefficientofgradation,Cz
Cu andCc21
D
10
60
DDCu =
2D( )1060
30
DDDCc =
Cu andCc22
Grainsizedistributioncurve 23
A fi i d ilA:finegrainedsoilsCu =5Cc =0.8
B i d ilB:coarsegrainedsoils
Cu =13Cc =0.83
Grainsizedistributioncurve 24
C l d d
Cu =1.63Cc =0.96
C:poorlygradedoruniformlygraded
Grainsizedistributioncurve 25
D ll d d il
Cu =190Cc =1.18
D:wellgradedsoils
E:gapgradedsoils
Cu =111C = 0 18Cc =0.18
Grainsizedistributioncurve 26
Wellgraded poorlygradedsoil (uniformlygraded)soil
27Grain-size distribution curve
3/4 in No 4 No 10 No 40 No 200 Hydrometer3 in
80
1003/4 in. No. 4 No. 10 No. 40 No. 200 Hydrometer 3 in.
60
s
s
i
n
g
D10 =0.10D30 =0.18
40
P
e
r
c
e
n
t
p
a
s
D60 =0.25
Cu = 2.50
0
20Cu 2.50Cc =1.30
0.0010.010.1110
Grain diameter (mm)
GRAVEL SANDSILT or CLAY
Coarse Fine Coarse Medium Fine
28
Hydrometeranalysis 29
Basedontheprincipalofsedimentationofsoilgrainsinwater.
Particlesinwatersettleatdifferentvelocities,dependingontheirshape,size,weight,andviscosity of waterviscosityofwater.
Particlesareassumedtobespheresandtheirvelocities can be expressed by Stokes law.velocitiescanbeexpressedbyStokeslaw.
2
18Dv ws
=where v = velocity(cm/s)
s = densityofsoilparticles(g/cm3) = density of water (g/cm3)w = densityofwater(g/cm3) = viscosityofwater(gsec/cm2)D = diameterofsoilparticles(cm)
Hydrometeranalysis 30
StokesLaw 31
2
18Dv ws
= where v = velocity s = densityofsoilparticlesw = densityofwater = viscosityofwaterD = diameterofsoilparticles
L1818 LDi
p
tLvD
wsws
==1818
tLv ==
TimeDistance where
wss G = ( ) tL
GD
ws 1
18=
StokesLaw 32
IfLisincm,tisinmin,w ing/cm3, ingsec/cm2,andDinmm,thenequationcanbewrittenas
( )( )
18mm = LD ( ) 60110 tG ws LL30
( ) tLK
tL
GD
ws
== 1
30
( )130 where, = sG
K Assumingw =1g/cm3
Stokeslaw 33
ValuesofKforseveralspecificgravityofsolidsandtemperaturecombinations
Hydrometeranalysis 34
Whenahydrometerisplacedinthesoilsuspensionatatimet,measuredfromthestartofsedimentation it measures the sp gravity in thesedimentation,itmeasuresthesp.gravityinthevicinityofitsbulbatadepthL.
Hydrometersaredesignedtogivetheamountofsoil,ingrams,thatisstillinsuspension.Theyarecalibratedforsoilsthathaveasp.gravityGsof2.65(forotherGsvalues,correctionisrequired).
Byknowingtheamountofsoilinsuspension,L,andt,wecancalculatethe%ageofsoilbyweightfinerthan a given diameterthanagivendiameter.
ForASTM152Hhydrometer(cm)16402916(cm) RL = (cm)164.029.16 (cm) RL =
whereR=hydrometerreadingcorrectedformeniscus
Particleshape 35
Theshapeofsilt,andgravelsvariesfromveryangulartowellrounded.
Angularparticlesarefoundneartherockfromwhichtheyaref d hil d d ti lformed,whileroundedparticlesarefoundfartherawayfromtheirorigin.
Angularparticleshaveagreaterstrengththansmoothones,because it is more difficult tobecauseitismoredifficulttoslidepastoneanother.
Thisiswhycrushedaggregateiss s y c us ed agg egate susedasthebasematerialinpavements.
Soil TexturesSoilTextures
To determine the texture of To determine the texture ofa soil sample, find itspercent for sand, silt, andclay.
The texture of the soil willbe where all three linesintersect.
Claysoils 37
Soilsthatconsistofsilt,sand,orgravelareprimarilytheresultofphysicalandmildchemicalprocessesandretainmuchofthechemicalstructureoftheirparentrocks.
Claysoilsexperienceextensivechemicalweatheringandareh d i i l i diff f hchangedintoanewmaterialquitedifferentfromtheparentrocks.
A lt th i i ti d b h i f l Asaresult,theengineeringpropertiesandbehaviorofclaysquitedifferentfromothersoils.
Clays 38
Physicalcharacteristicsofclaysare:
They can absorb water or lose water from simple humidity Theycanabsorbwaterorlosewaterfromsimplehumiditychanges.
When water is absorbed clays will often expand as the Whenwaterisabsorbed,clayswilloftenexpandasthewaterfillsthespacesbetweenthestackedsilicatelayers.
Due to the absorption of water the specific gravity of clays Duetotheabsorptionofwater,thespecificgravityofclaysishighlyvariableandisloweredwithincreasedwatercontent.
Claystendtoformfromweatheringandsedimentaryprocesseswithonlyafewexamplesofclaysformingin
hprimaryigneousormetamorphicenvironments.
Claysoils 39
Clayparticles
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