Lecture notes, lecture all PDF

Preview

Summary

Origin of soils.pdf
Compressibility of soil.pdf
In situ stresses.pdf
Mechanical analysis.pdf
Permeability.pdf
Plasticity and structure.pdf
Seepage.pdf
Soil classification.pdf
Soil compaction.pdf
Stresses in a soil mass.pdf

Description

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

Origin of Soil and Grain Size [Chapter 2] Soil formation: Soil is formed by the process of Weathering of rock. Weathering: disintegration and decomposition of rocks and minerals at or near the earth surface.

Two types of weathering: a) 1. 2. 3. 4.

Mechanical Weathering: Temperature change Freezing and thawing Splitting action of plant roots Abrasive movement (mass movement of by means of wind, water or ice may cause an erosion and disintegration of rocks) 



ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

b) Chemical weathering 1. Oxidation (oxygen ions combine with some minerals in the rock which finally decomposes as rusting of steel) 2. Carbonation (carbon dioxide and water from carbonic acid, which decompose the mineral containing iron, sodium or calcium) 3. Hydration (water convert some mineral into new mineral) 4. Vegetation (decaying vegetation produces organic acid, carbon dioxide or oxygen, which mixed with water penetrate through the rock and change the chemical contents. For example, silcate change to silica in this manner.

Two types of soils: a) Residual soils: Weathered soil remains in place. Engineering behaviour is different from transported soil. One important characteristics is that the size gradation. Fine grained soil near the surface and grain size increases with depth b) Transported soils: Glacial soils: formed by transportation and deposition of glaciers Alluvial soils: Transported by running water and deposited along stream Lacustrine soils: formed by deposition in quiet lake Marine soil: formed by deposition in the seas Aeoline soils: transported and deposited by wind Colluvial soils: formed by the movement of the soil from its original place by gravity, such as during landslide.

 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

Typical residual soil profiles

 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

Soil Particle Size [§2.4] Gravel: pieces of rocks with quartz and feldspar and other minerals Sand: Mainly quartz and feldspar Silts: (microscopic) fine quartz grain & flake shaped particles of micaceous minerals Clay: Kaolinite, Illite, Montmorillonite [§.5].

 



Notes: 1. Sometimes particles 0 Pw < P

 



   

                 

 

 = + u





One‐DimensionalLaboratoryConsolidationTest[11.5]   FirstsuggestedbyTerzaghi  Performedinaconsolidometer(sometimesknownasOedometer)  Soilspecimenplacedinametalring,withtwoporousstones(onetopandone bottom)  Specimensize:64mm(2.5inch)dia,25mm(1inch)thick  Loadsareappliedusingaleverarm  Compressionismeasuredusingadialguage  Specimenskeptiswater  Eachloadiskept24hours,afterthatloaddoubled  Attheenddryunitweightofthespecimeniscalculated                              

 

            

 Stage I: Initial compression, mostly by preloading

Stage II: Primary consolidation, excess pore water pressure gradually transferred to effective stress because of expulsion of water

Stage III: Secondary consolidation, after completion of excess pore water dissipation, deformation takes place because of plastic rearrangement of soil fabric

          

VoidRatio‐PressurePlots[11.6]

  Calculationofvoidratio: 

1. Calculatetheheightofthesolid,Hs,inthesoilspecimenFig.11.9 Hs 

Ws  AG s  w

Where,Ws=dryweightofthespecimen A=areaofthespecimen G=Specificgravityofsoilsolids w=Unitweightofwater 2. CalculatetheinitialheightofthevoidsHv Hv  H  Hs   H=initialheightofthespecimen 3. Calculated initial void ratio of the specimen e0 

Vv H v A H v    Vs H s A H s

4. If the deformation is H1 for first load increment1,thechangein thevoidratio e1iscalculatedas e1 

H 1  Hs

5. Calculatethenewvoidratioe1andstrain1as

e1  e0  e1

1 

 e1  (1  e0 )

Note:forthenextloadfirstloadincrement2(cumulativestress)causeadditional settlementH2 

 Example: Heightofthesample=25mm Diameterofthesample=63.5mm Dryweightofthesample=0.586N=0.586/1000kN SpecificgravityGs=2.65 0.586 / 1000 Ws   0.00712 m  0.712 cm AGs  w   63.5  2   2.65  9.81 4  1000

Heightofsolid H s 

Initialvoidratio e0 

H v H  H s 2.5  0.712    2.5  Hs Hs 0.712

Appliedstress (kPa)

Heightof specimen(cm)

e=H/Hs

Currentvoid ratio

10 20

2.5 2.478

2.500 2.470

40 80 160 320 640 1280

2.457 2.357 2.142 1.927 1.712 1.497

 (2.5‐2.478)/0.712=0.030 (2.478‐2.457)/0.712 =0.030 0.140 0.302 0.302 0.302 0.302

2.440 2.299 1.997 1.695 1.393 1.091

2.60

Void ratio (e)

2.20

1.80

1.40

1.00 1

10

100

Applied stress (kPa)

1000

10000

NormallyConsolidatedandOverconsolidatedClays[11.7]  Normallyconsolidated:Presenteffective overburdenpressureisthemaximum effectiveoverburdenpressurethesoilwas subjected(=’c)   Overconsolidated:Presenteffective overburdenpressureislessthanthe effectiveoverburdenpressuresoil experiencedinthepast(’v  



Page7of21 

Factorsaffectingthepermeabilityofsoil 1)Soiltype 

  2)Thepropertiesofporefluid(viscosity)

k 

w K 

 w =unitweightofwater

(kN/m3)  =viscosityofwater(kg/m.s) 2 K =Absolutepermeability(m ) k=permeability(m/s) 

kT 0C  k 20 0 C

20 T

0C



0

C

   3)Thevoidratioofthesoil(willbediscussedlater)  4)Theshapeandarrangementofpore‐verydifficulttodescribemathematically  5)Degreeofsaturation‐increaseindegreeofsaturationincreasesthe permeability  Page8of21 

LaboratoryDeterminationofHydraulicConductivity(§7.5)  a)Constantheadpermeability ‐isusedforcoarse‐grainedsoils

Q  Avt  A( ki )t  i  h/L  h Q  A k t   L

k

QL  Aht

Q=volumeofwatercollected A= cross sectional area of the specimen t=durationofwatercollection L=lengthofspecimen h=hydraulichead   Example‐2:Duringaconstantheadpermeabilitytestonasoilsample260mlof waterwascollectedin2minutes.Ifthelengthofthesampleis10cm,diameter4 cm,andthemaintainedhead20cm,whatisthecoefficientofpermeability?  



Page9of21 

b)Fallingheadpermeability ‐isusedforfine‐grainedsoils dh h A a  dt L

qk

dt  t

aL  dh    Ak  h 

 dt 

0

t



aL Ak

h2

 dh   h  h1

 

h aL h1 aL ln log10 1  2.303 Ak h2 Ak h2 x x  ln  2.303 log10 

k  2.303

h aL log 10 1  At h2

q=flowrate A=crosssectionalarea ofthespecimen a=crosssectionalareaofstandpipe L=lengthofspecimen  Example‐3: In a falling head permeability test, the hydraulic head dropped from 90 cm to 40 cm in 20 minutes.  The cross‐sectional area of the standpipe was 1 cm2.Thesamplewasof4cmdiameterandhadalengthof18cm.Determinethe coefficientofpermeability. 



Page10of21 

DirectionalVariationofPermeability(§7.8 

     

 



Page11of21 

EquivalentHydraulicConductivityinStratifiedSoil(§7.9) Anexcellentexampleofnaturallydepositedlayeredsoilisvarvedsoil.

    



Page12of21 

Horizontalflow Totalflowthroughunitwidthinunittimeis   . 1.      . 1.    . 1.     . 1.   ⋯ . .  . 1.   v=dischargevelocity v1,v2,v3,..vn=dischargevelocityineachlayer 󰇛󰇜      󰇛   󰇜  󰇛   󰇜  󰇛    󰇜  ⋯  󰇛   󰇜.              ⋯   󰇛󰇜   󰇛         ⋯    󰇜/

      Page13of21 

Verticalflow 

 

  



Page14of21 

Example‐4: A sandy soil with k=3x10‐2 cm/s contains a series of 5 mm thick horizontalsiltlayersspaced300mmoncenter.Thesiltlayershavek=3x10‐6cm/s. Compute equivalent coefficient of permeability in the horizontal and vertical directions. Whendrillingaboreholethroughthissoil layer,howeasywouldit betomiss the siltlayers?Ifyoumissit,howmuch effectwouldtheignoranceofsiltlayerhave oncomputationofwaterflowintheverticalandhorizontaldirections.  

Page15of21 

PermeabilityTestintheFieldbyPumpingfromWells[§7.10]

Hydrolo giccycle

Definitions:

Soilprofileshowingcomp lexnatureofgroundwater

Phreaticzone:Portionbelowgroundwatertable. Aquifer: Some soils, such as sands and gravel, can transmit large quantity of groundwater. Theseareknownasaquifer. Aquicludes:Othersoilssuchasclaytransmitwaterveryslowly,whichiskownasaquicludes. Aquitards:Intermediatesoils,suchassiltysand,canpasswaterslow‐to‐moderate rateandare calledaquitards. Unconfinedaquifer:Upperaquifer.Bottomflowboundaryis definedbyanaquicludewhilethe upperflowboundary(groundwatertable)isfreetoreachitsnaturalstate. Confined aquifer: Lower aquifer(s). Both upper and lower flow boundaries are defined by aquiclude.Mostconfinedaquifersareartesian,which meanswateratthetopofthe aquifer is underpressure. Page16of21 

UnconfinedAquifers Lowerflowboundaryisfixedbuttheupperflowboundaryisthegroundwater table,whichisfreetoseekitownlevel.  dh  q  k   2 rh   dr  r1





dr

2k

h2

  q  hdh  r2 r h2 r 2.303q log10  1  r2 k 2 2  h1  h2





  



  Example‐5:Apumpingwelltestwascarriedoutinasoilbedof15mthickandthe followingmeasurementwererecorded.The rateofpumpingwas10.6x10‐3m3/s; drawdownsintheobservationwellslocatedat15mand30 m fromthecenterof thepumpingwellwere1.6mand1.4m,respectivelyfromtheinitialgroundwater level. The initial groundwater level was located at 1.9 m below the ground surface.Calculatethehydraulicconductivity. Page17of21 

ConfinedAquifers Bothlowerandupperflowboundariesarefixed. Watercanenteronlyfromaquiferthickness,H.  dh  q  k  2rH   dr 

dr 2kH h2   q  dh  r2 r h2 r1

r  2.303q log10  1   r2  k 2H h1  h2 





 



Page18of21 

FieldInstrumentations

 Openstandpipepiezometer 

  ObservationWell Page19of21 

  Essential Points - Flow of water through soils is governed by Darcy’s law, which states that the velocity is proportional to the hydraulic gradient (v=ki). - The proportionality constant is the hydraulic conductivity. - The hydraulic conductivity depends on soil type, particle size, pore fluid properties, void ratio, pore size, homogeneity, layering and fissuring, and entrapped gases. - In coarse-grained soils, the hydraulic conductivity is determined using a constant-head test while for fine-grained soils a falling-head test is used. - In the field, a pumping test is used to determine the hydraulic conductivity. - Wellpoints are used at a construction site to lower the groundwater level.  



Page20of21 

Further study from textbook Das, B.M. & Sobhan, K. (2014) Principles of geotechnical engineering 8th ed 1. Sections:7.1to7.5;7.8to7.10  2. Exampleproblems:7.1to7.5,7.12,7.13,  3. Practiceproblemsfromtextbook:7.1to7.6,7.17  

Page21of21 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

PLASTICITY AND STRUCTURE OF SOIL [CHAPTER 4] 

The presence of water in fine‐grained soils can significantly affects associated engineeringbehavior,soweneedareferenceindextoclarifytheeffects. 

 SwedishscientistAtterbeg(1846‐1916)developedthismethod  When moisture content is very high, soil behaves like fluid, and when very lowitbehaveslikesolid  Soilcanhavefourbasicstates:solid,semi‐solid,plastic,andliquid  Definitions:  LiquidLimit(LL,wl):Theminimumwatercontentatwhichthe soil willflowunder itsownweight PlasticLimit(LL,wp):Theminimumwatercontentatwhichthesoilcanbe rolled intoathread3mm(1/8")diameterwithoutbreakingup Shrinkage Limit (SL, ws): The maximum water content at which further loss of moisturecontentdoesnotcauseadecreaseinthevolumeofthesoil 

LL,PLandSLareknownasAtterbergLimits. 

Page1of21 

ENGI 4723 – Geotechnical Engineering I

 Moisture(water) content  (Increasing) 







Instructor: Dr. Bipul Hawlader

Liquidstate:deformseasily; Consistencyofpeasouptosoftbutter

 

LiquidLimit(LLorwl)

    

Engineering

PlasticityIndex (PI)

 Plasticstate:deformswithoutcracking; Consistencyofsoftbuttertostiffputty

PlasticLimit(Plorwp)

    

Semi‐solidstate:deformspermanently; Consistencyofcheese ShrinkageLimit(SLorw s) Solidstate:breaksbeforedeformation Consistencyofhardcandy  

 

 Page2of21



ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

LiquidLimit[Section4.2] 1) Casagrandecup

  

Twotypesofbase: a)USbase ‐Micarta b)UKbase ‐Rubber LLUS  0.94LLUK 

(Norman1958)

   Page3of21 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

 Watercontentatwhich25blows close the grove 0.5 inch (12.7 mm)calledLiquidLimit. Obtainwatercontentfor0.5inch closurebetween15‐35blowsand plottheminasemi‐logplot.  Thisisknownasflow curve (line).Slopeof theflowlineisflowindex(positive value).       Page4of21 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader



One‐pointmethod: U.S.CropsofEngineers(1949) N  LL  w N    25 

0.121



N=Numberofblow(20‐30) wN=moisturecontentatNblow  CommentsonOne‐pointmethod:  UsedwhenonlyoneLLtestisrun  Fairlygoodassmall changeinmoisturecontentinvolveswithinN=20toN=30, andverysmallchangein 

N   25 

0.121

forN=20toN=30. 0.121

ForN=20 

N    25 

ForN=30 

N    25 

 0.973  0.121

 1.022 

 CanbeusedtochecktheLLtestresult Shearstrength: Cassagrande(1932):eachblowcorrespondstosu=0.1kPa 

Therefore,for25blowsatLL,shearstrengthsu(LL)=2.5kPa S u( PL) 100 Su( lL) =250kPa

       Page5of21 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

 Example‐1: The data shown in the table were Numberofblowsfor½“ Water obtainedfromaliquidlimittest. closureofthegroove content(%) 10 55 Determine: Liquid limit (ii) flow index 45 (iii)verifytheresultwithonepointtest 16 43 with N=20, (iv) what is the expected 20 22 42 shearstrengthatLL. 27 41  40 36 

Flowline

LL=41.5%

25

        Page6of21 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

2) Fallconemethod(BS‐1377):  ‐PopularinEuropeandAsia ‐Aconeofapexangle30andweight0.78Npenetrate20mmin5sec ‐ Perform at least 4 tests at different water content and determine penetration depth ‐Plottheresults(semi‐logarithmic) ‐Moisturecontentatd=20mmistheLiquidLimit ‐Flowindex(IFC): I FC 

w 2 (%)  w1(%)  log d 2  log d1

 

 Page7of21



ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

PlasticLimit[Section4.3]  ‐ Plastic Limit is the moisture content in percentage at which soil crumbles when rolled into a thread of3.2mm(1/8inch) indiameter ‐ Performed by rolling by hand on a glassplate       FallConeMethod ‐Sameas LL testusing fall cone,butthemassof the coneis2.35N(not0.78N asusedinLLtest).          Page8of21 

ENGI 4723 – Geotechnical Engineering I







Instructor: Dr. Bipul Hawlader

 PlasticityIndex(PI): plastic. PI=LL‐PL

Describestherangeofwatercontentoverwhichasoilis

         Typical Atterberg Limits for Soils (Budhu 2007)

 The Atterberg limits depend on the type of predominant mineral in the soil. If montmorillonite is the predominant mineral, the liquid limit can exceed 100%. Why? Because the bond between the layers in montmorillonite is weak and large amounts of water can easily infiltrate the spaces between the layers. In the case of kaolinite, the layers are held relatively tightly and water cannot easily infiltrate between the layers.

 Page9of21 

ENGI 4723 – Geotechnical Engineering I
<...