Vacuum Consolidation in Peat

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Vacuum Consolidation in Peat

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  • VACUUMCONSOLIDATIONINPEAT

    EricRFarrellAGLConsultingandTrinityCollege

    BrendanOKelly,TrinityCollege,Dublinand

    JuanPabloOsorioSalasUniversidaddeAntioquia,Colombia,formerlyTrinityCollege

  • StructureofPresentation Background

    Principlesofvacuumconsolidation

    Currentpracticalapplications

    TCD/NRAtestsite Geotechnical/geology/hydroge

    ologyoftestsite Constructionoftestarea Instrumentation Benchmarkgroundmovement

    readings.

    Resultsfromvacuumconsolidationtest Vacuumachieved Settlementversustime Practicaldifficulties

    Numericalmodelling Laboratoryparameters Soilmodel Comparisonoffield

    performancewithpredictions

    Conclusions

  • PRINCIPLESOFVACUUMCONSOLIDATION Vacuumconsolidationisaconstructionmethodusedtoaccelerate

    groundsettlementbyreducingtheairpressureatthegroundsurface.

    Normallyatmosphericpressure(patm ) istakenasthebaselinewhencomputing = u.

    patm isabout100kPa,andthepwp canreducedbelowatmosphericusingvacuumpumps.Thisincreasestheeffectivestresswithoutgenerallyincreasingtheshearstresses.

    VacuumconsolidationwasoriginallyproposedbyKjellman inSwedeninthe1950s.

  • Background Rampartroads Theconstructionofroadsoverpeat

    bogsinthe18th &19th centuriesopenedupthebogforharvesting

    Theeasiestplacetoharvestwasadjacenttotheroads,roadsendedupelevatedabovetheadjacentground.ThesearecalledRampartRoads.Heightsof9mhavebeenrecorded.

    Manyoftheseroadsareusedtoday,resultinginnarrowandverydangerousroads.

    Vacuumconsolidationmayassistinovercomingsomeofthechallengesinwideningtheseroads.

  • VACUUMCONSOLIDATION

    Pump

    PVDs

    Granular bed

    Airtight membrane

    Atmospheric pressure = 100kPa

    Pump creates a suction of up to 80kPa

    u up to -80kPa (20kPa absolute)

  • VACUUMCONSOLIDATION

    Pa

    Porepressurereduction

    Vacuumpressure

    D

    e

    p

    t

    h

    ,

    z

    PVD

    zo

    Initialpressure

    Reducedpressure

  • CurrentapplicationsMenardvacuum

    Baudrain Cofra bv

  • Hayashietal,2003

    Shang,Tang&Miao(1998)

  • TCD/NRATESTSITE

  • RaisedBogundermilledpeatproduction,thereforesomepeathasbeenremoved.Siteisatedgeofbog.

    Top1m veryclayeysandyGravelf%30%Below slightlyclayeysandyGravelf%411%K=1.96x106 to1.15x105 m/s

  • Insituvanes(55mmx110mm)

  • Hydrogeology

  • Drainwithin8mofedgeoftestarea.

  • Layer Depth (m) Description Observations and properties

    1 0 0.7 Manmadefill Blackpeat;occasionalplasticbags,gravel,piecesofgeotextile,machineparts.

    2 0.7 4.0 Pseudofibrouspeat

    w=561 1340%GS =1.38 1.59LOI=87 99%h =9.57 10.56kN/m3eo =6.78 14.83Sr =94 100%pH=4.5 6.2VonPost=H4 H7Cc =2.2 6.4

    3 4.0 7.0 Glacialtill Theclayfractionreduceswithdepth,about30%finesintopmetre,reducingto411%.

    Table1 Simplifiedsoilprofile

    TCD/NRAVACUUMCONSOLIDATIONFIELDTRIAL

  • TCD/NRAVACUUMCONSOLIDATIONFIELDTRIAL

    0.85msq.spacing

    1.2msq.spacing

  • Instrumentation 10No.Vibratingwire

    (VW)piezomters (alsocalibratedforsuction)

    6No.pushinVWsettlementcells0.9m,1.5m&2.65m

    Settlementplates

    Standpipes

    Barometricpressure/temp.

    Raingauge

    Watermeter

    Airpressuregauges

    AcknowledgeassistanceofNVMIrelandLtd.

  • TCD/NRAVACUUMCONSOLIDATIONFIELDTRIAL

  • 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66

    26

    30

    34

    38

    42

    46

    50

    54

    58

    62

    66

    26

    30

    34

    38

    42

    46

    50

    54

    58

    62

    66

    26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66

    Test area

    StandpipesBoreholes

    Shearvanes tests

    21

    16

    10

    1415

    26

    12 11

    24

    25

    13

    17

    BH-1

    BH-2

    Vane 3

    Vane 2

    Vane 1

    N

    123

    78 9

    45

    6

    18

    19

    2023

    22

    A

    A'

    B

    B

  • Pumping system: 30th Nov 2009 23rd Jun 2010 1.5kW Centrifugal pump 38mm diameter jet pump

  • Pumping system: 29th Jul 2010 29th Oct 2010 2.2kW Liquid ring pump 1.5kW Centrifugal pump 38mm diameter jet pump

  • MONITORINGBEFOREPUMPING

    PriortostartingtheTCD/NRAvacuumconsolidationfieldtrial,fourmonthsofbaselinemonitoringwereconducted.

    Thevacuumconsolidationtrialwasrunfrom30thNovember2009andwasterminatedon29thOctober2010

  • Btm ofditch

  • FloodinginAugust2009

  • MONITORINGDURINGPUMPING TheTCD/NRAvacuumpreloadingfieldtrialcommencedonthe30thNovember2009.

    Theresultsfortheelevenmonthsofpumpingpresentedhere.

    Rain,watertable,vacuum,settlementandporewaterpressurearepresented.

  • Sett=1.11mfor0.85mspacingSett=1.09mfor1.20mspacing

  • Volumeofwaterextracted

  • umax =38.9 53.2kPafordepths0.90to2.65m.umax =13.9kPafor4.05mdepth.

  • MAXIMUMSURFACESETTLEMENT1.23m(Strainof33%)

  • Visual1st June2010

  • CRACKSATEDGE

    Chaietal.(2005)

  • 00.5

    1

    1.5

    2

    2.5

    3

    3.5

    4

    4.5

    5

    5.5

    6

    6.5

    7

    7.5

    8

    8.5

    9

    9.5

    10

    10.5

    11

    11.5

    12

    12.5

    0.1 1 10 100 1000 10000

    C

    h

    a

    n

    g

    e

    i

    n

    H

    e

    i

    g

    h

    t

    m

    m

    Log Time minTIME SETTLEMENT

    12.5 kN/m2

    100kPa

    200kPa

    400kPa

    800kPa

    Primary24hr

    creep

    SETTLEMENTOFPEAT

    Rateofsettlement

    cv;cvh;chh

    k;kv;kh

  • e=Ck logtCk 0.25eo

  • 0.00

    10.00

    20.00

    30.00

    40.00

    50.00

    60.00

    70.00

    0 50 100 150 200 250 300

    S

    t

    r

    a

    i

    n

    %

    Stress

    2.4mto3.2mOed4 Spreadsheet

    0.00

    2.00

    4.00

    6.00

    8.00

    10.00

    12.00

    14.00

    1 10 100 1000

    V

    o

    i

    d

    r

    a

    t

    i

    o

    e

    EffectivestresskPa

    2.4mto3.2mOed4 CcModel

  • Soilmodelsinvestigated SimpleEOPCs &Cc (CR =Cc/{(1+eo)}&RR)andvc

    SoftSoilmodel(SS)Plaxis (*=Cc/{2.3(1+eo)},*)

    SoftSoilCreepmodelPlaxis (*=Cc/{2.3(1+eo)},*,*)

  • 0.00

    10.00

    20.00

    30.00

    40.00

    50.00

    60.00

    70.00

    1 10 100 1000Stress

    2.4mto3.2mOed4 Spreadsheet 1.5mto2.3m

    1 10 100

    S

    t

    r

    a

    i

    n

    %

    Stress

    E'=100kPa

    E'=100kPa

  • Modellingvacuumconsolidation

    x

    y

    1

    1

    1

    2

    2

    2

    3

    3

    3

    4

    4

    4

    5

    5

    5

    6

    6

    6

    7

    7

    7

    8

    8

    8

    9

    9

    9

    10

    10

    10

    AA AA

    A

    A

    A

    A

    A A

    0 1

    23

    4 5

    67

    89 10

    11

    1213141516

    17 18 19 20 21

    22 23 24 25

    26 27 28 29

    30 313233 34 35 36 37 38 39 40 41

    42 434445 46 47 48 49 50 51 52 53

    5455

    56

    5758

    59

    60 61

  • Upperpeat Middlepeat LowerpeatPOP 10 5 11l* 0.125 0.16 0.16k* 0.05 0.055 0.04m* 0.0065 0.0078 0.009g 10.45 10.1 10.06eo 7.42 13.81 12.05

    kv=kH(m/day)

    0.3 0.2 0.01

    ck 1.8 3.44 3.44

    VALUESUSEDONBACKANALYSISWITHSSCMODEL

    PARAMETERSINTERPRETEDFROMLABORATORYOEDOMETERTESTS

    Upperpeat Middlepeat LowerpeatPOP 10 5 11l* 0.11to0.125 0.14to0.16 0.16to0.2k* 0.022to0.05 0.03to0.055 0.033to0.04m* 0.0065to

    0.0080.0078to0.01 0.0065to0.009

    g 10.45 10.1 10.06eo 6.78 8.69 13.43 14.5 11.5 12.5

    kv=kH(m/day)

    0.011to0.128 0.0053to0.011 0.0015to0.0029

    ck 1.8 3.44 3.44

  • Swellusingsimplesoilmodel=0.161m

    Swellmeasured=0.13m

    EquivalentE100kPaatthisveryloweffectivestress.

    0

    0.5

    1

    1.5

    2

    2.5

    3

    3.5

    4

    0 5 10 15 20 25 30 35 40

    18thAugust

    18thSept

    pwpkPa

    D

    e

    p

    t

    h

    (

    m

    )

  • TheTCD/NRAvacuumpreloadingfieldtrialwasimplementedandshowedthatthistechniquecanbesuccessfullyusedinpeatsoils.

    ThedrainagesystemcomprisingPVDs,horizontaldrainsandagranularbed,waseffectiveindistributingtheappliedvacuumpressureandcollectingthedrainedwater.

    CONCLUSIONS

  • Practicaldifficulties/Observations Summerconditionsgeneralwatertablewaslowerthansidebarrier.

    Highersuctionsachievedwithvacuumpumpbutmorestablevalueswithliquidringpump

    Porepressurereductionatedgeswasslightlylower(2kPato8.5kPa)thanatcentre.

    PWProughlyuniformwithdepth Vacuumpressureadequatelytransmittedin0.85mand1.2mspacings.

  • continued

    Calcificationofpumpsasignificantissue

    Freezing,tears,burstselectricalcutsaffectedtheperformance.

    Airtightcovercouldbeimproved,useofwatersealshouldbeconsidered.

    Vacuumconsolidationhadlittleeffectonthewatertableoutsidethetestarea.

    Behaviourcanbesimulatedusingstandardsoilmodels.

  • ACKNOWLEDGEMENTS NationalRoadAuthority.

    TrinityCollegeDublin.

    GeotechnicalTrustFundAward(EngineersIreland).

    UniversidaddeAntioquia

    Bord na Mna