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Stebbins Wind Project Geotechnical Report - May 2017 - REF Grant 7071068
GEOTECHNICALREPORT For StebbinsWindTurbineProject Stebbins,Alaska PreparedBy: DougP.Simon,P.E. GeotechnicalServicesManager ReviewedBy: LorieM.Dilley,Ph.D.,CPG,P.E. GeotechnicalPrincipal 3335 Arctic Blvd., Ste. 100 Anchorage, AK 99503 Phone: 907.564.2120 Fax: 907.564.2122 May26,2017 GEOTECHNICALREPORT For StebbinsWindTurbineProject Stebbins,Alaska Preparedfor: AlaskaVillageElectricCooperative,Inc. HDLProjectNumber14Ͳ023 Preparedby: DougP.Simon,P.E. GeotechnicalServicesManager Reviewedby: LorieM.Dilley,Ph.D.,CPG,P.E. GeotechnicalPrincipal HDLEngineeringConsultants,LLC 3335ArcticBoulevard,Suite100 Anchorage,AK99503 Phone:907.564.2120 Fax:907.564.2122 AELC831 May26,2017 Stebbins Wind Turbine Project Stebbins, Alaska TABLEOFCONTENTS 1.0INTRODUCTION ........................................................................................... 1 2.0SITEANDPROJECTDESCRIPTION ................................................................... 1 3.0EXISTINGLITERATUREREVIEWED .................................................................. 3 4.0REGIONALSETTING ...................................................................................... 5 4.1GENERALGEOLOGY .................................................................................................. 5 4.2CLIMATOLOGY ........................................................................................................ 6 5.0FIELDEXPLORATIONS ................................................................................... 6 6.0LABORATORYTESTING ................................................................................. 9 7.0SUBSURFACECONDITIONS ............................................................................ 9 7.1PEATANDORGANICSILT .......................................................................................... 11 7.2SILTANDICE ......................................................................................................... 11 7.3SAND ................................................................................................................. 11 7.4SUBSURFACETEMPERATURES .................................................................................... 11 7.5GROUNDWATER .................................................................................................... 12 8.0ENGINEERINGANALYSIS&RECOMMENDATIONS ......................................... 12 8.1SITEWORK .......................................................................................................... 13 8.1.1SitePreparation ............................................................................................. 13 8.1.2TurbinePad ................................................................................................... 13 8.1.3AccessRoadandStagingArea ......................................................................... 15 8.1.4FillandCompaction ........................................................................................ 16 8.2SEISMICANALYSIS .................................................................................................. 16 8.3FOUNDATIONANALYSES .......................................................................................... 17 8.3.1Piles .............................................................................................................. 17 8.3.2Slurry ............................................................................................................ 18 8.3.3SlurryFreezeback ........................................................................................... 18 8.3.4Settlement..................................................................................................... 18 8.3.5FrostHeaveForces ......................................................................................... 18 8.3.6LateralDeflection ........................................................................................... 19 8.4DRAINAGEANDDEWATERING.................................................................................... 19 8.5CONSTRUCTIONCONSIDERATIONS .............................................................................. 19 9.0CLOSUREANDLIMITATIONS ....................................................................... 20 LISTOFFIGURES Figure1VicinityMap Figure2MeanAnnualTemperatureforNomefrom1949to2014 Figure3BoreholeLocationMap Figure4SubsurfaceProfile Figure5SubsurfaceTemperatureRecordingsfromSeptember2014 Figure6TurbinePadSketch Stebbins Wind Turbine Project Stebbins, Alaska LISTOFTABLES Table1StructuralFillSpecifications Table2SeismicDesignCriteria LISTOFAPPENDICIES AppendixAFigureA1UnifiedSoilClassificationSystem FigureA2FrostDesignSoilClassification FigureA3FrozenSoilClassification FigureA4ͲA7BoringLogs FigureA8,A9GrainSizeDistributionCurves AppendixBChemicalTestingResults Stebbins Wind Turbine Project Stebbins, Alaska Page 1 GEOTECHNICALREPORT STEBBINSWINDTURBINEPROJECT ALASKAVILLAGEELECTRICCOOPERATIVE STEBBINS,ALASKA 1.0 INTRODUCTION TheexistingAlaskaVillageElectricCooperative’s(AVEC’s)powergenerationfacilitiesinthe villageofStebbins,Alaskaarecurrentlybeingupgraded.Windturbinepowergenerationis includedaspartoftheupgradestoreducethedependencyonimporteddieselfuelandprovide analternativepowersource.Astudywasconductedtoevaluatepotentialturbinesitesbasedon thewindcharacteristicsofthearea.InSeptemberof2013,HDLEngineeringConsultants(HDL), formerlyHattenburgDilley&LinnellpreparedaConceptDesignReport(CDR)forAVECthat analyzedwindturbinealternatives,costs,andwindturbinesites.AVEChasretainedHDLto providewindturbinedesignincludingthisgeotechnicalevaluationtounderstandthesubsurface conditionsattheselectedwindturbinesitelocatedapproximately1ͲmilenorthofStebbins. Thisreportpresentstheresultsofthegeotechnicalevaluationfortheselectedwindturbinesite inStebbins.Includedinthisreportaredescriptionsofthesiteandproject,boringlogs, laboratorytestresults,interpretationofthesubsurfaceconditions,andgeotechnical recommendationsfortheproposedwindturbinefoundation. 2.0 SITEANDPROJECTDESCRIPTION TheproposedsitewasdiscussedwithintheCDRpreparedbyHDLanddatedSeptember6,2013. Basedontherecommendationsfromthisreport,theproposedturbinewillbelocatedatthe previouslydefinedStebbinsSite1,locatedat63°31’56.58”northlatitudeand162°16’50.64” westlongitude,atanapproximateelevationof155feetabovesealevel.Refertotheattached Figure1,VicinityMap,foradditionaldetails. ThesiteislocatedalongaridgelineadjacenttotheexistingStebbinsͲSt.MichaelRoad,nearthe intersectionoftheStebbinsͲSt.MichaelsRoadandtheStebbinsLandfillAccessRoad.Thesite hasnotbeenpreviouslydevelopedandtopographygenerallyslopestothenorthwith approximately10feetoftopographicrelief.Tundraispresentatthesurfaceandwatercollects inlowlyingareasasitflowsgenerallytothenorth. Accesswillbeprovidedbyaroadapproximately300feetlongbetweentheexistingStebbinsͲSt. MichaelRoadandtheproposedturbinesite.Theroadwillconsistofasinglelane,gravelroad witha26Ͳfoottopwidth.Agravelpadwillberequiredaroundtheproposedturbinetofacilitate constructionandfuturemaintenance.Boththeaccessroadandgravelpadwillprovideonly periodicaccessforlighttrucksandATV’safterconstructioniscomplete. EXISTING LANDFILL TO ST. MICHAEL NORTON SOUND NORTON SOUND EXISTING SEWAGE LAGOON STEBBINS PROJECT LOCATION PROJECT LOCATION Figure 1 STEBBINS WIND TURBINE PROJECT STEBBINS, AK VICINITY MAP ALASKA VILLAGE ELECTRIC COOPERATIVE STEBBINS, AK Stebbins Wind Turbine Project Stebbins, Alaska Page 3 Theoriginaldesignconceptsincludeduptofoursmallturbines.Theprojectcurrentlyproposed consistsoferectingone(1)500kWEmergyaWindTechnologiesBV(EWT)DW52windturbine witha164feet(50meter)hubheight(collectivelyEWT52/50).Theturbinewillbesupportedby asteelmonopoleboltedtoaconcretefoundationsupportedwithsteelpiles.Themanufacturer hasprovidedthefollowingtowerbasereactionsbasedona133mileperhour,3secondwind gust(50yearreturnperiod): Horizontal 64kips Vertical192kips Overturning 8458kipͲfeet Torsion639kipͲfeet Giventhesubsurfaceconditionsdescribedlaterinthisreport,BBFMEngineers,Inc.(BBFM, structuralengineer),AVEC,andHDLconcludedthatthepreferredfoundationtypewouldbea steelpilegroup.Usingtheproposedtowergeometryandbasereactions,thestructuralengineer developedexpectedmaximumdownwardandupliftloadsforindividualpilesof140kipsand69 kips,respectively. 3.0 EXISTINGLITERATUREREVIEWED Thegeotechnicalevaluationbeganwiththecollectionandreviewofexistinggeotechnicaldata inthearea.Summariesofthegeotechnicaldatareviewedareprovidedbelow. o SoilsInvestigationforCityofStebbins,AlaskaEnvironmentalControlServices,Inc., December1984. SixtyͲfourtestholeswereadvancedin,andnear,StebbinsbybackhoeinOctoberof 1984.Thetestholesgenerallyencountered,andwereunabletopenetrate,frozensoils between3.4and4.5feetbelowtheexistinggroundsurface(bgs).Thesoils encounteredgenerallyconsistedofwellsorted,blackandredsandandgravel.Anarea northofFultonStreetencounteredthawedsoilstoatleast8feetbgs.Groundwater appearedtobeabout6feetbgs;mostlikelyperchedonunderlyingfrozenground. Fifteentestholesweredrilledtodepthsof9to12feetbgsinNovemberof1984. Permafrostwasobservedinallbut2holes;bothholesthatdidnotencounter permafrostwerelocatednortheastofthecommunitybulkfuelstoragetanksandwere advancedto9feetbgs. o GeotechnicalStudies,ProposedNewSchool,Stebbins,Alaska,Shannon&Wilson,Inc., July1990. TenboringswereadvancedinMayof1990tosupportdesignofaschoolwithin Stebbins.Theboringstypicallyencounteredorganicsoiloverfinetomediumsands.Ice rich,slightlyclayeytoclayeysiltswerepresentbelowthesands.Theseasonalthawwas typically2.5feetattheboringlocations.However,thawedsoilswereencounteredin Stebbins Wind Turbine Project Stebbins, Alaska Page 4 threeoftheboringsstartingatdepthsbetween4to6feetbgs.Thethicknessofthese thawedsoilsrangedfrom2.5to14feet,ormore.Wherethawedsoilswerepresent, groundwaterwasencounteredbetweenabout5.5and8feetbgs.Temperaturesinthe frozensoilsrangedfromapproximately30.5°Fto31.5°F,withtypicalsalinitieslessthan 3partsperthousand(ppt). o FinalFieldInvestigationResultsandRecommendationsforProposedWaterReservoirin Stebbins,Alaska,GolderAssociatesInc.,April2008. Six(6)boringswereadvancedtoadepthof61.5feetbgsatalocationsouthofthe proposedturbineinMarchof2008.Ingeneral,theboringsencounteredlayersofsilt, siltysand,andice.Organicswereencounteredatthesurfaceofallbutoneboringand werepresenttodepthsrangingfrom2.5feetto7.5feetbgs.Frozensoilswere encounteredatdepthsrangingfromthegroundsurfaceto2.5feetbgsandextendedto theterminationdepthoftheborings.Nothawedgroundwaterwasrecorded,andthe thawedsoilsampleshadameasuredsalinitylessthan0.2ppt.Thetemperatureofthe permafrostwastypicallyabout31°F. o FinalReportonGeotechnicalInvestigationforWastewaterLagoon,Stebbins,Alaska, GolderAssociatesInc.,June2009. Five(5)boringsweredrilledtoadepthof32.0feetbgsforalagoonlocatedwestofthe proposedturbineinAprilof2009.Ingeneral,theboringsencounteredorganicmaterial atthesurfacethatextendedtoadepthofupto6.1feetbgs,followedbyirregularlayers ofsiltandsandysilttodepth.Soilsweregenerallyfrozenthroughoutthedepthexcept inG09ͲB02.Soiltemperaturesmeasurementswithintheboringsindicatesubsurface temperaturesweretypicallybetween31.0°Fand32.0°FintheboringsexceptG09ͲB02. ThawedconditionsnotedinG09ͲB02wereattributedtothawfromtheexistingsewage lagoon.Nothawedgroundwaterwasrecorded.Thereportedsalinityoftheporefluids waslessthan0.1ppt. o GeotechnicalInvestigation,Water&SewerImprovements,Stebbins,Alaska,Duane Miller&Associates,July2000. TwentyͲsix(26)boringsweredrilledtoamaximumdepthof49.5feetbgsinMarchof 2000inStebbins.Ingeneral,discontinuouspermafrostconsistingofsiltandsandlayers wasencounteredintheborings.Icelensesandicerichsoilswerecommoninthe shallowerportionsoftheboringswherepermafrostwasencountered.Thesalinityof theporefluidswastypicallylessthan4pptinboringsthatwerenotnearthelake. Groundwaterwasencounteredatdepthsof3feetto11feetbgsinunfrozensoils.No measurementsofsoiltemperatureswerereported. o StebbinsRoadRehabilitationProject,FinalGeotechnicalMemo,Stebbins,Alaska,Bristol Environmental&EngineeringServicesCorporation,March2010. TwentyͲfour(24)boringsweredrilledtoevaluatetheexistingroadwaysectionsin Stebbinsandnearaproposedroadstraighteningeastoftheproposedturbine.The Stebbins Wind Turbine Project Stebbins, Alaska Page 5 boringsweredrilledtodepthsbetween5feetand11.5feetbgs.Ingeneral,theroad structuralsectioneastoftheproposedturbineconsistedofsiltandsiltysand.Thesoils werefrozenatadepthof2.5feetinTPͲ1andappearedtobeicerich.Nomeasurements ofsoiltemperaturewerereported. o GeotechnicalReport,PowerPlantandBulkFuelUpgrades,Stebbins,Alaska,Hattenburg Dilley&Linnell,June2010. Nine(9)boringsweredrilledtoamaximumdepthofapproximately50feetbgsinApril of2010.Siltsandsiltysandwasgenerallyencounteredtothedepthsoftheborings.The soilswerediscontinuouslyfrozenwithmorethawedsoilstypicallyencounteredtothe easternportionofthesite.Themeasuredsoiltemperaturesvariedfromapproximately 25.5°Fto33.4°Fbutweretypicallywithin0.5degreesof32°F.Thesalinityofthepore fluidswaslessthan0.1ppt. o GeotechnicalReport,StebbinsWaterandSewerFacilities,Stebbins,Alaska,Hattenburg Dilley&Linnell,September2010. Seventeen(17)boringsweredrilledtoamaximumdepthofapproximately50feetbgs inAprilof2010.Siltsandsiltysandwasgenerallyencounteredtothedepthsofthe borings.Thesoilswerediscontinuouslyfrozenandthemeasuredsoiltemperatures variedfromapproximately29.5°Fto31.5°Finthefrozensoilsbelowtheseasonalthaw bulb.Thesalinityoftheporefluidswaslessthan3pptexceptinoneboringdrillednear thecoast. Basedonpreviousboringsandtestholesadvancedintheareabetween1984and2010, subsurfaceconditionsintheareagenerallyconsistedofdiscontinuoustocontinuous permafrost.Discontinuous,warmpermafrostwaslikelytobeencounteredinareasthathad previouslybeendisturbed.Continuous,warmpermafrostwasgenerallyencounteredwherethe surfaceorganicshadnotbeendisturbed.Salinitiesweregenerallylowanddidnotappearto causelargefreezingpointdepressionsinthesoilsinmostlocations.Basedonthe reconnaissancecompletedaspartoftheCDR,bedrockoutcropswereobservedalongthe shorelinetothenorthandacinderconeeastoftheproposedturbinesite. 4.0 REGIONALSETTING 4.1 GeneralGeology ThesiteislocatedonSt.MichaelIslandwhichisprimarilycomprisedofquaternarybasaltic flows,tuffs,cones,andcraters.SurficialgeologyintheareaofStebbinsiscomposedofsands, silts,andorganicmaterialworkedbyoceanwaves,shiftingrivers,andwind.Lakesinthe volcaniccratersareusedaswaterreservoirsandwastewaterlagoons. Permafrostisgenerallycontinuousandwarm(above30°FintheStebbinsarea),thoughisnoted asdiscontinuouswherethawbulbsfromwaterbodiesanddisturbedgroundhavedegradedthe permafrost. Stebbins Wind Turbine Project Stebbins, Alaska Page 6 Stebbinsislocatedinaregionofrelativelylowseismicity,despitebeingwithin50milesofthe KaltagFault.BasedoninformationobtainedfromtheAlaskaEarthquakeInformationCenter, therehavebeentworecordedeventsaboveRichtermagnitude4withinapproximately50miles ofStebbinsfrom1903to2004.Therearenorecordedeventslargerthanamagnitudeof5in theimmediatevicinity. 4.2 Climatology Stebbinshasasubarcticclimatewithamaritimeinfluenceduringthesummer.NortonSoundis generallyiceͲfreefromJunetoNovember,butcloudsandfogarecommon.Averagesummer temperaturesrangefrom40°Fto60°F;wintertemperaturesrangefromͲ4°Fto16°F.Extremes havebeenmeasuredfromͲ55°Fto77°F.Annualprecipitationis12inchesofrainfalland38 inchesofsnowfall.Datapublishedon“TheAlaskaClimateResearchCenter”website,indicatesa generalwarmingtrendintheareafromtheinitialreadingscollectedin1949throughthemost recentdatacollectedin2014.Refertothechartbelowsummarizingtheaverageannual temperaturesforthenearbycityofNome,Alaska. Figure2–MeanAnnualTemperatureforNomefrom1949to2014 5.0 FIELDEXPLORATIONS HDLstaffobserveddrillingoffour(4)soilborings,designatedBHͲ01throughBHͲ04,fromJuly27 throughJuly29,2014toevaluatethesubsurfaceconditionsneartheselectedwindturbinesite. Trend Line 5-Year Moving Average Stebbins Wind Turbine Project Stebbins, Alaska Page 7 BoringsBHͲ01throughBHͲ03weredrilledatalternateturbinelocations.BoringBHͲ04was drilledneartheproposedturbine.TheboringswerelocatedinthefieldbyahandheldGlobal PositioningSystem(GPS)deviceandbymagneticcompassbearingandtapemeasurements. RefertoFigure3,BoreholeLocationMap,fortheapproximatelocationoftheborings.Fieldwork wasperformedingeneralaccordancewiththeproceduresoutlinedintheAlaskaDepartmentof TransportationandPublicFacilities(DOT&PF)“AlaskaGeotechnicalProceduresManual”,and recoveredsoilswereclassifiedinthefieldingeneralaccordancewithASTMInternational Standard(ASTM)D2488.SampleswerecollectedanddeliveredtoHDL’slaboratoryforfurther testing. TheboringsweredrilledbyGeoTekAlaska,IncofAnchorage,Alaska,workingasasubcontractor toHDL.AtrackmountedGeoprobe®6620DTwith3Ͳ½Ͳinchcontinuousflight,hollowstem augersandanairͲrotaryhammerwasusedtodrilltheborings.AtwoͲinchoutsidediameter (O.D.)splitͲspoonsamplerwasusedtocollectsamples.Soilsamplescollectedduringdrilling wereplacedintotwolayersofplasticbagsandsealedtopreventmoistureloss.Sampleswere thentransportedtoHDL’slaboratoryfortestings. SamplingwiththesplitͲspoonwasconductedusingtheStandardPenetrationTest(SPT) procedureingeneralaccordancewithASTMD1586.InSPTsampling,samplesarerecoveredby drivingatwoͲinchO.D.splitͲspoonsamplerintothebottomoftheadvancingholewithblows froma140ͲpoundhammerfreeͲfalling30inchesontothedrillrod.Thenumberofblows requiredtoadvancethesamplerthemiddle12inchesofa24Ͳinchsampleristermedthe PenetrationResistance,designatedasthe“NͲvalue”,whichwasrecordedforeachsample depth.Thevaluesgiveameasureoftherelativedensity(compactness)orconsistency (stiffness)ofthawedcohesionlessorcohesivesoils,respectively. Thesoilboringswereadvancedtodepthsrangingfrom37.8feetto50.0feetbgs.SPTtestswere generallyconductedintheboringsat2.5Ͳfootintervalstoadepthof10.0feetbgs,thenat5Ͳ footintervalstodepth.TherecoveredsoilswereclassifiedinthefieldinaccordancewithASTM D2488andwerealsoclassifiedperAKDOT&PF’s“AlaskaFieldGuideforSoilClassification”, Figure6Ͳ1,FrozenSoilClassifications,aspresentedasFigureA3.AnexperiencedHDLengineer waspresentduringdrillingtolocatetheborings,observedrillingaction,collectsamples,log subsurfaceconditions,andobservegroundwaterdepths,whereencountered. CrossͲlinkedPolyethylene(PEX)tubingwasplacedinboringsBHͲ02throughBHͲ04tofacilitate groundtemperaturedatacollection.Temperatureswererecordedimmediatelyafter completionoftheboringsusingthreefactorycalibratedTemperatureAcquisitionCables(TACs). TheoperationoftheTACswasconfirmedpriortomobilizationusinganicewaterbath. TemperaturesweremeasuredagaininSeptemberof2014afterthesoilhadrecoveredfromthe effectsofdisturbanceduringdrilling. Figure 3 STEBBINS WIND TURBINE PROJECT STEBBINS, AK. BOREHOLE LOCATION MAP ALASKA VILLAGE ELECTRIC COOPERATIVE STEBBINS, AK Stebbins Wind Turbine Project Stebbins, Alaska Page 9 6.0 LABORATORYTESTING LaboratorytestingofthesoilsampleswasconductedatourAmericanAssociationofState HighwayandTransportationOfficials(AASHTO)MaterialsReferenceLaboratory(AMRL) accreditedandUSACEcertifiedlaboratory.Selectlaboratorytestswereperformedonsamples recoveredfromtheboringstoconfirmormodifyfieldclassificationsinaccordancewiththe UnifiedSoilClassificationSystemaspresentedinAppendixA,FigureA1.Asappropriate,the soilsweregivenafrostdesignclassificationbasedonamodifiedversionoftheUnitedStates ArmyCorpsofEngineers(USACE)systemaspresentedasFigureA2,FrostDesignSoil Classification.Theresultsofthelaboratorytestsaredepictedontheboringlogsandgrainsize distributioncurvesinAppendixAasFiguresA4throughA9. Forty(40)naturalmoisturecontenttestswereperformedonselectsamplesfromtheborings. ThemoisturecontenttestswereconductedinaccordancewithproceduresdescribedinASTM D2216.Themoisturecontentsprovideevidenceoffrostbehaviorandsaturationconditions. Moisturecontentsnearorabove100percentgenerallyindicatethatalargeamountoforganics and/oricewaspresentinthesample. Ten(10)P200tests,whichmeasurestheamountofmaterialfinerthanthe#200sieve,were performedonselectsamplesinaccordancewithASTMD422.Theresultsarepresentedonthe boringlogs. Six(6)grainsizedistributionanalyseswereperformedonselectsamplesinaccordancewith proceduresdescribedinASTMD422.Theresultswereincorporatedintotheboringlogsand includedinAppendixA,FiguresA8andA9.Theresultswereusedtoestimateengineering characteristicsandthefrostsusceptibilityofthesoils. Nine(9)salinitycontenttestswereperformedonselectsamples.Theresultsareincorporated intotheattachedboringlogs. Two(2)sampleswereselectedforchemicalanalysestomeasurethepH,resistivity,andchloride andsulfatecontent.AnalyseswereperformedbySGSNorthAmerica,Inc.andwereperformed inaccordancewithUnitedStatesEnvironmentalProtectionAgencymethodsSW9045D,SM19 2510A,andSW9056A,respectively.Resultsofthechemicalanalysesarepresentedonthe boringlogsandinAppendixB. 7.0 SUBSURFACECONDITIONS Theboringsweredrilledinapreviouslyundisturbedareathatwasvegetatedwithtundra.In general,theboringsencounteredtundra,underlainbyfrozensiltandsandthatextendedto depthintheborings.SPTsamplingwasperformedwithinthefrozensoilsbutthereportedNͲ valuesdonotaccuratelyrepresenttherelativesoildensitiesofthesubsurfaceconditions.The subsurfaceconditionsarefurtherdescribedbelowandgraphicallydepictedonFigure4, SubsurfaceProfile;detailedinformationcanbefoundontheboringlogs. Figure 4STEBBINS WIND TURBINE PROJECTSTEBBINS, AKSUBSURFACE PROFILEALASKA VILLAGE ELECTRIC COOPERATIVESTEBBINS, AK Stebbins Wind Turbine Project Stebbins, Alaska Page 11 7.1 PeatandOrganicSilt Tundraconsistingofpeatandorganicsiltwasencounteredatthegroundsurfaceoftheborings. Themeasuredthicknessofthetundrarangedfrom1.6feetto3.5feet,andaveraged2.6feet. Themeasuredmoisturecontentofthetundrarangedfrom83.8%to342.9%,andaveraged 155.2%. 7.2 SiltandIce Frozensiltandicewasencounteredbelowthetundra.ThesiltwastypicallyiceͲrichand includedlayersofmassiveiceinmostborings.Thefrozensiltandicewasencounteredtodepths rangingfrom7.8feetto13.0feetbgs.Thelayersofmassiveicewereupto5.2feetthick.The measuredmoisturecontentofthesiltrangedfrom30.3%to279.6%andaveraged97.4%.The measuredsalinityrangedfrom0.12pptto0.37pptandaveraged0.25ppt.Basedonasample fromboringBHͲ02,themeasuredpHwas7.96,chloridecontentwas308mg/kg,sulfatecontent was70.7mg/Kg,andtheresistivitywas15.0ohmͲm.Theicestrata,whereencountered,was describedascloudyandcolorless.BoringBHͲ03encounteredalayeroffrozensandwithinthe siltfrom8.0feetto9.5feetbgs;thesandhadamoisturecontentof25.7%. 7.3 Sand Astratumoffrozensandwithvaryingamountsofsiltwasencounteredbelowthesiltandice andtypicallyextendedtodepthoftheborings.Thesandwasgenerallydescribedaspoorly gradedandfinetomediumgrainedwithlittletonoexcessice.Themeasuredmoisturecontents rangedfrom25.1%to62.7%andaveraged41.7%.Themeasuredsalinityrangedfrom0.09ppt to1.00pptandaveraged0.38ppt.BasedonasamplefromboringBHͲ03,themeasuredpHwas 8.31,chloridecontentwas21.6mg/kg,sulfatecontentwas5.00mg/Kg,andtheresistivitywas 140ohmͲm.AlayerofsiltwasencounteredwithinthesandinboringBHͲ01from27.0feetto34 feetbgs.Themeasuredmoisturecontentofthesiltwas84.1%. 7.4 SubsurfaceTemperatures Permafrostwasencounteredinthesoilboringsatdepthsrangingfrom0.8feetto2.0feetbgs andwaspresenttotheboringterminationdepth.Temperaturedatawascollectedatthe completionofdrillingonJuly30th,2014,thenagainonSeptember16,2014.Thetemperatures measuredinJulyof2014appearedtohavebeencollectedbeforethesubsurfacetemperatures hadrecoveredfromdisturbanceduringdrilling.Therefore,thetemperaturesmeasuredin Septemberareassumedtobemostrepresentativeoftheactualsubsurfaceconditions.Thawed conditionswereobservedintheupperfewfeet,theeffectsofseasonalwarmingwasobserved todepthsofapproximately10feet,andthesubsurfacetemperaturesweretypicallylowerthan 30.5°Fatdepthsgreaterthan10feet.RefertotheFigure5foragraphicaldepictionofthe subsurfacetemperatureswithdepth. Stebbins Wind Turbine Project Stebbins, Alaska Page 12 Figure5–SubsurfaceTemperaturesMeasuredSeptember16,2014 7.5 Groundwater Freegroundwaterwasnotencounteredduringdrilling.Groundwaterlevelsintheactivelayer fluctuatedependingontheseason,temperature,andprecipitation.Groundwaterlevelsduring constructionmaydifferfromthatobservedintheborings.Significantgroundwatermaybe releasediftheicerichpermafrostisallowedtothaw. 8.0 ENGINEERINGANALYSIS&RECOMMENDATIONS Designofanystructure’sfoundationmustconsidertheloadbearingcapabilitiesofthe subsurfacesoilsaswellasexpectedsettlements,theeffectsofseasonalfrostaction,andthe potentialimpactsofclimatechanges.ThesubsurfacesoilsgenerallyconsistedoficeͲrichsiltand sandpermafrost.Basedonsubsurfacetemperaturedata,thepermafrostisconsideredwarm butcontinuouswithminimalseasonalthawdepth. Severalfoundationtypeswereconsideredincludingshallowanddeepfoundations.Basedon theexpectedloadsandthesubsurfaceconditionsencountered,werecommendadeep foundationsystemwithpreͲdrilled,slurried,steelpipepileswiththermosiphonrefrigerationto maintainpermafrosttemperatures.Theproposeddesignrequiresthepermafrostbeprotected duringconstructionandthroughoutthedesignlife. Stebbins Wind Turbine Project Stebbins, Alaska Page 13 8.1 SiteWork Thefollowingsectionsprovideasummaryofgeotechnicalconsiderationsforthesite development. 8.1.1 SitePreparation IceͲrichsiltisexpectedtounderlietheorganicmatalongtheproposedaccessroadandthe turbinepad.Removaloftheorganicmatwouldeliminatetheinsulatingeffectsandwould exposetheunderlyingsoilstohigherlevelsofthawdegradationduringandafterconstruction. Therefore,werecommendprotectionofandconstructionovertheorganicmat.Someminor isolatedremovalofhummocksmaybenecessary.Iftheorganicmatismodified,itshouldbe limitedinextentandbeconductedinwinterorearlyspringandtheareashouldbeprotected with4Ͳinchesofinsulation. Thesiteshouldbeclearedofdebris,snow,surfaceice,orotherdeleteriousmaterialprior constructionoftheturbinepadandroad. 8.1.2 TurbinePad ThepadfortheproposedturbineshouldconsistofaninsulatedsectionofNFSfillthatis underlainbygeotextile.Foreaseofconstruction,alevelingcourseconsistingofNFSfillmaybe placedovertheexistingsurface.Alayerofgeotextileshouldbeplacedontheexistingtundra priortoplacementofthelevelingcoursetoprovideseparationbetweenthefillandthe underlyingtundra.Theseparationgeotextileshouldconsistofawovengeotextilethathasan apparentopeningsizeof0.425mmorsmallerandatensilestrengthof300lbsorgreater. Winfab315isoneproductthatmeetstheintendedrequirements. Theconstructionofthepadwillresultinachangeofthealbedothatwillincreasethepotential forthawoftheicerichsiltsbelowtheorganicmat.Tolimitthepotentialforthawdegradation oftheunderlyingsoils,werecommendinsulationbeplacedatthebaseofthestructuralsection andbeneaththeconcretepilecap.Theinsulationatthebaseofthestructuralsectionshouldbe aminimumof4inchesthickandconsistofhighdensityhydrophobicextrudedorexpanded polystyrenewithaminimumcompressivestrengthof40psi.Theinsulationshouldextenda minimumdistanceof20feetbeyondtheedgeofthepilecap.Theinsulationimmediatelybelow thepilecapshouldbeaminimumof6Ͳinchesthickandconsistofhydrophobicextrudedor expandedpolystyrenewithcompressivestrengthof10psi.Theinsulationimmediatelybelow thepilecapshouldextendaminimumdistanceof1footbeyondthepilecap. NFSfillshouldbeplacedbetweentheinsulationlayersandbecompactedtoestablishrequired grade.Careshouldbetakenduringconstructiontoavoiddamagingtheinsulation.Typically, damagecanbeavoidedbyplacing18inchesofNFSfillbeforeoperatingheavyequipmentand avoidingsuddenorsharpequipmentmovement.Protectionoftheinsulationshouldbethe responsibilityofthecontractor.TheintendeddesignisdepictedinFigure6,TurbinePadSketch. Figure 6 STEBBINS WIND TURBINE PROJECT STEBBINS, AK TURBINE PAD SKETCH ALASKA VILLAGE ELECTRIC COOPERATIVE STEBBINS, AK Stebbins Wind Turbine Project Stebbins, Alaska Page 15 8.1.3 AccessRoadandStagingArea Theconstructionoftheaccessroadwillresultinachangeofthealbedothatwillincreasethe potentialforthawoftheicerichsiltsbelowtheorganicmat.Anuninsulatedorinsulatedsection maybeusedforconstructionoftheaccessroadandstagingareas.However,theamountof settlementexpectedwillvarydependingontheoptionselected. InsulatedEmbankment.Oneoptionisfortheproposedaccessroadandstagingareastoconsist ofaninsulatedsectionofstructuralfillthatisunderlainbygeotextile.Foreaseofconstruction,a levelingcourseconsistingofstructuralfillmaybeplacedovertheexistingsurface.Withthis option,alayerofgeotextileshouldbeplacedontheexistingsurfacepriortoplacementofthe levelingcoursetoprovideseparationbetweenthefillandtheunderlyingtundra.Theseparation geotextileshouldconsistofawovengeotextilethathasanapparentopeningsizeof0.425mm orsmallerandatensilestrengthof300lbsorgreater.Winfab315isoneproductthatmeetsthe intendedrequirements. Ifaninsulatedembankmentisselected,theinsulationshouldbeplacedbeneaththeentire roadwayandshouldextendtowithin1footofthetoeofthefillembankment.Theinsulation layershouldbeaminimumof4inchesthickandconsistof40Ͳpsicompressivestrength extrudedorexpandedpolystyrene.Iftheaccessroadisconstructedwithaninsulated embankment,weanticipatesettlementsoflessthan0.5feetduringthefirstthawseason. Aminimumof4feetofcompactedstructuralfillshouldbeplacedovertheinsulationand compactedtoestablishfinalgrade.Careshouldbetakenduringconstructiontoavoiddamaging theinsulation.Typically,damagecanbeavoidedbyplacing18inchesofstructuralfillbefore operatingheavyequipmentandavoidingsuddenorsharpequipmentmovement.Protectionof theinsulationshouldbetheresponsibilityofthecontractor.Theedgesoftheroadway embankmentshouldbeconstructedwithslopesthathaveahorizontaltoverticalrelationshipof 2to1(2H:1V)orflatter. EmbankmentWithoutInsulation.Asecondoptionisfortheproposedaccessroadandstaging areastoconsistofanembankmentwithoutinsulation.Withthisoption,alayerofgeotextile shouldbeplacedontheexistingsurfacetoprovideseparationbetweenthefillandthe underlyingtundra.Theseparationgeotextileshouldconsistofawovengeotextilethathasan apparentopeningsizeof0.425mmorsmallerandatensilestrengthof300lbsorgreater. Winfab315isoneproductthatmeetstheintendedrequirements. Aminimumof4feetofcompactedstructuralfillshouldbeplacedoverthegeotextileand compactedtoestablishfinalgrade.Tomatchthethermalperformanceoftheinsulated embankment,theembankmentwithoutinsulationwouldneedtobeapproximately12feet thick.Theedgesoftheroadwayembankmentshouldbeconstructedwithslopesthathavea horizontaltoverticalrelationshipof2to1(2H:1V)orflatter. Iftheaccessroadisconstructedwithoutinsulationandwithoutsufficientdepthtobethermally stable,itislikelythattheunderlyingiceͲrichpermafrostwillbesubjecttothaw.Thiswillresult Stebbins Wind Turbine Project Stebbins, Alaska Page 16 inashorterusablelifecomparedtoaninsulatedsection.Thawsettlementsforthisoptionwill behighestinareaswhereicelensesarepresent.Settlementcouldexceed2.5feetinasingle seasonandmaymaketheaccessroadandstagingareasunusable.Maintenanceandregrading shouldbeexpected.Inaddition,theroadwayculvertsaremorelikelytosettleorjackifthis optionisselected. 8.1.4 FillandCompaction StructuralfillandNFSfillplacedtogradethesiteshouldbegranularandconsistofwellgraded mixtureofcleansandsandgravelstoprovidedrainageandfrostprotection.Thestructuraland NFSfillshouldconsistofsoilthatisfreeofdebris,ice,excessmoistureandotherdeleterious materials.TherecommendationfillgradationsareprovidedinTable1below. Table1–FillSpecifications NFSMaterialStructuralFill Sieve%Passing%Passing 1.5"100100 3/4"70Ͳ10070Ͳ100 3/8"60Ͳ9560Ͳ90 No.445Ͳ8535Ͳ75 No.825Ͳ7025Ͳ65 No.508Ͳ3010Ͳ35 No.200<3<15 Fillsshouldbeplacedinliftsnottoexceed10to12inchesloosethickness,andcompactedtoa densityofatleast95percentofthemaximumdrydensityasdeterminedbytheModified Proctorcompactionprocedure(ASTMD1557)orvibratorytable(ASTMD4253)asappropriate. Aninitialliftofmaterialthatisupto18inchesthickmaybeplacedovertheinsulationalongthe accessroadtofacilitateconstructionbutitmayresultinchallengesachievingthedesired compaction.Duringfillplacement,wealsorecommendthatlargecobblesorboulderswith dimensionsinexcessof2/3theliftthicknessberemoved. 8.2 SeismicAnalysis Thesitecharacterizationcriteriafoundinthe2009InternationalBuildingCode(Code)shouldbe usedfordesign.TheseismicdesigncriteriaarefoundinChapter16,Section1613oftheCode. TheCoderequiresthatthesitecharacterizationbedeterminedbysoilandrockparameters. Basedonthesubsurfaceconditionsencountered,werecommendthesitebeconsideredSeismic SiteClass“B”.However,ifthepermafrostwarmsorthaws,areducedsiteclasswillberequired. Themaximumconsideredearthquakegroundmotionspectralresponseaccelerationsforshort periodandforoneͲsecondpeakswereobtainedutilizingtheUnitedStatesGeologicalSurvey’s (USGS’s)EarthquakeHazardsProgram;theresultsaresummarizedinTable2. Stebbins Wind Turbine Project Stebbins, Alaska Page 17 Table2–SeismicDesignCriteria IBC2012SeismicDesignCriteriaValue SpectralResponseatShortPeriods,SS0.257 SpectralResponseat1ͲSecondPeriod,S10.084 SiteClassB SiteAdjustedSpectralResponseatShortPeriods,SMS0.257 SiteAdjustedSpectralResponseat1ͲsecondPeriods,SM10.084 8.3 FoundationAnalyses Basedoniterativeconversationswiththestructuralengineer,theproposedturbinefoundation willbesupportedbyaseriesofslurrybackpilesthatare12Ͳinchesindiameterandhave2Ͳinch helicesalongthelower26feet.ThepileswillbeplacedintoapreͲdrilledholeandasoilͲwater slurrywillbeallowedtofreezebetweenthepileandthesurroundingsoil.Thepileswillbe coupledtoacastͲinͲplaceconcretepilecapattheturbinetowerbase. 8.3.1 Piles Axialloadswillbetransmittedfromthepiletothenativesoilthroughadfreezealongthe sideofthepiles.Thepilemustresisttheupliftfromfrostjacking,shorttermoverturning moments,andlongtermverticalcompressionloadsthatwillgeneratecreepsettlementin thepermafrost.Fourteen(14)pilesarerecommendedtosupporttheanticipateddesign loads. Thepilesshouldbeaminimumof12Ͳinchesindiameterwith2Ͳinchhelicesthathavea 12Ͳinchpitchandbeinstalledtoaminimumdepthof35feetbelowtheinsulation.The pilesshouldbefittedforthermosiphons.Thehelicesshouldbelocatedalongthelower25 feetofthepiles. Tominimizegroupeffects,aminimumcenterͲtoͲcenterpilespacingofthree(3)pile diameters(outerdiameter)shouldbemaintained.Thepilesshouldbeinstalledindrilled holesthatareaminimumof6to8inchesindiameterlargerthantheoutermostdiameter ofthepile.Theholesmaybedrilledusingairrotaryordryaugertechniquesandshouldbe keptplumbandstraight.Drillingmudsorotherfluidsshouldnotbeusedtodrilltheholes. Inaddition,thawingofthepermafrostusingsteam,water,orothertechniquesshouldnot beallowed. Stebbins Wind Turbine Project Stebbins, Alaska Page 18 Duringdrilling,surfacewatermustbepreventedfromenteringthedrillholes.Iffree waterorunfrozensoilisencountered,thepilesmustnotbeinstalledandHDLmustbe contactedtoreviewthesiteconditions.Drillcuttingsshouldbeproperlydisposedof. Thepilesshouldbeinstalledplumbandstraightintothedrilledholesandaminimum spacingof2Ͳinchesshouldbemaintainedbetweenthepilesandthewallsofthedrill holes.Temperaturemonitoringtubesshouldbeinstalledbetweenthepilesandthedrill holewalls.Themonitoringtubesshouldconsistof¾inchdiameterPEXtubingthatis properlycappedatbothends.Alternatively,1Ͳinchdiametergalvanizedsteelpipingmay beused.Thepilecapsmustbeconstructedtoallowaccesstothemonitoringtubesuntil temperaturemonitoringiscomplete. 8.3.2 Slurry Oncethepilesandthetemperaturemonitoringtubesareinstalled,thedrillholesshould bebackfilledwithslurry.Theslurryshouldbeplacedinliftsthatareamaximumof3feet thickanddensifiedwithaconcretevibrator. TheslurryshouldconsistofawellͲgradedsandandgravelaggregatepreͲmixedwith potablewater.Theaggregatefortheslurryshouldbefullythawedpriortomixingandthe resultingslurrymixshouldhaveatemperatureof40°F+/Ͳ5°Fatthetimeofplacement. Theslurrymaterialsmustbesaturatedandshouldhaveaconsistencysimilartoaconcrete withaslumpof5inches+/Ͳ1Ͳinch. Theaggregateshouldnotcontaingravellargerthan1Ͳinchindiameterandlessthan10% (byweight)ofthematerialshouldbefinerthanthe#200sieve.Arepresentativesample oftheproposedslurryaggregateandwatersourceshouldbeprovidedtoHDLtoassure thattheymeettherequiredgradationanddonotcontainfreezingpointdepressing materials. 8.3.3 SlurryFreezeback Theslurrymustbeallowedtofreezeandreachatemperatureof31°Forcolderpriorto constructionofthepilecaptoallowforpotentialpilemovementduringfreezeback.The temperatureoftheslurryshouldbemeasuredusingthetemperaturetubesalongthe entirelengthofthepile. 8.3.4 Settlement Thetotalsettlementsthatwilldevelopdependontheactualloadsthatareappliedand thequalityofconstruction.Iftheloadsareasanticipatedandthefoundationsare constructedasdesigned,theestimatedsettlementduetocreepis0.5Ͳinchin30years. 8.3.5 FrostHeaveForces Frostheaveforcesareanticipatedtobeminimalassumingtheturbinepadisconstructed usingnonͲfrostsusceptiblestructuralfill,thepadisinsulated,andthermosiphonpilesare Stebbins Wind Turbine Project Stebbins, Alaska Page 19 used.Ifdifferentmaterialsareusedortheunderlyingpermafrostissubjecttoseasonal thaw,thefrostheaveforcescouldbesignificant. 8.3.6 LateralDeflection Lateraldeflectionwillresultfromthehorizontalloadsappliedatthebaseofthetower. Thedeflectionwillberesistedbypassivepressuresalongthepiles.Theamountof deflectionisafunctionofthepiledimensions,load,whetherthepileheadsarefixedor pinned,andthedepthtothepointoffixity.Inpermafrost,thepointoffixityisconsidered tobethemaximumthawdepth.Assumingthepilesaresubjectedtoalateralloadof10 kipsandhaveapointoffixity6inchesbelowtheinsulation,alateraldeflectionof0.25 inchesshouldbeexpected.Thecalculateddeflectionalsoassumesthatthepileshavea diameterof12Ͳinchesandthicknessof0.5inches. 8.4 DrainageandDewatering Groundwaterwasnotencounteredintheboringsanddewateringisnotanticipated.HDL recommendsthesitebegradedtopromotepositivedrainageawayfromthestructuresand compactionofthenearsurfacesoilstoreducethepermeability.Culvertsshouldbeinstalledas neededtokeepwaterfrompondingalongtheroadandturbinepad. 8.5 ConstructionConsiderations Schedulingandsequencingisacriticalelementtothesuccessfulconstructionoftheturbineand siteimprovements.Thefrozenconditionsmustbemaintainedundertheinsulationafter installationofthepiles.Inaddition,carefulattentiontositepreparation,pileinstallation,and insulationandfillplacementisneededtoassurethattheunderlyingpermafrostisthermally protected. Thereareseveralfeasibleoptionstoinstallthepilesandmaintainfrozengroundconditions.We haveassumedthatthepileswillbeinstalledduringwinterorearlyspringconditionsandthe turbinepadwillbeprotectedfromthawwithinsulationthatistemporarilycoveredwithfrozen soilthatconsistsofNFSmaterial.Thecoversoilshouldbeallowedtothawanddrainduring summerconstructionconditionsandbecompactedinaccordancewithourrecommendations. Theactualschedulingandsequencingoftheworkshouldbeproposedbythecontractorand reviewedbyHDLtoverifythattheunderlyingpermafrostwillbeprotected. APPENDIXA FigureA1UnifiedSoilClassificationSystem FigureA2FrostDesignSoilClassification FigureA3FrozenSoilClassification FiguresA4ͲA7BoringLogs FiguresA8,A9GrainSizeDistributionCurves Criteria for Assigning Group Symbols and Names Soil Classification Generalized Group Descriptions UNIFIED SOIL CLASSIFICATION SYSTEM COARSE-GRAINED SOILS More than 50% retained on No. 200 sieve FINE-GRAINED SOILS 50% or more passes the No. 200 sieve GRAVELS 50% or more of coarse fraction retained on No. 4 sieve SANDS More than 50% of coarse fraction passes No. 4 sieve SILTS AND CLAYS Liquid limit 50% or less SILTS AND CLAYS Liquid limit greater than 50% CLEAN GRAVELS Less than 5% fines GRAVELS with fines More than 12% fines CLEAN SANDS Less than 5% fines SANDS with FINES More than 12% fines INORGANIC ORGANIC INORGANIC ORGANIC HIGHLY ORGANIC SOILS Primarily organic matter, dark in color, and organic odor GW GP GM GC SW SP SM SC ML CL OL CH MH OH PT Well-graded Gravels Poorly-graded Gravels Gravel & Silt Mixtures Gravel & Clay Mixtures Well-graded Sands Poorly Graded Sands Sand & Silt Mixtures Sand & Clay Mixtures Non-plastic & Low Low-plasticity Clays High-plasticity Clays High-plasticity Silts Peat Plasticity Silts Non-plastic and Low Plasticity Organic Clays Non-plastic and Low Plasticity Organic Silts High plasticity Organic Clays High Plasticity Organic Silts LIQUID LIMIT (LL)PLASTICITY INDEX (PI)010201630 40 50 60 70 80 90 100 11010 20 30 40 50 6074ML or OL CL or O L MH or OH CH or O H "A" LIN E"U" LI N E For classification of fine-grained soils and fine-grained fraction of coarse-grained soils. Equation of "A" line Horizontal at PI= 4 to LL= 25.5, then PI= 0.73 x (LL-20) Equation of "U" line Vertical at LL= 16 to PI= 7, then PI= 0.9 x (LL-8) CL - ML Figure A1 STEBBINS WIND TURBINE PROJECT STEBBINS, AK UNIFIED SOIL CLASSIFICATION SYSTEM ALASKA VILLAGE ELECTRIC COOPERATIVE STEBBINS, AK GROUP FROST DESIGN SOIL CLASSIFICATION KIND OF SOIL P200 TYPICAL SOILS (Modeled after U.S. Army Corps of Engineers Standards) F1 F2 F3 F4 Gravelly Soils Gravelly Soils Sands Sands, except very fine silty sands Gravelly Soils Clays PI > 12 All Silts Very fine silty sands Clays, PI < 12 Varved clays and other fine-grained, banded 6 to 10 10-20 6-15 Over 20 Over 15 Over 15 GM, GW-GM, GP-GM GM, GW-GM, GP-GM SW, SP, SM, SW-SM, SP-SM GM, GC SM, SC CL, CH ML, MH SM CL, CL-ML CL and ML CL, ML, and SM; CL, CH, and ML; CL, CH, ML, and SM P200 = percent passing the number 200 sieve NFS Sand or Gravel 0 to 6 SW, SP GW, GP sediments CL, CH Figure A2 STEBBINS WIND TURBINE PROJECT STEBBINS, AK FROST DESIGN SOIL CLASSSIFICATION ALASKA VILLAGE ELECTRIC COOPERATIVE. 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