4 COX_Seismic Site Characterization(11!18!10)

Geotechnical Characterization for Seismic Design: Standard Penetration Testing and Shear WaveVelocityProfiles Shear Wave Velocity ProfilesBradyR. Cox, Ph.D., P.E. Brady R. Cox, Ph.D., P.E.The University of ArkansasDepartment of Civil EngineeringGeotechnical Earthquake Engineering for Seismic Design Workshop,Department of Civil EngineeringPort-au-Prince, Haiti, November 18-19, 2010Geotechnical Investigation: Standard Objectives Determine the depth and thickness of soil layers (including depth to bedrock if possible) Determine the location of the ground water table Obtain soil samples for testing Most common method used around the world is the Standard PenetrationTest Penetration Test (SPT)McCarthyStandard Penetration Test (SPT): EquipmentDrill RigCoduto(2001)S litCoduto (2001)Split-spoon Sampler5 ODCoduto (2001)5 cm OD3.5 cm IDSPT: Procedure Drill to the desired depth Drop a 63.5 kg mass on top of the drill rod from a height of 0.75 m Count the number of hammer blows to drive the split-spoon sampler 3 separate 15 cm intervals Sum of blows over the last 2 increments (i.e. the last 30 cm) is the blow count or N-valueC d t (2001) Coduto (2001) Stop if> 50 blows are needed for any 15 cm increment (refusal) Removethesplit spoonandretrievesoil samplefor characterization Remove the split-spoon and retrieve soil sample for characterization Repeat the test at desired depth interval (typically every 1 1.5 m)SPT: Example Boring LogN = 11N = 17N = 16N = 11SPT: Example N-valuesIn-Situ Shear Wave Velocity (Vs) Measurements Earthquake damage is considered to be caused primarily by vertically propagating shear waves The velocity at which these shear waves travel throughagivenmaterial (i e rockvs soil) through a given material (i.e. rock vs. soil) strongly influences the response of the material becauseV isdirectlyrelatedtoshear modulus because Vsis directly related to shear modulus Therefore, a very important part of Geotechnical Earthquake Engineering is dynamic site characterization to obtain in-situ measurements of VsSeismic Investigation: Additional Objectives Obtain a shear wave velocity (Vs) profile to a depth of at least 30m0 0600 500 400 300 200 100 0Shear Wave Velocity (m/s)30 m Vs reflects the shear modulus (G) of thesoil accordingto:5010(G) of the soil according to: G = *Vs2 Vs used to obtain simplified 100epth (ft)3020Depth (pSeismic Site Classification via the average shear wave velocity over thetop30m(Vs30or Vs)150De40(m)Vs =Vs30 =325 m/sover the top 30m (Vs30 or Vs) Vs profile also needed for more advancedgroundmotion2006050advanced ground motion prediction via site response analysis2002000 1600 1200 800 400 0Shear Wave Velocity (ft/sec)In-Situ Shear Wave Velocity (Vs) Measurements Intrusive (Borehole Methods)C h l Crosshole DownholeS i L i Suspension Logging Non intr si e(S rfaceWa eMethods) Non-intrusive (Surface Wave Methods) Spectral Analysis of Surface Waves (SASW)Multi channel Analysisof SurfaceWaves(MASW) Multi-channel Analysis of Surface Waves (MASW) Refraction Microtremor (ReMi) Crosshole: Setup and EquipmentHorizontal (H1)GeophoneH i t l (H2)Horizontal (H1)GeophoneH i t l (H2) Horizontal (H2)GeophoneVertical (V)GeophoneHorizontal (H2)GeophoneVertical (V)GeophoneReceiverCaseReceiverCase3D ReceiverCrosshole HammerCrosshole: Shear Wave Records2 Downward Impact Upward ImpactT i0agnitudeTrigger Vertical ReceiverinOneBorehole-2ormalized Ma in One BoreholeVertical Receiverin Second Borehole-4NoDenotesArrival Time-60.010 0.008 0.006 0.004 0.002 0.000 -0.002Time, secDenotes Arrival TimetVs = d / t = m/s Crosshole: Vs Profile0050Thin Limestone 100surement Depth, ft Layer (?)

150Meas150Site 2 Boreholes 41C-41A Crosshole 41C-41B Crosshole20010000 8000 6000 4000 2000 0SV-Wave Velocity, fpsDownhole: Setup and EquipmentBHG-3 Borehole ReceiverBoreholeBH-4Downhole: Setup and EquipmentInstrumented SledgehammerShear WaveTraction Plank BHG-3Control BoxLaptopDynamic Si l A l Signal AnalyzerDownhole: Travel Time vs. DepthDownhole: Vs ProfileSuspension Logging: Setup and WaveformsCable Head7-Conductor cableDisketteOYO PS-160Logger/RecorderHead ReducerUpper GeophoneWinchwith DataLower GeophoneFilter TubeSourceSource DriverWeightOverall Length~25ft Overall Length 25 ftDepth Sequential Waveform Arrivals Courtesy of GeoVisionSurface Wave MethodsVertically Oriented Source Vertically Oriented SourceSASW SetupReceiver 1Receiver 2d d(Impact, Random, or Steady-State Vibration)Receiver 1Receiver 2d d(Impact, Random, or Steady-State Vibration)Vertically Oriented Velocity TransducersLayer 1Vertically Oriented Velocity TransducersLayer 1Multi-Layered SolidLayer 2Multi-Layered SolidLayer 2MASW SetupSASW EquipmentDynamic Signal Analyzer Geophones and SledgehammerMASW Equipment12 60 GeophonesSledge Hammer & Drop Weight SourcesVibroseis SourceSurface Wave DispersionLow frequencyLayer 1 Layer 1VerticalParticle MotionVertical Particle Motion1AirLayer 1 Layer 1VerticalParticle MotionVertical Particle Motion1AirLayer 1 Layer 1VerticalParticle MotionVertical Particle Motion1AirLow frequency surface waves have long wavelengths Layer 2Layer 1Layer 2Layer 121Layer 2Layer 1Layer 2Layer 121Layer 2Layer 1Layer 2Layer 121(), while high frequency waves have short Depth DepthLayer 3Layer 3Depth DepthLayer 3Layer 3Depth DepthLayer 3Layer 3wavelengthsW i hDepth Deptha. MaterialProfilec. Longer Wavelength, 2b. Shorter Wavelength, 1Depth Deptha. MaterialProfilec. Longer Wavelength, 2b. Shorter Wavelength, 1Depth Deptha. MaterialProfilec. Longer Wavelength, 2b. Shorter Wavelength, 1Waves with different frequencies/qwavelengths sample different depthsSurface wave velocity (Vr) is close to shear wave velocity (Vs):Vs~ 1.1*VrExample SASW Dispersion Curve5000Wavelength (m)1 10 100 1000Experimental Disp. Curvesec)4000/sec)1200ReceiverSpacings=5,10,20,25,40,50,150,300,450,and600ft.Velocity (ft/s3000Velocity (m/800Phase V10002000Phase V4001 10 100 1000 10000010000Wavelength (ft)1 10 100 1000 10000Inversion to Obtain Vs ProfileWavelength (m)c)400050001 10 100 1000c)1200Experimental Disp. CurveTheoretical Disp.CurveVelocity (ft/sec20003000Velocity (m/sec800Shear Wave Velocity (ft/sec)0 2000 4000 6000 8000Phase 10002000Phase V40001000200Wavelength (ft)1 10 100 1000 100000 0epth (m)100200Depth (ft)400600D300D8001000max/2Shear Wave Velocity (m/sec)0 500 1000 1500 20001200Seismic Site ClassificationRequired by Seismic Provisions in Building Codes Required by Seismic Provisions in Building CodesIBC (2009) ASCE 7-05IBC & ASCE Codes Seismic Site ClassificationVsN SuSite Class: A - F>1500m/s >1,500 m/s760 1,500 m/s360 760 m/s180 360 / 180 360 m/s