Geotechnical Characterization for Seismic Design: Standard Penetration Testing and Shear Wave Velocity Profiles Brady R. Cox, Ph.D., P.E. The University of Arkansas Department of Civil Engineering Geotechnical Earthquake Engineering for Seismic Design Workshop, Port-au-Prince, Haiti, November 18-19, 2010 Geotechnical 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 Penetration Test (SPT) McCarthy Standard Penetration Test (SPT): Equipment Drill Rig Coduto (2001) Split-spoon S lit Sampler Coduto (2001) 5 cm OD 3.5 cm ID SPT: 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-value C d t (2001) Coduto • Stop if > 50 blows are needed for any 15 cm increment (refusal) • Remove the split split-spoon spoon and retrieve soil sample for characterization • Repeat the test at desired depth interval (typically every 1 – 1.5 m) SPT: Example Boring Log N = 11 N = 17 N = 16 N = 11 SPT: Example N-values In-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 through a given material (i.e. (i e rock vs. vs soil) strongly influences the response of the material because Vs is directly related to shear modulus • Therefore, a very important part of Geotechnical Earthquake Engineering is dynamic site characterization to obtain in-situ measurements of Vs Seismic Investigation: Additional Objectives • Obtain a shear wave velocity (Vs) profile to a depth of at least 30 m Shear Wave Velocity (m/s) 0 200 300 400 500 600 0 10 • Vs reflects the shear modulus (G) of the soil according to: G = *Vs2 50 Deepth (ft) 20 30 100 Vs = Vs30 = 325 m/s 40 150 50 • Vs profile also needed for more advanced ground motion prediction via site response analysis 200 0 400 800 1200 1600 Shear Wave Velocity (ft/sec) 60 2000 Depth ((m) • Vs used to obtain simplified p Seismic Site Classification via the average shear wave velocity over the top 30m (Vs30 or Vs) 100 0 In-Situ Shear Wave Velocity (Vs) Measurements • Intrusive (Borehole Methods) –C Crosshole h l – Downhole – Suspension S i Logging L i • Non-intrusive Non intr si e (Surface (S rface Wave Wa e Methods) – Spectral Analysis of Surface Waves (SASW) – Multi-channel Multi channel Analysis of Surface Waves (MASW) – Refraction Microtremor (ReMi) Crosshole: Setup and Equipment Horizontal (H1) Geophone H i t l (H2) Horizontal Geophone Vertical (V) Geophone Receiver Case 3D Receiver Crosshole Hammer Crosshole: Shear Wave Records 2 Downward Impact Upward Impact Noormalized Maagnitude Ti Trigger 0 Vertical Receiver in One Borehole -2 Vertical Receiver in Second Borehole -4 Denotes Arrival Time -6 -0.002 0.000 0.002 0.004 0.006 0.008 Time, sec t Vs = d / t = m/s 0.010 Crosshole: Vs Profile 0 Meassurement Depth, ft 50 Thin Limestone Layer (?) 100 150 Site 2 Boreholes 41C-41A Crosshole 41C-41B Crosshole 200 0 2000 4000 6000 SV-Wave Velocity, fps 8000 10000 Downhole: Setup and Equipment Borehole BH-4 BHG-3 Borehole Receiver Downhole: Setup and Equipment Shear Wave Traction Plank Instrumented Sledgehammer BHG-3 Control Box Laptop Dynamic Signall Analyzer Si A l Downhole: Travel Time vs. Depth Downhole: Vs Profile Suspension Logging: Setup and Waveforms 7-Conductor cable OYO PS-160 Logger/Recorder Cable Head Diskette with Data Head Reducer Winch Upper Geophone Lower Geophone Filter Tube Source Source Driver Weight Overall Length ~ 25 ft Courtesy of GeoVision Depth Sequential Waveform Arrivals Surface Wave Methods Vertically Oriented Source (Impact, Random, or SteadyState Vibration) d Receiver 1 d Receiver 2 SASW Setup Vertically Oriented Velocity Transducers Layer 1 Layer 2 Multi-Layered Solid MASW Setup SASW Equipment Dynamic Signal Analyzer Geophones and Sledgehammer MASW Equipment 12 – 60 Geophones Sledge Hammer & Drop Weight Sources Vibroseis Source Surface Wave Dispersion Low frequency surface waves have long wavelengths (), while high frequency waves have short wavelengths Air Layer Layer11 Vertical Particle Motion 1 Vertical Particle Motion 2 Layer22 Layer Layer 3 Layer Waves with W ih different frequencies/ q wavelengths sample different depths a. Material Profile Depth Depth c. Longer b. Shorter Wavelength, 2 Wavelength, 1 Surface wave velocity (Vr) is close to shear wave velocity (Vs): Vs ~ 1.1*Vr Example SASW Dispersion Curve Wavelength (m) 1 5000 10 100 1000 Experimental Disp. Curve Receiver Spacings = 5, 10, 20, 25, 40, 50, 150, 300, 450, and 600 ft. 1200 3000 800 2000 400 1000 0 1 10 100 Wavelength (ft) 1000 0 10000 Phase V Velocity (m//sec) Phase V Velocity (ft/s sec) 4000 Inversion to Obtain Vs Profile Wavelength (m) 1 5000 10 100 1000 Experimental Disp. Curve Theoretical Disp.Curve 1200 3000 800 2000 400 Phase V Velocity (m/sec c) Shear Wave Velocity (ft/sec) 0 1000 0 2000 4000 6000 8000 0 200 100 1 10 100 Wavelength (ft) 1000 0 10000 400 600 200 800 max/2 1000 1200 0 500 1000 1500 2000 Shear Wave Velocity (m/sec) 300 Depth (m) 0 Depth (ft) D Phase Velocity (ft/sec c) 4000 Seismic Site Classification Required by Seismic Provisions in Building Codes IBC (2009) ASCE 7-05 IBC & ASCE Codes – Seismic Site Classification Site Class: A - F Vs N Su > 1,500 1 500 m/s 760 – 1,500 m/s 360 – 760 m/s 180 – 360 m/s / < 180 m/s ASCE 7-05 Vs is i preferred f d because b it is i directly di tl related l t d to t the th shear stiffness of the soil deposit (G = Vs2) Preview Importance of Seismic Site Classification IBC and ASCE Code – Design Response Spectra Little Rock, Arkansas Soft Soil ((Site Class E)) Horizontal Earthquake Force 70% of the Structure Weight AR Hard Rock (Site Class A) Horizontal Earthquake Force 25% of the Structure Weight 0.2-sec (~ 2-story building) Seismic Site Classification via Vs (i.e. Vs30) ASCE 7-05 Example Sites Shear Wave Velocity (m/s) Shear Wave Velocity (m/s) 0 500 1000 1500 0 2000 0 100 200 300 400 0 0 0 5 5 20 20 10 10 400 Depth (ft) 60 15 60 20 20 80 25 100 Vs = Vs30 = 1015 m/s 0 2000 4000 6000 Shear Wave Velocity (ft/sec) Site Class B 30 8000 80 25 100 Vs = Vs30 = 250 m/s 0 400 800 1200 Shear Wave Velocity (ft/sec) Site Class D 30 1600 Depth (m) 15 Depth (m) Depth (ft) 40 Seismic Site Classification via N ASCE 7-05 Seismic site classification via blow count (N) is possible, but classification via Vs is preferred because Vs is a material property that strongly influences ground motions Questions?