Geotechnical Site Characterization by Cone Penetration Testing Paul W. Mayne, PhD, P.E. Professor, Geosystems Civil & Environmental Engineering Mid-America Earthquake Center FHWA
Standard Penetration Test Rotary-drilled Borehole Standard Penetration Test (SPT) Procedures: ASTM D 1586 N = measured Number of Blows to drive sampler 300 mm into soil.
Representative SPT Profile
Corrections to SPT N-value Effect Variable Term Value Overburden Stress svo' CN (Pa/svo')0.5 but < 2 Energy Ratio1 · Safety Hammer · Donut Hammer · Automatic Hammer CE 0.6 to 0.85 0.3 to 0.6 0.85 to 1.0 Borehole Diameter · 65 to 115 mm · 150 mm · 200 mm CB 1.00 1.05 1.15 Sampling Method · Standard sampler · Sampler without liner CS 1.0 1.1 to 1.3 Rod Length · 10 m to 30 m · 6 to 10 m · 4 to 6 m · 3 to 4 m CR 0.95 0.85 0.75 Particle Size Median Grain Size (D50) of Sand in mm CP 60 + 25 log D50 Aging Time (t) in years since deposition CA 1.2 + 0.05 log (t/100) Overconsolidation OCR COCR OCR0.2 1 Obtain by energy measurement per ASTM D4633
Corrections to SPT N-value Nmeasured = Raw SPT Resistance (ASTM D 1586). N60 = (ER/60) Nmeasured = Energy-Corrected N Value where ER = energy ratio (ASTM D 4633). Note: 30% < ER < 100% with average ER = 60% in the U.S. N60 CE CB CS CR Nmeas = Estimated corrected N (N1)60 = CN N60 = Energy-Corrected SPT Value normalized to an effective overburden stress of one atmosphere: (N1)60 = (N60)/(vo’)0.5 with stress given in atm. (Note: 1 atm = 1 bar = 100 kPa = 1 tsf).
Cone Penetrometers Electronic Steel Probes with 60° Apex Tip ASTM D 5778 Procedures Hydraulic Push at 20 mm/s No Boring, No Samples, No Cuttings, No Spoil Continuous readings of stress, friction, pressure
Cone Penetration Tests (CPT) Cone Trucks Mobile 25-tonne rigs with enclosed cabins to allow testing under all weather conditions
Corrections to CPT Tip Stress correction for porewater pressures on unequal areas – Important in Intact Clays & Silts Sleeve Friction correction for porewater pressures – Use equal end areas on friction sleeve Baseline corrections – Obtain before & after sounding
Anchored Cone Rig Mud Island, Downtown Memphis, TN 6-tonne Geostar truck with 20-tonne hydraulic pushing system No special license Twin earth anchors Has penetrated to depths over 32+ meters at sites in SC, AL, MO, TN, & AL
CPT Profile, Downhole Memphis fs ub qt
Comparison CPT and SPT Downtown Memphis
Seismic Piezocone Test Obtains Four Independent Measurements with Depth: Vs Cone Tip Stress, qt Penetration Porewater Pressure, u Sleeve Friction, fs Arrival Time of Downhole Shear Wave, ts fs u1 u2 60o qc
Downhole Shear Wave Velocity Anchoring System Automated Source Polarized Wave Downhole Vs
Automated Seismic Source Electronically-actuated Self-contained Left and right polarization Modified beam uses fin to enhance shear wave generation Successfully tested to depths of 20m Capable of being used with traditional impulse hammer
Downhole Shear Waves Left Strike Right Strike CROSSOVER Method Shear Wave at 8.15 m Shear Wave at 9.20 m CROSSCORRELATION Maximum crosscorrelation at Dt = 6.75 ms Shear wave velocity = 155 m/s Time shift (ms)
Sod Farm, Charleston, SC
CPT Soil Behavioral Classification Soil Behavior Type (Robertson et al., 1986; Robertson & Campanella, 1988) 1 – Sensitive fine grained 5 – Clayey silt to silty clay 9 – sand 2 – Organic material 6 – Sandy silt to silty sand 10 – Gravelly sand to sand 3 – Clay 7 – Silty sand to sandy silt 11 – Very stiff fine grained* 4 – Silty clay to clay 8 – Sand to silty sand 12 – Sand to clayey sand* *Note: Overconsolidated or cemented
Sounding - Shelby County, TN
CPT-Related Websites The Liquefaction Site (and CPT site): www.liquefaction.com Link page to manufacturers, suppliers, and CPT services: http://www.usucger.org/insitulinks.html Listing of available videos on CPT and other in-situ tests: http://www.geoinstitute.org/in-situ.html The book Cone Penetration Testing in Geotechnical Practice (Lunne, Robertson, & Powell, 1997): Review: http://geotech.civen.okstate.edu/magazine/books/ Order: http://www.routledge-ny.com/