Adventures in Transportation Geophysics Where The Rubber Meets the Road Bill Owen, CEG, RGP California Department of Transportation Geophysics and Geology Branch

Caltrans’ Geophysics and Geology Branch Geophysical Services Offered:Geophysical Services Offered: –Surface Methods Ground Penetrating Radar MagnetometryGround Penetrating Radar Magnetometry Seismic RefractionConductivitySeismic RefractionConductivity Refraction TomographyResistivityRefraction TomographyResistivity –Borehole Geophysical Logging GammaGamma DensityResistivityGammaGamma DensityResistivity CaliperSonic InductionCaliperSonic Induction PS Suspension Acoustic TeleviewerPS Suspension Acoustic Televiewer

Advantages of Geophysics Cost Effectiveness Caltrans’ Experience

Advantages of Geophysics Larger Volume of Investigation –Improved Representativeness Increased Mobility/Portability –Better Accessibility “Environmentally Friendly” Increased Sample Density In Situ Measurements Reduced sample storage requirements

Advantages of Geophysics (Continued) Equivalent Geotechnical DataEquivalent Geotechnical Data –SPT “N” Values –Bulk Density –Porosity –Elastic Moduli –Poisson’s Ratio

Borehole Geophysical Logs Collect unbiased continuous and in-situ data Sample a larger volume than core and cuttings Correlate between boreholes Provide information on unsampled boreholes Why Log?

Borehole Geophysical Log Interpretation No unique interpretation of a single geophysical log Multiple logs are collected and interpreted together Qualified with lithologic logging, sampling and laboratory testing Most geophysical interpretations are qualitative instead of quantitative

Cost Savings Using Borehole Geophysics More thorough site characterization Reduce the number of required borings Reduce the number of laboratory analyses

Caliper Nominal boring diameter 9.4 cm Washouts to 35.5 m bgs Collapsed borehole in fractured rock 35.5 m to 43 m, fractured rock to 49 m bgs Borehole Diameter (cm) Collapsed Zone - No Data

Natural Gamma Coarse grain, thick bedded sands and gravels from surface to 4 m bgs Fine grain, thin bedded silts and clays below 4 m bgs Gamma (counts per second)

Resistivity Low gamma and elevated resistivity suggest coarse grained material at 25.5 m to 27 m bgs Elevated gamma and elevated resistivity suggest sand derived from granite at 27.5 m to 30 m bgs GammaResistivityResistance

Induction Log collected in PVC cased boring Material change from soil to rock noted by increase in resistivity at 35.5 m bgs Spike in conductivity at 23 m probably due to saline groundwater GammaConductivityResistivity

Full Wave Form Sonic Log Collected in open borehole Porosity, Slowness, Waveform quality Decreased porosity at 88 m and 94 m bgs Increased porosity at 79.5 m bgs

Acoustic Televiewer Harder rock shown as darker color In this example, bedding appears as low amplitude sine waves and fractures appear as high amplitude sine waves that cross other features Image orientation N E S W N E S W N

Acoustic Televiewer Log Fracture/Bedding Orientation Stratigraphic Delineation Compressional- Wave Velocity Borehole Deviation Borehole Eccentricity Acoustic Televiewer Pseudo-Core “Front” View“Back” View NSN 2 feet

Stereonet Plot of Acoustic Televiewer picks Stereonet plot of features picked as bedding, plotted as poles-to-planes Preferred orientation of south dipping bedding planes

P-S Suspension Logger

Caltrans’ Seismic Retrofit Program A major effort to retrofit the state toll bridges to resist earthquake forces well into the 21st century –Investigation –Design –Retrofit

Caltrans’ Seismic Retrofit Program (Continued) Need for velocity log information –Site response analysis –Foundation load modeling –Lithologic correlation –Soil and rock properties (density, elastic moduli)

The Suspension Logging System Theory of Operation - Source –Dipole force –Wavelength >> borehole diameter –Flexural wave

The Suspension Logging System (Continued) Theory of Operation - Receiver –No clamping to borehole wall –Average density of receiver section must be close to that of the borehole fluid –Assures nearly flat response

S P S P Dipole Source Monopole Source Wavelength < Borehole Diameter

Frequency Velocity S-wave velocity Tube-wave velocity

12 Khz Source 4 KHz Source From Kurkjian and Chang, 1986

1 Khz Source From Kurkjian and Chang, 1986

Example: Open Hole S P

Example: Good Casing Bond S P

S formation S casing P Example: Free Casing

Data: Measured vs. Calculated Measured –Compressional and shear wave velocity Calculated –Density/Porosity –Poisson’s ratio –Elastic moduli (Gmax)

Comparison of Measured and Calculated Density Values Calculated Density (g/cc) Measured Density (g/cc) "Zero Residual” Line mean error = -4.5%   V p 0.25

P-S Suspension Logger

Project Design:Project Design: Geophysical Data = Reduced CostGeophysical Data = Reduced Cost –preliminary information prior to drilling or construction permits –focus conventional sampling methods on identified areas –continuous profiling –BETTER INFORMATION more accurate, less conservative designmore accurate, less conservative design more accurate cost estimatingmore accurate cost estimating

Project construction:Project construction: Geophysical data = money saved Geophysical data = money saved –fewer costly surprises –better bids –better claims defense –materials QC testing = more value for construction $

No Geophysics = Increased Cost No dataNo data Misinterpreted dataMisinterpreted data Discontinuous dataDiscontinuous data = Underdesign= Underdesign = Overdesign= Overdesign –Rule of Thumb for Seismic Design increase construction cost by 20% = no foundation quality assurance = potential shortened life span= no foundation quality assurance = potential shortened life span = Construction claims= Construction claims

Selection Criterion for Geophysics The Project Engineer/Geologist:The Project Engineer/Geologist: “Will It Help Me Solve My Problem?” The Project Manager:The Project Manager: “How Much Does It Cost?”

Evaluating Cost-Effectiveness For Geophysics Savings to the ProjectSavings to the Project –Drilling vs. Geophysics –Costs of Not Having Geophysical Data In-House vs. Contract CostsIn-House vs. Contract Costs

Comparison of Borehole Geophysics Field Costs: Contract vs. In-House (1995) In-House vs. Contract Costs

Comparison of Borehole Geophysics Field Costs: Contract vs. In-House (2000) In-House vs. Contract Costs

Results of Caltrans’ In-House Geophysics Marketing Identification of ClientsIdentification of Clients NetworkingNetworking The “Familiarity Factor”The “Familiarity Factor” AccessibilityAccessibility

Caltrans’ Geophysics Demand,

Comparison of Projects and Actual Borehole Geophysical Logs

020 m  Boring 02-1 Seismic Profile Boring 02-2 Seismic Tomography: NB I-80 at Willow Creek

? ? ? ? W.T. Undifferentiated Sand, Silt and Clay Very Weathered Sandstone Moderately Weathered to Fresh Sandstone Ground Surface Shot Location Geophone Culvert B 02-1 B 02-2 S N

? ? ? Shot Location Geophone Culvert B 02-1 B 02-2 S N Pseudo Raypath Model, NB I-80 at Willow Creek

? ? ? ? Shot Location Geophone B 02-1 B 02-2 S N

Some Conclusions Selective Use of Geophysics Can Result In Significant Cost Savings and Improve the Quality of Geotechnical InvestigationsSelective Use of Geophysics Can Result In Significant Cost Savings and Improve the Quality of Geotechnical Investigations At Caltrans, Cost Savings Are Realized Through Use of In-house Expertise for Routine Geophysical ServicesAt Caltrans, Cost Savings Are Realized Through Use of In-house Expertise for Routine Geophysical Services Effective Marketing Strategy and Favorable Investigation Results Have Increased Demand for Geophysical Services Within CaltransEffective Marketing Strategy and Favorable Investigation Results Have Increased Demand for Geophysical Services Within Caltrans

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