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Pore-Pressure Generation During CPT Probe Advancement By Michael Fitzgerald

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CPT Overview: The Cone Penetration Test (CPT) : in-situ technique used to determine various soil parameters. The CPT : a cone on the end of a series of rods constant rate (~2 cm/s) Electronic sensors measure parameters Parameters: cone penetration resistance pore-pressure measurement (static and excess) sleeve friction. characteristics of the soil: hydraulic conductivity grain size bearing capacity The Cone Penetration Test (CPT) : in-situ technique used to determine various soil parameters. The CPT : a cone on the end of a series of rods constant rate (~2 cm/s) Electronic sensors measure parameters Parameters: cone penetration resistance pore-pressure measurement (static and excess) sleeve friction. characteristics of the soil: hydraulic conductivity grain size bearing capacity

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CPT Overview:

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Soil Liquefaction: Cyclic loading caused by earthquakes: excess Pore-Pressures can be generated methods being developed to determine potentially liquefiable soils Pore-pressure is function of: permeability of the soil penetration rate of the probe When pore-pressure equals weight of the overburden soil: Soil is potentially unstable and may lose it’s bearing capacity ability to support a load, such as a building Cyclic loading caused by earthquakes: excess Pore-Pressures can be generated methods being developed to determine potentially liquefiable soils Pore-pressure is function of: permeability of the soil penetration rate of the probe When pore-pressure equals weight of the overburden soil: Soil is potentially unstable and may lose it’s bearing capacity ability to support a load, such as a building

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Governing Equations: FEMLab - Incompressible Navier-Stokes Seed and Booker 1 - Generation/Dissipation Equations the volume strain u = excess pore-pressure u g = earthquake generated u w = unit weight of water k h,v = coeff. of permeability m v = coeff. of vol. compressibility r = radius N = number of seismic cycles, with CPT Generated Pore-Pressure: (2-D) Earthquake Generated Pore-pressure: (radial symmetry) 1H.B. Seed and J.R Booker, “Stabilization of Potentially Liquefiable Sand Deposits Using Gravel Drains”, Journal of the Geotechnical Engineering Division. July 1977

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Formulation: 20kg/m 3 382 kg/m 2 Slip No-slip Outflow velocity = 0.02 m/s Initial pressure = 0 kPa Inflow velocity = 0.02 m/s Inflow pressure = 17,680 kPa Probe is ~1.5” diameter

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Solution: Pressure Profile

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Solution: Velocity Profile

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Validation: Field Data Data from GEMS site in KS, property of PSU-Energy and Geo-Environmental Engineering Model pressure at tip: ~ 291 kPa Pore-pressure measured at tip: 102 kPa Model Pressure - effective stress = excess pore pressure (291 kPa -193.9 kPa) = 97 kPa 97 kPa ≈ 102 kPa

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Validation: Strain Path Method by Baligh 2 :

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Parametric Study: Pressures at different advancement rates 2.0 m/s0.02 m/s Rate increased by 100 times; Pressure increased by about 10-20 times

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Parametric Study: Density = 1000 kg/m 3 Viscosity = 2000 kg/m 2 Density = 20 kg/m 3 Viscosity = 382 kg/m 2

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Conclusions: Pore-pressures are generated through soil strain FEM can be an effective tool in modeling the pressures induced at the tip of a CPT cone If the soil compressibility is known (tri-axial test) then pressure can be converted to strain Strain can then be converted to pore-pressure using the permeability of the soil Pore-pressures are generated through soil strain FEM can be an effective tool in modeling the pressures induced at the tip of a CPT cone If the soil compressibility is known (tri-axial test) then pressure can be converted to strain Strain can then be converted to pore-pressure using the permeability of the soil

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FHWA CPT Workshop Goal Assist DOT’s to start and increase use of CPT in Highway applications by developing, presenting and discussion information on CPT.

FHWA CPT Workshop Goal Assist DOT’s to start and increase use of CPT in Highway applications by developing, presenting and discussion information on CPT.

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