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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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