Pore-Pressure Generation During CPT Probe Advancement

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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

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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 its
  bearing capacity
• ability to support a load, such as a building

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Governing Equations
FEMLab - Incompressible Navier-Stokes
Seed and Booker1 - Generation/Dissipation
Equations
????the volume strain u excess
pore-pressure ug earthquake generated u ?w
unit weight of water kh,v coeff. of
permeability mv coeff. of vol.
compressibility r radius N number of seismic
cycles
(2-D)
Earthquake Generated Pore-pressure
(radial symmetry)

• H.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
Probe is 1.5 diameter
Outflow velocity 0.02 m/s Initial pressure 0
kPa
No-slip
??? 20kg/m3 ????382 kg/m2
Slip
Slip
Inflow velocity 0.02 m/s Inflow pressure
17,680 kPa
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Solution Pressure Profile
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Solution Velocity Profile
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Validation Field Data
Model Pressure - effective stress excess pore
pressure
(291 kPa -193.9 kPa) 97 kPa
97 kPa 102 kPa
Data from GEMS site in KS, property of PSU-Energy
and Geo-Environmental Engineering
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Validation

• Strain Path Method by Baligh2

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Parametric Study Pressures at different
advancement rates
2.0 m/s
0.02 m/s
Rate increased by 100 times Pressure increased
by about 10-20 times
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Parametric Study
Density 1000 kg/m3 Viscosity 2000 kg/m2
Density 20 kg/m3 Viscosity 382 kg/m2
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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