Pore-Pressure Generation During CPT Probe Advancement By Michael Fitzgerald

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

CPT Overview:

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

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

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

Solution: Pressure Profile

Solution: Velocity Profile

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 kPa) = 97 kPa 97 kPa ≈ 102 kPa

Validation: Strain Path Method by Baligh 2 :

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

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

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