Self CPTu Measurements for Determining su and OCR in Clay Soils: Simulation and Practical Applications
Publication: International Journal of Geomechanics
Volume 22, Issue 11
Abstract
The undrained shear strength (su) and overconsolidation ratio (OCR) of clayey soils are two key parameters that are frequently used in geotechnical engineering practice. In comparison to laboratory tests that are expensive and time-consuming, the fast evaluation of these parameters using the field piezocone penetration test (CPTu) is favorable. The available literature contains a number of correlations that have been suggested by several researchers to estimate the su and OCR from CPTu; however, these correlations were mostly empirical and were constants multiplied by CPTu measurements. In addition, these correlations were site-specific and provided rough estimations of these important parameters. In this paper, the process of a CPTu in clayey soils was modeled via finite-element (FE) formulations, and the predictions for excess pore water pressures (EPWPs) that were generated around the penetrating piezocone were elaborated on and used to develop two new relationships for the estimation of su and OCR from CPTu data. A major advantage of the proposed relationships is that they could be employed using CPTu measurements and no other information from laboratory experiments or calibrations with field benchmarks or reference values may be needed. The utilized simulation procedure and the proposed relationships were compared and validated with the laboratory and field measurements and the existing relationships in the literature.
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Data Availability Statement
All data, models, and codes generated or used during this paper appear in the published article.
Notation
The following symbols are used in this paper:
- Af
- Skempton pore water pressure parameter at failure;
- Bq
- Pore water pressure parameter;
- e
- current void ratio;
- eo
- initial void ratio;
- eN
- void ratio at p′ of 1 kPa on normal compression line;
- Fr
- normalized friction ratio;
- fs
- unit sleeve friction;
- G
- shear modulus;
- Gmax
- maximum shear modulus;
- g
- gravitational acceleration constant;
- Ic
- soil behavior type index;
- Ir
- rigidity index;
- K0
- coefficient of earth pressure at rest;
- K0,nc
- K0 for normally consolidated state;
- k
- constant parameter, calibration factor for overconsolidation ratio;
- k1
- excess pore water pressure coefficient;
- k2
- excess pore water pressure coefficient;
- ls
- length of the friction sleeve;
- M
- slope of the critical state line;
- Nkt
- cone factor, which relates the cone tip resistance to the undrained shear strength;
- Nm
- cone resistance number;
- NΔu
- cone factor, which relates the excess pore water pressure at cone shoulder to the undrained shear strength;
- PI
- plasticity index;
- p′
- mean effective stress;
- Pa
- atmospheric pressure;
- Q
- normalized cone tip resistance;
- q
- deviatoric stress;
- qc
- measured cone tip resistance;
- qnet
- net cone tip resistance;
- qt
- corrected cone tip resistance;
- R
- isotropic overconsolidation ratio;
- R2
- R-square;
- Rf
- friction ratio;
- r
- piezocone radius;
- St
- soil sensitivity;
- su
- undrained shear strength;
- u
- pore water pressure;
- uo
- hydrostatic pore water pressure;
- u1
- pore water pressure on the cone;
- u2
- pore water pressure at the cone shoulder;
- u3
- pore water pressure behind the friction sleeve;
- Vs
- shear wave velocity;
- wL
- liquid limit;
- α
- nondimensional parameter that affects the magnitude of excess pore water pressure at u3 position;
- γt
- total soil unit weight;
- Δu
- excess pore water pressure (u – uo);
- strain rate;
- ηɛ
- correction factor for the strain rate;
- κ
- slope of the swelling line;
- Λ
- plastic volumetric strain ratio;
- λ
- slope of the normal compression line;
- μR
- ratio of R to overconsolidation ratio;
- ρt
- total soil mass density;
- σvo
- total vertical stress;
- vertical effective stress; and
- ϕ′
- effective friction angle.
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International Journal of Geomechanics
Volume 22 • Issue 11 • November 2022
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© 2022 American Society of Civil Engineers.
History
Received: May 23, 2021
Accepted: Jun 5, 2022
Published online: Aug 30, 2022
Published in print: Nov 1, 2022
Discussion open until: Jan 30, 2023
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