Effect of Total Stress Path and Gas Volume Change on Undrained Shear Strength of Gassy Clay
Publication: International Journal of Geomechanics
Volume 21, Issue 11
Abstract
Clay with free gas bubbles can be frequently encountered in the seabed. Gassy clay is an unsaturated soil, but its mechanical behavior cannot be described using conventional unsaturated soil mechanics because it has a composite internal structure with a saturated soil matrix and gas bubbles. The gas bubbles can have either a detrimental or beneficial effect on the undrained shear strength of clay. New lower and upper bounds for the undrained shear strength of gassy clay are derived by considering the effect of total stress path and plastic hardening of the saturated soil matrix. For the upper bound, it is assumed that there is only bubble flooding, and the shear strength of an unsaturated soil sample is the same as that of the saturated soil matrix. Bubble flooding makes the saturated soil matrix partially drained and increases the undrained shear strength. The amount of bubble flooding is calculated using the modified Cam-Clay model and Boyle's law for ideal gas. The lower bound is derived based on the assumption that the entire soil fails without bubble flooding and the gas cavity size evolves due to plastic hardening of the saturated soil matrix. Compared with Wheeler's upper and lower bounds that do not consider plastic hardening of the saturated soil matrix, the new theoretical results give a better prediction of the undrained shear strength of gassy clays, especially for the upper bound. Implications for constitutive modeling of gassy clay are discussed based on the new research outcomes.
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Acknowledgments
The authors acknowledge the comments given by Professor Simon Wheeler at the University of Glasgow during their discussions with him.
Notation
The following symbols are used in this paper:
- a
- slope of the total stress path;
- initial void ratio for the saturated soil matrix;
- f
- volume fraction of gas;
- ff
- gas volume fraction at failure;
- f0
- initial volume fraction of gas;
- M
- critical state stress ratio;
- N
- value of Vm at a unit mean effective stress for the normal compression line in the Vm − lnp′ space;
- p
- total stress;
- pa
- atmospheric pressure;
- p′
- mean effective stress;
- mean effective stress at failure;
- initial mean effective stress;
- q
- deviator stress;
- qf
- deviator stress at failure;
- R
- overconsolidation ratio;
- su
- undrained shear strength;
- Vg
- specific volume of free gas;
- specific volume of gas at failure;
- initial specific volume of free gas;
- Vm
- specific volume of the saturated soil matrix;
- specific volume of the saturated soil matrix at failure;
- initial specific volume of the saturated soil matrix;
- ug
- gas pressure;
- initial gas pressure;
- uw
- pore-water pressure;
- pore-water pressure at failure;
- initial gas pressure;
- undrained shear strength of the saturated soil;
- Γ
- value of Vm at a unit mean effective stress for the CSL in the Vm − lnp′ space;
- κ
- slope of the swelling line; and
- λ
- slope of the NCL.
References
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Published In
International Journal of Geomechanics
Volume 21 • Issue 11 • November 2021
Copyright
© 2021 American Society of Civil Engineers.
History
Received: Mar 16, 2021
Accepted: Jul 19, 2021
Published online: Sep 15, 2021
Published in print: Nov 1, 2021
Discussion open until: Feb 15, 2022
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Cited by
- Zhiwei Gao, Hongjian Cai, Yi Hong, Dechun Lu, A critical state constitutive model for gassy clay, Canadian Geotechnical Journal, 10.1139/cgj-2020-0754, 59, 6, (1033-1045), (2022).