Micromechanical Analysis of Internal Instability during Shearing
Publication: International Journal of Geomechanics
Volume 23, Issue 6
Abstract
Internal instability means that finer particles pass through the constrictions of coarser particles at a hydraulic gradient well below that of heave or piping, rendering the soil ineffective for its intended purpose. The soil could make a transition from an internally stable state to an unstable state or vice versa due to shear-induced deformation. The discrete element method (DEM) is adopted in this study to examine and quantify soil behavior by simulating the quasi-static shear deformation of internally stable and unstable soils at the micro- and macroscales. The dense bimodal specimens were sheared under drained conditions following a constant mean stress path in order to investigate the influence of stress heterogeneity. At the macroscale, the peak deviatoric stress was found to be a function of the fines content and the initial void ratios of the specimens. The development of the average number of contacts per particle and the stress transfer to the finer fraction during shearing are discussed. The simulation results innovatively show that a dense specimen could undergo a transition from an internally stable to an unstable soil as it dilates during shear. These numerical results have significant implications on the importance of real-life situations, such as predicting mud pumping in railroad tracks.
Practical Applications
Microscale investigations enabled a better understanding of the mechanism of internal instability of soil, which can be helpful in the design and construction of substructures in railways. To demarcate internally unstable and stable soils, the microscale criterion based on the coordination number and the stress reduction factor can be used to estimate the probability of internal instability in the field at different stress state. Internal instability is a problem in railways that results in significant maintenance costs. The findings of this study can be used to avoid the huge maintenance cost associated with internally unstable capping layer in railways.
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Acknowledgments
The authors gratefully appreciate the assistance of Prof. Catherine O’Sullivan (Imperial College, London) in reading the manuscript and making constructive comments for its improvement, including the plots in Fig. 8. Financial support from the Transport Research Centre, University of Technology Sydney (UTS), Sydney NSW, Australia, is greatly appreciated.
Notation
The following symbols are used in this paper:
- davg
- average diameter of the coarser fraction of PSD;
- G
- shear modulus;
- In
- inertial number;
- Np
- number of particles;
- Nc
- number of contacts;
- number of contacts between fine particles;
- number of contacts between fine and coarse particles;
- number of fine particles;
- number of coarse particles;
- number of contacts between coarse particles;
- p′
- effective mean stress of the specimen;
- q
- deviatoric stress;
- Rd
- relative density;
- rmin
- minimum particle radius;
- Z
- coordination number;
- Zfine–coarse
- fine-coarse coordination number;
- Zcoarse–coarse
- coarse-coarse coordination number;
- α
- stress reduction factor;
- strain rate;
- axial strain;
- volumetric strain;
- µs
- coefficient of sliding friction;
- ρ
- solid particle density;
- major principal stress;
- intermediate principal stress;
- minor principal stress; and
- ν
- Poisson’s ratio.
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Published In
International Journal of Geomechanics
Volume 23 • Issue 6 • June 2023
Copyright
© 2023 American Society of Civil Engineers.
History
Received: Jul 27, 2022
Accepted: Jan 6, 2023
Published online: Apr 13, 2023
Published in print: Jun 1, 2023
Discussion open until: Sep 13, 2023
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