Puran Sinha
Observing hysteresis in stress-strain curves while using the Mohr-Coulomb model under static vertical loading in a caisson foundation could stem from several factors related to the soil behavior, model limitations, and numerical issues:
Soil Behavior and Model Limitations
1. Plastic Deformation: The Mohr-Coulomb model captures linear elastic-perfectly plastic behavior. When the soil undergoes plastic deformation under static loading, energy dissipation occurs, which can manifest as hysteresis in stress-strain curves. This is especially relevant if the soil experiences significant yielding.
2. Complex Soil Behavior: Real soils often exhibit more complex behavior than what the Mohr-Coulomb model can represent. Factors such as strain hardening or softening, anisotropy, and non-linear elastic behavior are not captured by this model, potentially leading to hysteresis effects.
3. Cyclic Loading History: If the soil has a history of cyclic or dynamic loading, residual stresses and strains from these past events can influence its response under current static loading, resulting in hysteresis. This reflects the soil’s memory of past deformations and the inherent path dependency of its behavior.
Numerical and Computational Factors
4. Numerical Artifacts: Numerical issues in computational simulations can introduce artifacts that resemble hysteresis. Mesh discretization, time-stepping methods, and convergence criteria in the finite element analysis can affect the results. Ensuring fine mesh quality and proper numerical parameters might mitigate such issues.
5. Boundary Conditions and Model Setup: The setup of boundary conditions and load application in the numerical model can influence the observed stress-strain response. Improperly defined constraints or load application procedures can cause non-physical responses, including hysteresis.
Material Properties and Time-Dependent Effects
6. Soil Heterogeneity and Anisotropy: Natural soils are inherently heterogeneous and anisotropic. Variations in soil properties across the foundation and directional dependencies can lead to complex stress-strain responses that are not fully captured by the Mohr-Coulomb model, resulting in hysteresis.
7. Viscoelastic and Viscoplastic Effects: Some soils exhibit viscoelastic or viscoplastic behavior, where time-dependent deformation occurs under sustained loading. Even under static loading conditions, these time-dependent effects can cause a hysteresis-like response in the stress-strain curves.
Recommendations for Addressing Hysteresis
1. Advanced Constitutive Models: Consider using more sophisticated constitutive models that account for strain hardening/softening, anisotropy, and time-dependent behavior. Models such as the Hardening Soil Model, the Modified Cam-Clay model, or viscoelastic/plastic models might provide a more accurate representation of soil behavior.
2. Calibration and Validation: Calibrate the chosen model with experimental data specific to the soil type and conditions at the caisson foundation site. Validate the model by comparing simulation results with observed field behavior to ensure accuracy.
3. Improving Numerical Precision: Refine the numerical model by using a finer mesh, appropriate time-stepping, and ensuring convergence criteria are met. Conduct sensitivity analyses to understand the influence of numerical parameters on the stress-strain response.
By considering these factors and improving the modeling approach, you can achieve a more accurate representation of the soil behavior and mitigate the occurrence of unexpected hysteresis in the stress-strain curves.
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