How to simulate the undrained behavior?
Simulating the undrained behavior of soil can be carried out by means of three different approaches:
- Effective stress analysis using Consolidation driver type for Deformation+Flow problem type
- Effective stress analysis using Driven Load-Undrained driver type for Deformation+Flow problem type
- Total stress analysis using Driven Load driver type for single-phase Deformation problem type
These approaches are described below.
Driver setup: Deformation+Flow, Time Dependent, Consolidation
Input parameters: always effective strength and stiffness parameters
- undrained or partially drained conditions arise naturally depending on action time and soil permeability
- partial saturation effects are included
- consolidation analysis can be followed by any analysis type
- CPU time for very large boundary value problems
DRIVEN LOAD – UNDRAINED
Driver setup: Deformation+Flow, Time Dependent, Driven Load (Undrained)
Input parameters: effective strength and stiffness parameters
- fully undrained regime (no volume change) is obtained regardless defined soil soil permeability
- useful for dynamics
- Undrained driver cannot be followed by any other driver
Using Undrained driver type, special attention must be paid on appropriate selection of undrained driver settings. Undrained behavior can be disabled for material layers for which undrained behavior is not relevant.
TOTAL STRESS ANALYSIS
Driver setup: Deformation, Time Dependent, Driven Load
Input parameters: total stress strength and stiffness parameters
- relatively fast in terms of CPU
- undrained shear strength c = Su is stress independent (i.e. constant over depth; typically, not the case in natural conditions)
- although « undrained » conditions imply no volume change, model’s preconsolidation pressure pc may evolve so not relevant for normally- and lightly overconsolidated soils
- stiffness depends on φc term because c cot φ = 0
The use of the Hardening-Soil model to simulate undrained behavior with the total stress approach demands the parameter setup which presented below.
For Consolidation analysis, pore pressure values can be standardily visualized using nodal quantities. In the case of Undrained driver, the results are stored in the Gauss points and visualization of pore pressure can be done using “Undrained pressure” under continuum element results.
Suction pressure (S*p)
Magnitudes of suction pressure are stored in the Gauss points and can be read using S*p + <dp_undr> under continuum element results.