In geotechnical engineering, sometimes it is tricky how to model a problem.
Should it be an effective soil stiffness?
Should it be total stress?
All of this relates to a bigger issue which is; the drained or undrained modelling condition (soil permeability condition).
To understand, first, one has to knew what type of soil involve in the problem, and either is it the problem analyse for a short term or long term (stability)condition?
Drained condition
Drained condition almost always exists for coarse-grained material such as gravels and sands under static/monotonic loading. In this condition, it is assumed that material have large permeability so that the pore water can quickly drain out.
Eg. of the conditions can be:
- Soil with seismic loading condition due to the quick rate of loading saturated loose sands can experience undrained loading condition resulting in liquefaction.
- In case of liquefaction where unloading condition exists, the common approach is to either use residual strength parameters of liquefied soil per Idriss and Boulanger (2008) guidelines or adopt advanced constitutive models for which the input parameters are drained parameters.
Drained parameters (E’, \nu’, \phi’E′,ν′,ϕ′) must be always used for the analysis of coarse-grained materials (i.e., gravels, sands).
Undrained condition
On the other hand, due to the low material permeability, undrained condition almost always exist for clays and silts when subjected to quick static loads and earthquake loads.
For more;
http://geotechsimulation.com/2018/05/18/soil-drained-undrained-behavior/
Speed Notes:
Slope stability analysis
Short-term condition: undrained parameters (E_u, \nu_u, S_uEu,νu,Su)*
Long-term condition: drained parameters (E’, \nu’, c’, \phi’E′,ν′,c′,ϕ′)
Pseudo-static condition: undrained parameters (E_u, \nu_u, S_uEu,νu,Su)*
* If generation of pore water pressure is simulated in the analysis, drained stiffness parameter (E’, \nu’E′,ν′) must be used instead of E_u, \nu_uEu,νu. This is because the analysis accounts for the pore water stiffness (K_wKw) in the calculation of soil volume stiffness.
Static settlement analysis
Immediate settlement: undrained parameters (E_u, \nu_u, S_uEu,νu,Su)*
Consolidation settlement: drained parameters (E’, \nu’, c’, \phi’E′,ν′,c′,ϕ′)
* If generation of pore water pressure is simulated in the analysis, drained stiffness parameter (E’, \nu’E′,ν′) must be used instead of E_u, \nu_uEu,νu. This is because the analysis accounts for the pore water stiffness (K_wKw) in calculation of soil volume stiffness.
Seismic time history analysis
Case #1 – the analysis is cable of simulating build-up and/or dissipation of pore water pressure over time: drained parameters (E’, \nu’, c’, \phi’E′,ν′,c′,ϕ′)
Case #2 – the analysis is formulated based on fully undrained conditions (in programs like PLAXIS): drained stiffness and undrained strength (E’, \nu’, S_uE′,ν′,Su)
Note: The undrained analysis accounts for the pore water stiffness (K_wKw) in calculation of soil volume stiffness.
