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Undrained Behaviour of Soil

RS2 allows you to model both drained and undrained behaviour of soils. By default, the material behaviour is set to Drained, which can be defined by the Define Hydraulic Properties option from the Properties menu. In order to model undrained behaviour, three approaches are available in RS2, involving the strength and stiffness parameters of the material, as well as the material/stage behaviour.

Background

In geotechnical engineering, the behaviour of soil can be either drained or undrained. It refers to the soil’s ability to dissipate excess pore pressure induced due to loading. Two factors determine whether the soil is under drained or undrained condition: 1) soil permeability and 2) rate of loading. The soil permeability differs by soil types. For instance, sand has high permeability and clay has low permeability.

For drained conditions, when the soil is subjected to loading, excess pore pressure can dissipate out freely. The rate of dissipate is governed by the soil permeability. More permeable soils allow faster dissipation. For drained conditions, loads should be applied slowly, allowing soils to respond and dissipate water out, and eventually achieve equilibrium with respect to pore pressure. In addition, some time might be required after the loading process for fully dissipation. The slow loading is essentially crucial to low permeable soils, as soils can experience undrained condition under quick loads.

For undrained conditions, excess pore pressure is not able to dissipate out from the pores of soil mass. For cases where the rate of dissipation is very low, they will as well be considered as undrained conditions. Undrained condition can be due to zero or low permeable soils, or (and) quick loads. The undrained strengths are obtained when the soils are loaded to failure.

It is up to the practicing engineer to assess the soil properties and loading conditions of a project to determine whether they should use drained or undrained strength parameters.

Undrained Behaviour Modelling

To model undrained behaviour, the following three approaches are provided in RS2 (the material/stage behaviour is specified in one section below):

  1. Using undrained strength parameters and undrained stiffness parameters. Material behaviour set to drained.
  2. Using undrained strength parameters and effective stiffness parameters. Undrained behaviours will be accounted for by program either by material or stage ”Drained” behaviors setting.
  3. Using effective strength parameters and effective stiffness parameters. Undrained behaviours will be accounted for by program either by material or stage ”Drained” behaviors setting.

The first approach will not generate excess pore pressure. The second approach can generate excess pore pressure, but it may not be accurate. The third approach can generate accurate excess pore pressure, depending on the constitutive model utilized.

All three approaches can be used to model short term behaviours in clays. However, for consolidation analysis, the third approach will be the most appropriate method to apply since it allows consolidation analysis and generates accurate excess pore pressure. Effects of consolidation and changes in pore pressure will be considered.

The undrained behaviour setting can be defined by material, or by stage for consolidation analysis. The compatibility between the two depends on consolidation options. See more information on the Drained / Undrained Settings topic.

For an example of short-term behaviours in clay modelling, see the RS2 Excavation Example #1. For an example of undrained behaviour in consolidation analysis, see the #32 Undrained behaviour modelling in consolidation analysis of strip footing in the RS2 Stress Analysis Manual.

Undrained Calculations

When an undrained model is defined, a set of calculations will be run to account for undrained behaviours. An undrained effective stress analysis combined with effective strength parameters will be activated to model undrained behaviours. See the Effective Stresses Calculation in Undrained Materials theory manual for details. It will be applicable when the model is set to:

  • Undrained behaviour by material
  • Undrained behaviour by stage, consolidation option = uncoupled

However, for coupled consolidation, instead of the abovementioned method, the undrained behaviour will be accounted for in the global matrix. See the Coupled Consolidation document for details. It will be applicable when the model is set to:

  • Undrained behaviour by stage, and consolidation option = coupled
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