Full form of the van Genuchten soil water retention curve (SWRC)
The current upgrade offers a full form of the van Genuchten’s law including 3 parameters. It is expressed in the following form:
The m parameter depends on n number (m = 1 – 1/n), and n no longer needs to be fixed to value 2 as it was in the previous versions of ZSoil (versions < 23.50).
In order to help the user to define three parameters Sr, alpha and n, a dedicated calculator is proposed in the material interface under group Flow. Based on basic geotechnical data i.e. eo, d60, d10 for coarse grained soils, and eo , LL (liquid limit) for fine grained ones, a best fit of the full van Genuchten model to the Modified Kovacs one can be obtained. The new user interface for Flow properties is illustrated in Fig.1, whereas the van Genuchten’s parameters estimator is shown in Fig.2. It makes it possible to find van Genuchten’s parameters and visualize an agreement with the Modified Kovacs model.
In addition, the effective suction stress which appears in the Bishop’s effective stress principle, is displayed in order to anticipate the resulting maximal apparent cohesion number. The effective suction stress may appear in a partially-saturated medium. Parameter alpha is the only one which affect the maximal value of the apparent cohesion. In order to keep back compatibility with previous ZSoil versions, the user may set n=2 and modify Sr, alpha values.
Modified Bishop’s effective stress principle
The current upgrade introduces a modified formulation for the Bishop’s effective stress principle. The new formulation makes it possible to keep control over the resulting apparent cohesion, and it takes following form:
where the modified effective saturation is expressed as follows:
In addition, the Biot’s coefficient (only elastic one) has been introduced to make numerical analyses in rock geomaterials more realistic.
In order to keep back compatibility with previous ZSoil versions (<23.50), the user can always use the previous form of the Bishop’s principle. The decision can be taken under the new menu item Control/two-phase formulations (see Fig.3). All older projects by default will use standard saturation ratio in the Bishop’s principle while in every new project the modified effective one will be set as default. It is important to say that the modified effective saturation preserves monotonic and asymptotic behavior of the resulting apparent cohesion with suction increase.
Fig.1 The new user interface for setting seepage-related properties
Fig.2 Calculator for SWRC parameters based on the best fit of van Gencuhten’s curve to the Modified Kovacs model
Fig.3 Defining Bishop’s principle form under Control/two-phase formulations menu
Ko cut off in the Hardening Soil model for large OCR values
In the Hardening Soil model the initial Ko coefficient can automatically be calculated based on a given OCR or POP (preoverburden pressure) profile using a commonly recognized formula
Sometimes, for the shallow depths, the OCR values computed from POP definition may lead to very high Ko values. Starting from the version 23.50, the user may set the upper limit for the Ko coefficient for computing the initial in situ stresses (see Fig.4). Note that the default upper limit is defined by the passive earth pressure coefficient Ko.
Fig.4 Setting initial Ko for the Hardening Soil model including the user-defined upper limit for Ko