ZSoil 2023

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 S_r, alpha  and n, a dedicated calculator is proposed in the material interface under group Flow. Based on basic geotechnical data i.e. e_o, d₆₀, d₁₀ for coarse grained soils, and e_o , 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 S_r, 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

Saturation state profiling

In addition, using the Initial State Profile tool which is included in the Virutal Lab, allows the user to inspect the profiles of initial state variables that control the new formulations. It makes it possible to test the sensitivity of each parameter describing material behavior (see Fig.4) in terms of:

• Total stress (gravity analysis)

• Saturation (van Genuchten’s model for the soil water retention curve), e.g. Fig.5

• Effective stress (Bishop’s principle),

• Overconsolidation (preconsolidation state depending on mechanical constitutive model)

• Permeability (Irmay or Mualem model for permeability of partially-saturated medium), e.g. Fig.5

Fig.4 Definition of parameters in the initial state profile tool in Virtual Lab

Fig.5 Visualization of partial-saturation effects in the initial state profile tool in Virtual Lab

K_o cut off in the Hardening Soil model for large OCR values

In the Hardening Soil model the initial K_o 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 K_o values. Starting from the version 23.50, the user may set the upper limit for the K_o coefficient for computing the initial in situ stresses (see Fig.6). Note that the default upper limit is defined by the passive earth pressure coefficient K_o.

Fig.6 Setting initial Ko for the Hardening Soil model including the user-defined upper limit for K₀

A new non-local FE formulation for modelling piles and barrettes

A new non-local FE formulation designed for modelling piles and barrettes enhances ZSoil in the domain of deep foundation design.

Contrary to the previous local formulation, the new formulation offers a significantly reduced mesh dependency as well as good convergence features for fine meshes. The new formulation is designed for two standard cross sections i.e., circular and quadrilateral one. The latter makes it possible to model a barrette.

The pile/barrette elements can be linked with the shell elements eliminating another spurious effect of strong mesh dependency of the bending moment and shear forces in piles/barrettes in the connection zone.

The plot below illustrates a comparison of force-displacement diagrams obtained with a reference true 3D geometry model of an axially loaded pile and two models based on the embedded beam approach with the local and the non-local formulation. The results are obtained for different mesh sizes for 800mm circular pile show how the new formulation eliminates the strong mesh dependency.

Fig.7 Predictions of force-displacement curves for local and non-local technique

 

A very good match between the true 3D model and the simplified non-local one is proven for a horizontally loaded circular pile.

Fig.8 Predictions of bending moment and deflection for a horizontally loaded pile using local and non-local technique

 

The image below shows a distribution of settlements for a an axially loaded barrette (cross section 1.6m x 0.8m). It can be noticed that the non-local embedding technique is able to reproduce the deformation pattern of a real geometry of barrette in a very realistic manner.

Fig.9 Distribution of settlements around a barrette accounting for its real geometry geometry

 

The new formulation proves to give reasonable predictions for horizontally loaded barrettes in terms of bending moment and deflection profiles.

Fig.10 Bending moment and deflection in a horizontally loaded barrette using local and non-local technique

 

Reduction of computational time

A significant improvement in the calculation module makes it possible to reduce the overall computation time of a simulation by 10 to 30%, depending on the analyzed problem and the number of models which are run in parallel.

Preprocessing

ZSoil 2023 brings a few new features which aim at improving model creation such as :

• coloring line objects
• improved selection dialog box for elements
• customized verification of model data and geometrical consistency

ZSslope

The new ZSoil 2023 brings a simplified template for ZSoil 2D with a special reference to fast modelling of slopes for stability analysis.

The application offers:

• stability checks based on incremental tan(ϕ)-c reduction algorithm
• rapid pre-processing of different-shaped slopes based on automated finite element mesh generation including locking-free finite elements
• importing geometry from DXF files
• creating simple slopes using pre-defined geometries
• definition surface loads, pseudo-static body load, rain flux
• enforcement of user-defined slip surfaces
• control of actions by means of time-dependent functions (loads, fluxes and ground water table changes)
• modelling of soil behavior by means of advanced elasto-plastic constitutive model for soils (HSS)
• time-dependent (consolidation) or steady-state analysis including partially-saturated effects in soil
• analysis of infinite slopes
• automatic PDF reporting for the computed results

Virtual Lab

A number of improvements enhance Virtual Lab in ZSoil 2023.

Material database manager

Users who used the previous versions of Virtual Lab can connect to previously created databases. The user can also create material databases in a user-defined location.

From now, generation of a full user-configured or a simple PDF report with a list of model parameters only is possible from the database manager level. The latter allows the user to print list of parameters for the material models which are exported to the database manager from the ZSoil’s material definition list.

Parameter identification

The improvements in the parameter identification module have been focused on eliminating singularities which in the previous versions, used to halt the parameter identification process in the case of poorly or un-processed laboratory data. An automatic detection of docking phase for triaxial test measurements and a new dedicated toolbox for trimming the non-processed data has been integrated on this occasion.

Docking data trimmer tool

 

Lab test simulator

A few modifications have been introduced in the lab test simulator with respect to the oedometer test for the Hardening Soil model.

 

Automatic/Interactive parameter selection

A minor update of the correlations database has been focused on upgrading a few correlations for the friction angle in granular soils. The revisions are listed in the Correlation Database Log.