### How to define Solid Boundary Conditions in Local Base (defined by vector)

Go to **FE Model\Boundary Conditions\Solid BC\On node** choxe X direction, select * Local base* and press

**Define local base**button.

In Local base dialog box choose

**Vector**and enter vector coordinates.

Use

**Get defined vector**button when vector was defined by picking face/edge or were defined by another graphical tool.

### How to define Solid Boundary Conditions in Cylindrical Local Base

### How to use Data Super Elements to set spatial variations of material properties

### Why two different unit weight numbers define material weight?

Two different unit weights define material weight for two different types of numerical analysis:

1. **Deformation** analysis for which material is considered as a single-phase medium

2. **Deformation + Flow **analysis where material is a double-phase medium which consists of skeleton and fluid that fills voids

The **Deformation + Flow **analysis is always considered as the effective stress analysis. In this case, material consists of skeleton and voids which can be filled with water.

Therefore, an apparent weight of each finite element depends on unit weight of dry material *g**D*, its current porosity *n*, and the saturation degree *S*. It means that the weight may vary for the same material depending on saturation conditions.

In the case of **Deformation **analysis (single-phase continuum), material’s weight remains constant throughout the entire simulation. If the material is considered as a solid-fluid mixture, one has to decide which type of analysis is intended to be carried out:

1 . **Total stress analysis **– used when modelling undrained behavior using total stress parameters, e.g. φ = 0, c = Su (undrained shear strength)

or

2.** Effective stress analysis** – using effective stress parameters and assuming free drainage conditions in the real conditions.

In both cases, a ground water table (GWT), if such exists, should be taken into account. Therefore, geotechnical layer should be discretized in the model with the aid of two different materials although they share the same mechanical parameters.

The following unit weights can be assumed:

1.Total stress analysis – **unit weight of soil with ****natural moisture** *γ* above GWT, and **unit weight of saturated soil **** γ_{SAT}** below GWT

2.Effective stress analysis – unit weight of dry soil, * γ_{D}*, should be given to compute the profile of effective stresses above GWT, whereas buoyant weight of soil

**should be defined for the fully saturated soil**

*γ*_{B}= γ_{SAT}– γ_{F}In some cases the initial stress can be imposed by the user be means of so-called super-elements.

Imposing initial stresses can solve convergence issues in the case of the boundary value problems that include construction works, e.g. an embankment. The initial stresses should be then imposed on the introduced finite elements. Imposing initial stresses is mandatory for the Modified Cam-clay model since the initial shear modulus is computed based on the mean effective stress.

Initial stresses super-elements can be defined in PreProcessor (Initial Conditions -> Initial Stresses), by selecting 4 nodes (2D) or 8 nodes (3D) . Note that the compressive stresses are negative numbers. In the case of Deformation+Flow analysis the imposed initial stresses always define effective stresses.

### 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.

**CONSOLIDATION**

*Driver setup: *Deformation+Flow, Time Dependent, Consolidation

*Input parameters: *always effective strength and stiffness parameters

*Advantages:
*

- 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

*Disadvantages:*

- 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

*Advantages:*

- fully undrained regime (no volume change) is obtained regardless defined soil soil permeability
- useful for dynamics

*Disadvantages:*

- 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

*Advantages:*

- relatively fast in terms of CPU

*Disadvantages:*

- undrained shear strength
*c = S*is stress independent (i.e. constant over depth; typically, not the case in natural conditions)_{u} - although « undrained » conditions imply no volume change, model’s preconsolidation pressure
*p*may evolve so not relevant for normally- and lightly overconsolidated soils_{c} - stiffness depends on
*φ*term because_{c}*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.

**POST-PROCESSING**

*Pore** pressure*

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.