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Setting Material Properties

Material Types

LimitState:GEO provides a selection of built in materials primarily based on the Mohr-Coulomb failure envelope. This encompasses the majority of soil types likely to be encountered and also allows many structural materials to be reasonably represented. Later on it will be seen that more sophisticated material models can be generated using the ’multi-material’ technique. These materials may be assigned to Solids and also interfaces i.e. Boundary Objects.

In addition LimitState:GEO provides a special Engineered Element material which may only be assigned to Boundary Objects.

LimitState:GEO comes with a number of predefined materials. These may be viewed in the Materials Explorer:

The materials provided have typical properties according to their type and are provided to allow an easy introduction to LimitState:GEO and may be used as templates for user defined materials. The properties are not intended to correspond to those in any specific soil description standard (these could be defined by the user if required). It is not anticipated that these materials will be used for any ‘real’ design calculations. Note that the set of predefined materials provided is determined by the chosen system of units to be used in the software. If the software has been set to work in Imperial units, then a separate set of predefined ‘Imperial’ materials are provided. The reason for this is simply convenience in terms of providing materials with typical properties with rounded values. Thus a ’Soft Clay [Imperial]’ will not have quite the same properties as a ’Soft Clay’.

A material may be modelled as a:

1. standard Mohr-Coulomb material with directly defined cohesion () and friction (),
2. derived Mohr-Coulomb material whose cohesion () and friction () properties are defined with reference to another standard Mohr-Coulomb material,
3. a cutoff material (tension and/or compression),
4. a Mohr-Coulomb material for which the cohesion varies linearly with depth.
5. a Rigid material
6. an Engineered Element (formerly a Soil Nail).

Click on any material in the Materials Explorer to bring its properties up in the Property Editor. Values may be interrogated and edited as described under Property editor.

Assigning a Material to a Solid or a Boundary

Materials may be assigned to a Solid object or Boundary Object by the following methods:

Drag and drop

Select a material in the Materials explorer and drag it onto the required Geometry Object

the Change dialog

Select the required Geometry Object. In the Property Editor click on the right hand box in the row ‘Materials’ to display the ‘Change’ button (see below). Click this to display the ‘Edit Object Material(s)’ dialog (see below). Check or uncheck the relevant boxes to include or remove materials from the object.

If a material is assigned to an object which already has one or more materials assigned to it, a warning message appears asking if the material is to replace the existing one, or be added to the materials in the object. For further information on the use of multi-materials see here.

Standard Mohr-Coulomb Material

Setting Soil/Material Strength Properties

For a standard Mohr-Coulomb material (e.g. medium-dense sand), the properties depicted in the figure below may be edited:

It may be seen that it is possible to enter drained and undrained properties for each material. The Analysis mode allows the user to switch between long and short term (drained and undrained) analyses without having to redefine all the material properties in the problem.

For clay soils, typically undrained shear strength would be entered for the undrained cohesion. The effective stress properties of the clay, and , are entered for the drained cohesion and drained angle of shearing resistance.

Setting Short Term/Long Term Behaviour

For each material it is necessary to define how it behaves when subjected to short or long term loading. Table indicates the scenarios that might arise. It must be emphasized that this table is indicative and the user is free to choose the behaviour as they see fit. In the context of unfissured rock, the behaviour would be undrained in that its deformation up to the point of failure is unaffected by water pressure and thus undrained. At failure the rock may fracture and water, if present, would affect the analysis. How this would be modelled depends on the specific scenario.

Note that water pressures only affect soil behaviour where soils have a non-zero angle of shearing resistance. Thus an undrained material with zero behaves in the same way irrespective of the pore pressures.

The Drainage behaviour setting is provided for clarity and to avoid unnecessary pore pressure computations for undrained materials.

Mohr-Coulomb Material with Linear Variation of Strength with Depth

Frequently it is found that the undrained shear strength of clay soils varies with depth and that in many cases this variation can be approximated as a linear function of depth.

It is necessary to enter the datum level for where the baseline undrained strength is found, and in addition the strength gradient (kN/m/m). These values are found under the Advanced setting in the Property Editor as shown in the figure above. Partial factors are applied to both the baseline strength and the strength gradient to give the correct factored strength at any depth. The baseline undrained strength is the value of used for constant strength bodies of soil.

If the baseline undrained strength and the strength gradient are defined such that the average undrained shear strength on any given slip-line is less than zero, then a zero value of average undrained shear strength is assigned to that slip-line.

For step changes in strength properties, it is necessary to define separate zones with differing materials.

Derived Mohr-Coulomb Material

Often when dealing with soil/structure interaction problems, it is desirable to define a soil/structure interface property that is a function of the adjacent soil. A typical example is a retaining wall where the interface adhesion and friction will be some multiple of the adjacent soil strengths. This may be set using this material type and entering values for cohesion and shear resistance () multipliers in the Property Editor as depicted below.

NB In many design codes the interface friction property is often set to be a function of , the critical state angle of shearing resistance. If this is not the actual value entered e.g. a peak value is entered, then it will be necessary either to make an additional modification to the multiplier or to use a ‘standard’ material instead of a ‘derived’ material type. The main advantage of the Derived material is that if the soil properties are altered, then the interface properties are automatically adjusted.

Cutoff Material

When a cohesive material is modelled using the Mohr-Coulomb failure envelope, the mathematical representation may give the material an unrealistically large tensile strength. For many problems dominated by compressive forces this is not an issue. However for e.g. slope stability or retaining wall problems, tensile stresses may arise. To model such cases, it is possible to specify a tension cutoff. The property defined is the normal stress at which tensile failure occurs and may be specified in the Property Editor as depicted below. Additionally a compressive cutoff can be specified (see Cutoff material theory).

The tension cutoff material is typically used on its own at the interface between a solid body e.g. a concrete block or rock and another solid or material with high cohesion. This allows the blocks to separate or undergo rocking displacement. The tension cutoff material is also often used in combination with other materials. For further information on this refer to the use of multi-material zones.

Rigid

This material type may be assigned to Solids, but not Boundaries. If it is the only material present in a Solid, it simply prevents the solver from assigning any nodes to that solid. Thus no deformation can take place within that solid. Where it is known that the solid will not deform, use of a Rigid material ensures efficient use of nodes in the overall problem.

Additionally nodes will not be assigned to Boundaries that lie between two Rigid Solids or between a Rigid Solid and an external boundary unless the Boundary has a material assigned to it.

Setting Soil/Material Unit Weight

Unit weights corresponding to material above and below the water table may be entered for each material. The corresponding Property Editor entries are listed in the below table.

For drained materials such as sands, different values of Unit Weight and Sat. Unit Weight will normally be specified. The Unit Weight will typically correspond to the dry unit weight.

For materials such as rock, concrete or steel, represented as an undrained material, the unit weight is normally unaffected by the location of the water table and the Unit Weight and Sat. Unit Weight values should be set to be the same.

Similarly, clay soils are often saturated above the water table due to capillary action and thus the Unit Weight and Sat. Unit Weight values would normally be set to be the same.

Unit weights are always defined as resulting in permanent loads or actions. They may be set to act in a Favourable, Unfavourable or Neutral way within a problem. It is also possible to find an Adequacy factor in terms of a unit weight. These parameters may be set in the Property Editor values for the Solid containing the material, under the entry Self Weight Loading. Click on the to display the entries Loading Type and Adequacy. Further information on modelling with the Loading Type settings may be found in Use of Partial Factors.

Engineered Element

Introduction

In LimitState:GEO Engineered Elements such as soil nails are modelled as a special material that may be assigned to a Boundary object. The theory behind their implementation in the software is given in Engineered Element theory.

For the Engineered element type, the properties depicted in the figure below may be edited. Engineered element pullout and lateral resistance properties are given in kN per metre length of element per metre width in the problem and may be specified absolutely and/or as a linear function of the effective stress acting at the element midpoint (see Engineered Element theory) . The bending resistance (used only at Vertices along the element) is given in kNm per metre width in the problem. Note that partial factors (if defined) are not applied to the properties of any Engineered element.

Engineered elements may represent items such as soil nails or reinforcing strips around which surrounding materials may flow, or items such as sheet piles walls through which soil does not flow. Individual elements may be connected to each other. With appropriate choice of parameters, an Engineered elements material can be used to represent:

1. Soil nails.
2. Soil reinforcement, such as within a reinforced earth wall (see below).
3. Connecting rods.
4. Soil anchor (the tendon and anchor may be modelled by two connected elements with differing properties)
5. A pile such as within a non-contiguous pile wall used to stabilise a slope.
6. A sheet pile wall.

However it is necessary to be take account of the constraints of the Engineered element model when representing these types. When creating a new Engineered element material LimitState:GEO provides pre-programmed and suggested values for common types of soil reinforcement. This feature is described further in Creating an Engineered Element

Defining Engineered element geometry

A simple example of an Engineered element modelled within a soil and an adjacent wall element is depicted in the below figure. To model an engineered element within the wall solid, it is first necessary to draw a boundary element such as AB, and then drag and drop an Engineered element material onto this boundary. To continue the engineered element into the soil, draw the boundary element BD and then draw the vertex C onto this boundary. Then drag and drop an Engineered element material onto the boundary BC. The engineered element now runs from A through B to C. Appropriate pullout and lateral resistances then need to be assigned to the engineered elements. Since these properties are likely to be a function of the material in which the Engineered element is embedded, it will usually be necessary to specify more than one Engineered element material each with the appropriate properties, and assign these to appropriate segments of the element. The pullout and lateral factors for the element embedded in the wall will typically be much higher than for the element in the soil. For elements to connect to each other, it is not necessary for the same material to be used in each element, as long as they are both Engineered element materials. Note that it is necessary for the boundary element CD to be present to satisfy the geometrical constraints of LimitState:GEO. For purely visual purposes it can be simply ’hidden’ by assigning the same soil type to it as the surrounding soil. This has no effect on the analysis.

Also note that when Engineered Elements do connect, they are modelled with a joint that may rotate with plastic moment . There is no restriction as to the initial angle of the joint. This means that several connected elements are modelled as an object that may pull through the soil as an undeformed rigid object if is set to a high value, or that may bend if is set to a lower value. Tensile failure is not modelled in an Engineered Element.

Post solve information

Additional information relating to a specific Engineered Element may be obtained via the Property Editor after solving (but before Unlocking). To obtain the information, it is necessary to drag the Animation Slider bar to any position beyond the starting position. Then using the mouse, select the required Engineered Element in the Viewer Pane. The properties depicted below will be displayed. This permits checking of the stresses assumed by the software in the computation of the lateral and pullout resistances for the Engineered Element. Note that these are not displayed if the corresponding coefficients are zero.

Creating and Deleting User Defined Materials

Materials Explorer Context Menu

The pre-defined materials are not editable (though those provided via wizards are editable).

To access material specific functions, with the mouse over a material in the Materials Explorer, right click to bring up the context menu depicted below.

Creating a New Material

Select ‘New material...’ in the context menu (or from the main menu, select Tools - Create New Material...) to display the Create New Material dialog.

Enter the required parameters and click OK. Note that only the core material parameters are requested in this dialog. Other parameters may be set subsequently using the Property Editor.

When ‘Engineered Element (1D)’ in entered in the ‘Type’ drop down box, it is possible to select a specific ‘Application’ as shown in the next Create New Material dialog

The available ‘Applications’ are further described in Modelling Soil Reinforcement

Creating a Duplicate Material

Select ‘Duplicate material’ in the context menu. A new material will be created with the name ‘Copy of X’ where ‘X’ was the parent material.

Deleting a Material

To delete a material, select ‘Delete material’ in the context menu. This will delete the selected material.

Exporting and Importing Materials

To export or import the materials listed in the Materials Explorer to a .csv (comma separated values) file, select ‘Export materials...’ or ‘Import materials...’ in the context menu. The file format allows the data to be easily edited within a text editor or spreadsheet if required.