Default parameter (General)

The input parameters and units for defining the default stiffness and initial condition of each model are listed in the table below

[Elastic modulus (E)]

This parameter defines the default initial stiffness of the material. The user can specify the Elasticity modulus, or use the Shear modulus (G) or Oedometer Elasticity modulus (Eoed) from the Oedometer test. The initial stiffness is very important because geo-materials display nonlinear behavior from the early stages of loading. The initial stiffness can be defined from the stress-strain curves of the triaxial compression test. It is realistic to use the E0 for materials that display linear (elastic) behavior until a large strain but for general geo-materials, E50, the slope of the tangent at 50% of the stress, is appropriate as an initial stiffness. When simulating unloading and reloading due to excavation during construction step analysis, it is better to use Eur instead of E50 to realistically simulate the ground behavior.

Hence, it is important to set the stress path and stress range (size) when using the initial stiffness to simulate the ground behavior. To simulate detailed behavior, various nonlinear material models can be used.

<Triaxial compression test result graph>

The use of K(bulk modulus) and G(shear modulus) may be debatable use to the continuity issues associated with the ground, but it can be expressed more simply and clearly than E or v and is convenient to use. The following figure briefly explains the mechanical significance of K and G.

<Various types of Elasticity modulus>

The Elasticity modulus values gained from on-site tests can be 1 of the many elasticity moduli discussed above and can be modified appropriately for real situations.

<The Elasticity modulus and Poisson’s ratio for rock and other materials>

The Elasticity modulus in the table above is for small, intact rock samples tested in the lab. Hence, when considering the site conditions, a reduced elasticity modulus needs to be used considering the discontinuous surfaces within large scale rocks. The figure below is a graph of actual data showing the relationship between the RQD (Rock Quality Designation) and the Elasticity modulus reduction ratio. An RQD is the percentage of the sum of the lengths of cracks that are over 10cm and exist on the 100cm situ core against the total length. An RQD of 100% does not mean the core is an intact rock. However, a higher RQD means a higher quality rock and the RQD decreases with more weathering.

<RQD- Modulus reduction ratio (EL/EM) relationship>

As shown on the figure, an RQD of 70% already needs to decrease the lab Elastic modulus by 20%.

[Increment of Elastic modulus]

In general, the strength properties of the soil change with depth and confining pressure, even within a ground layer composed of the same material. To take this characteristic into account, increase or decrease in the Elastic modulus can be simulated with reference to a reference height (standard height). If the elastic increase according to height is '0(zero)', the Elastic modulus has a constant value and if it is not '0(zero)', the Elastic modulus is calculated with reference to a standard height using the following equation.

Here,   :  Input elastic modulus value

 :   Incremental slope of elastic modulus

 :   Depth of   measurement

<Schematic diagram of Elastic modulus increment>

The in the equation represent the integral point positions of an element where the finite element method calculation occurs. If the integral point position is higher than , the elastic modulus value can be less than 0 in some places. To avoid this, use the    value instead of further decreasing the  value.

[Poisson’s ratio( )]

Poisson’s ratio is a proportional constant from the stress-strain relationship and displays the material volume change associated with loading. As approaches 0.5, the material becomes an incompressible solid and closer to 0 means the material is elastic, showing large volume changes even at small loads. The initial stress ratio due to self weight K0 = σh/ σv can be related to the ratio in the uniaxial compression state by K0 =. If K0 is not used to define the initial in-situ stress, the horizontal stress is calculated from the vertical stress using the entered . For geo-materials, the general Poisson range is within 0.3~0.4 and entering a value larger than 0.49 can cause numerical errors. Hence, if K0 is larger than 1, for example over-consolidated ground, the Poisson’s ratio cannot be calculated and the value must be entered directly.

Shear modulus(G)

The Shear modulus is automatically calculated from the Elastic modulus and Poisson's ratio using the following equation derived from Hooke's law. If the value is directly entered, the Elastic modulus changes.

[Oedometer Elastic modulus (Eoed)]

The Oedometer modulus can be calculated from the Elastic modulus and Poisson's ratio using the following equation.

[Initial stress (K0)]

K0 is the Coefficient of earth pressure, which is defined as the ratio of the initial vertical/horizontal stress (K0 = σh/ σv). The anisotropic property can be set with reference to the Global Coordinate System.

Firstly, select yes/no on whether the Global Coordinate System direction and anisotropic property match and set the lateral pressure index in each axis or any direction depending on the selected options.

When the 2 properties do match, the lateral pressure index is set in each axis direction but a value of ‘1’ in the direction of gravity cannot be defined depending on the work environment (2D/3D).

When the 2 properties do not match, the lateral pressure index direction is set by entering the angle with respect to the reference axis. The reference axis exists to set the lateral pressure index direction. For a 2D work environment, the ’X-Y’ plane is fixed and only the ‘X’ axis can be selected, with all initial shear stress at ‘0(zero)’. For 3D, each axis apart from the gravitational direction can be selected. For example, if the gravitational direction is the ‘Z’ axis and the reference axis is set as the ‘X’ axis, the angle can be entered on the ‘X-Z’ plane will be the maximum lateral pressure angle and all initial shear stress in the XY and YZ direction will be '0(zero)'.

The in-situ stress state, where the soil it not disturbed by excavation or fill, can be expressed using the Coefficient of earth pressure and self weight. In other words, realistic results can be obtained from applying K0 after modeling the in-situ ground for the 1st step of construction during analysis. This is true for flat foundations but for inclined foundations, it is recommended that another construction process be added to converge the stress found using K0 for equilibrium.

[Thermal Parameter]

Thermal Coefficient -  describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure.

Molecular vapor diffusion coefficient - the gas diffusion coefficient of a porous medium, which indicates the change in gas density over time.
This parameter will be used in a future release of FEA NX after additional analysis enhancement.

Thermal diffusion enhancement (factor) -  controls the degree of gas flow according to the temperature gradient (unit less).
This parameter will be used in a futurehttps://attendee.gotowebinar.com/register/5057000740216531467  release of FEA NX after additional analysis enhancement.

[Safety Result (Mohr-Coulomb)]

§Cohesion, Friction Angle and Allowable tensile strength (optional) can be defined as the failure criteria.

§Stress status of material for each construction stage can be represented by Factor of Safety based on Mohr-Coulomb failure criteria.

§The ratio of generated stress to stress at failure for each element will be calculated automatically. 

§Users can figure out stable, potential failure and plastic failure area directly.

§Check factor of safety for each element - (2D : Plain  Strain Stresses > SAFETY FACTOR , 3D : Solid Stresses > SAFETY FACTOR)

§In case that Safety Factor is less than 1(or 1.2), it can be identical with plastic failure region.

Porous: Seepage, Drained/Undrained Parameter

The input parameters for the permeability characteristics and drained/undrained conditions of the foundation are as follows.

<Permeability parameter>

[Initial void ratio (e0)]

The initial void ratio of the foundation used in consolidation analysis and stress-seepage coupled analysis. It is the volume ratio between the voids and soil particles within the soil and the value is less than 1 for most soils. The value can be larger than 1 for clays or organic soils, but the value depends greatly on the sampling method or compaction. Generally, coarse grain sand has a value of 0.6~0.8 and high density sand with an even size distribution has a value of 0.3. The void ratio can be even 2~3 for fine grained soils.

[Unsaturated Property]

Set to consider the unsaturated property of the foundation. It is a required property for unsteady infiltration analysis and is used to consider the partial degree of saturation of the foundation for nonlinear (construction step)/consolidation analysis etc. Because real foundations are unsaturated and have a certain ratio of air, unsteady infiltration analysis needs to consider unsaturated characters of the soil for more realistic results. If the unsaturated properties are not considered, it is assumed that the ground is saturated and hence, the infiltration analysis with time cannot be examined.

Unsaturated property defines the change in permeability coefficient and water content (Degree of saturation) in the unsaturated region depending on the size of the negative pore water pressure. There are 2 methods to define the unsaturated property; directly defining (define individually) the permeability function and water content function using the pressure head (negative pore water pressure) or defining the relationship between pressure head-volumetric water content (degree of saturation)-permeability ratio (define relationship). Refer to "Function>Unsaturated characteristic function" for more information.

[Drainage parameters]

The pore water pressure in stress analysis can be divided into normal state pore water pressure and abnormal state pore water pressure - the excess pore water pressure generated between soil particles due to external loading under undrained conditions. An excess pore water pressure of nearly 0 is called the drainage condition and is generally used for drainage analysis of sand, which has a large permeability. However when simulating clay, which has a very small permeability and water cannot be drained out during sudden loading, undrained analysis is appropriate.  The initial state, where the excess pore water pressure has not yet dissipated, is seen as the most unstable state and the pore water pressure is determined by the volume change of the foundation due to compressibility and permeability coefficient.

Undrained Poisson’s ratio and Skempton(B) coefficient are parameters used to calculate the bulk modulus of elasticity for water. The undrained Poisson’s ratio has a standard value of 0.495 with a compressibility of nearly ‘0(zero)’ and the Skempton coefficient expresses the saturation, with 1 meaning full saturation.

The materials for the unsaturated analysis are as follows.

Please refer to Ch.4 of the Analysis manual for more detailed information.

[Permeability coefficients (kx,ky,kz)]

The permeability coefficient represents the permeability characteristics (velocity) of the foundation and is used in infiltration analysis and consolidation analysis. The permeability coefficient for each direction can be defined on the GCS. The input value is the saturated permeability coefficient and becomes the standard for computing the permeability ratio (Kunsat / Ksat) due to negative pore water pressure when defining an unsaturated property function.

[Void ratio dependency permeability (ck)]

The permeability coefficient is a measurement of how much the groundwater within a foundation moves in a unit time and is dependent on the water content and the void ratio change . The larger water content, the larger the flow channel and hence, the value is largest when the foundation is saturated. The water content depends on the pore water pressure and hence, the permeability coefficient is also dependent on the pore water pressure. The change in void ratio is considered in consolidation analysis as well as stress-seepage coupled analysis and is calculated from the initial void ratio.

To express the change in pore water pressure, FEA NX uses the permeability ratio function depended on saturated pore water pressure coefficient and pore water pressure change and the void ratio dependent permeability ratio dependent on void ratio change . The unsaturated permeability coefficient depending on void ratio change is given by the following equation.

[Specific storativity(Ss)]

The Specific storage is the water volume inflow or outflows from the unit volume of the aquifer due to water level rise or fall in a confined aquifer. A coefficient can be directly entered or automatically calculated for compressible fluids.

The change in volumetric water content for pore water pressure in infiltration and consolidation analysis can be expressed by the porosity and degree of saturation.

The first clause is the slope of the volumetric water content under saturated conditions that can be expressed using the specific storage.

When the material drainage property is set to undrained, the specific storage is automatically calculated using the undrained Poisson's ratio (vu) and the Effective elastic modulus (E') and Poisson's ratio (v'), entered in the general parameters.

Thermal

Conductivity: the ability to conduct thermal energy.

Specific Heat: the amount of heat required to raise single unit mass of a substance by single temperature unit. (required for transient heat transfer problems)

Heat Generation Factor: the value of the heat load multiplied by the exothermic coefficient used as the load vector for heat transfer analysis is the total exothermic load applied to the object.

Unfrozen water content: indicates floating water content in soil / rock. It is given as a temperature-dependent function as a unique characteristics of the ground.

Time Dependent

This is to define Creep Formulation to simulate time-dependent behavior of concrete structures. Following constitutive models are available for concrete structures, Elastic, Tresca, von Mises, Mohr-Coulomb, Drucker Prager, and Hoek Brown.

Two types of creep formulation are available to define Time-dependent behavior of material, Age Dependent and Age Independent. Refer to analysis reference Ch.4-Section5 in detail.

[Age Dependent]

The stiffness of concrete changes with time, and the creep and shrinkage may cause unexpected deformation. The creep strain of concrete depends on the time of stress occurrence even under the same applied load. FEANX supports aging-Kelvin model and aging Viscous model excluding the spring from Kelvin model.

[Age Independent]

FEANX can take into account the primary and secondary creep. The user can use two types of empirical law to define the creep behavior.