1. Interface

The interface behavioral model was developed to simulate the boundary (interface) behavior between same or different materials. The interface behavioral model is not only used in geo-technology but also throughout architecture and civil engineering in general to define the behavior of various interfaces. The interface behavioral model is based on Coulomb's law of friction (1785) and follows the assumption that the frictional force of an interface is proportional to the coefficient of friction and the confining forces perpendicular to the normal direction acting on the interface.

This model is mostly used to simulate rock joints or structure-ground interfaces such as friction pile-ground interface, earth retaining wall-ground interface, lining-ground interface etc.   

<Coulomb Friction function>

The main nonlinear parameters of the interface model are as follows. The user can also define the coefficient of permeability or stiffness to simulate interface behavior.

[Normal stiffness modulus (Kn)]

Is the Elasticity modulus for bonding and un-bonding behavior in the normal direction to the interface element. The general range is 10~100 times the smallest value of the Elasticity modulus on the oedometer of adjacent elements.

[Shear stiffness modulus (Kt)]

Is the Elasticity modulus for slip behavior in the normal direction to the interface element. The general range is 10~100 times the smallest value of the shear strength of adjacent elements.

The nonlinearity of the interface needs to be computed by applying the Coulomb Friction criterion and using the stiffness parameters along with experimentation (relative displacement-frictional force curve), but an empirical formula can be used to predict the interface behavior between 2 materials. The empirical formula uses a virtual thickness (tv) and strength reduction factor (R). When creating an interface element, the following Wizard can be used for automatically calculate, according to the element properties of the neighboring ground element, using the 2 parameters (tv, R).

The interface material can be defined using the following equation. Using the stiffness of adjacent elements and nonlinear parameters, the virtual thickness (tv) and strength reduction factor (R) is applied. Interface material stiffness and parameters are applied differently according to the relative stiffness difference between neighboring ground and structural members. The Wizard can be used to simplify this process.

Kn = Eoed,i / tv

Kt = Gi/tv

Ci = R x Csoil

Here, Eoed,i = 2 x Gi x (1-νi)/(1-2 x νi)

(νi =Interface Poisson’s ration=0.45, the interface is used to simulate the non-compressive frictional behavior and automatically calculates using 0.45 to prevent numerical errors.)

tv = Virtual thickness(Generally has a value between 0.01~0.1, the higher the stiffness difference between ground and structure, the smaller the value)

Gi = R x Gsoil (Gsoil = E/(2(1+ νsoil)), R = Strength Reduction Factor

The general Strength reduction factor for structural members and neighboring ground properties are as follows.

• Sand/Steel = R : 0.6~0.7

• Clay/Steel = R : 0.5

• Sand/Concrete = R : 1.0~0.8

• Clay/Concrete = R : 1.0~0.7

In case of multiple soil layer the same structural component, the smaller value of R is recommended.

Checking the Element size consideration calculates the interface material properties considering the average length(line), average area(face) of the neighboring ground element when creating an interface. In other words, the average length(l), average area(A) are multiplies to the virtual thickness in the equation below to calculate the tangent, normal direction stiffness of the interface.

Kn = Eoed,i / (I or √A x tv ) , Kt = Gi / (I or √A x tv )

If the consideration is not checked, the unit length(area) is applied.

The thickness is defined separately for a line interface. The thickness is an important element when using the interface on a ground material that displays hardening behavior. Generally, the neighboring ground particle size is input, but if an accurate numerical value is not available, the default value from the program is used. For a 3D model, like the 1 in the example above, the surface interface does not need a thickness.

When defining the stiffness against seepage for an interface element, the “permeability coefficient” can be defined to be the same as the permeability coefficient of the ground. If the option is not checked, the layer is considered to be impermeable.

When the dilatancy angle and tensile strength is defined, a smaller or equal value needs to be defined for the interface element and the cohesion; friction angle can be multiplied with the strength reduction factor. For the interface dilatancy angle, the same angle can be applied as the ground when the ground is under rigid body motion without strength reduction (R=1). When considering strength reduction, entering '0(zero)' is the general definition for rigid body motion.

[Mode-II Model]

The Mode-II model expresses shear behavior and defines the tangential slip behavior or the interface. For the 2 models below, the failure envelopes are shown for when the ‘Constant Shear Retention’ function is considered suitable in terms of numerical analysis stability etc.

• Brittle model

The structure does not receive any loading if the vertical force is higher than the tensile strength.

• Constant Shear Retention

Apply the input value on the shear direction such that the structure can receive loading in that direction.

 : Reduced Shear Stiffness

[Multi-linear Hardening]

If a function is entered in the multi-linear hardening, the cohesion and friction angle used in the Coulomb friction failure criterion changes with plastic displacement. Note that the cohesion and friction angle both need to increase as the plastic displacement increases. This behavioral characteristic must be defined by experimentation and is mainly used for research purposes than practical purposes.

2.Shell Interface

The interface element was developed to simulate the interface behavior just like a general face element. Here, the interface element is also capable of resisting the rotational force between plates.

• Tensile force is not transmitted to the load or moment.

• Linear behavior is observed for small rotations or shear force.

• Nonlinear elastic behavior for large rotations. (Janssen’s law)

• Plastic behavior for large shear force. (Coulomb friction)

Nonlinear behavior at the plate interface element follows the Coulomb friction law for movement and Janssen’s law for rotation. The relative move displacement and interface force follows the Coulomb friction model with some restrictions. The tensile strength is set as 0 for the Tension Cut-off function, the dilatancy angle and interior friction angle are identical and the asymmetrical material property matrix is not defined. The stiffening function does not need to be defined separately.

For the User supplied shell interface, it is the same as the  "User Suplied Material" model.