Define Joint Properties

To define joint properties, select Define Joints from the toolbar or the Properties menu. You can define up to 20 different joint types in the Define Joint Properties dialog.

The Joint Properties which are defined in the Define Joint Properties dialog, can be assigned to the following entities in Phase2:

Explicitly modeled joint boundaries created with the Add Joint option in the Boundaries menu
Composite liners which include slip (i.e. an implicitly defined joint between two layers of the composite)
Structural Interface support (which always includes an implicitly defined joint on both sides of the support element)
A network of joint boundaries created with the Add Joint Network option

The following joint properties can be defined.

Normal & Shear Joint Stiffness

Each joint element has a normal and a shear spring stiffness which relate the normal and shear stress on the element to the normal and shear displacement.

Joint stiffness is usually not a well known parameter. Methods of estimating joint stiffness have been derived, see the Estimating Joint Stiffness topic for two possible methods (one method is based on the properties of the joint infilling material, the other is based on the deformation properties of the rock mass and the intact rock).

Slip Criterion

If the Slip Criterion = None, then joints will behave elastically, according to the normal and shear joint stiffness. Plastic slip cannot occur.

If Slip Criterion is set to one of the following joint strength models, then local joint slip can occur if the shear stress on a joint element exceeds the shear strength as defined by the Slip Criterion parameters.

Mohr-Coulomb

The Mohr-Coulomb Slip Criterion parameters are:

Cohesion
Friction Angle
Tensile Strength

Barton-Bandis

The Barton-Bandis Slip Criterion parameters are:

JCS
JRC
Residual Friction Angle

Geosynthetic Hyperbolic

The Geosynthetic Hyperbolic slip criterion can be used for modeling the shear strength of the interface between a geosynthetic (e.g. geotextile or geogrid) and soil.

The Geosynthetic Hyperbolic slip criterion is defined by the following equation:

Eqn. 1

This equation was derived from a study of the constitutive behaviour of geosynthetic interfaces [ Esterhuizen, Filz & Duncan (2001) ], and has been found to characterize the shear strength of soil / geo-synthetic interfaces, and other types of interfaces. Note the definition of the adhesion and friction angle parameters:

Adhesion is defined as the shear strength at (i.e. the Adhesion for a Hyperbolic shear strength envelope, is actually the limiting, maximum shear strength, for high normal stress).
Friction angle is defined as the friction angle at .

The following figure illustrates a hyperbolic shear strength envelope.

Hyperbolic shear strength envelope

In addition to the peak values of Adhesion and Friction Angle, you may also define residual values of Adhesion and Friction Angle. The residual values will be used in the analysis if the peak strength has been reached. If you do not wish to consider residual strength, then enter zero for these parameters.

NOTE: in order to use the Geosynthetic Hyperbolic slip criterion for modeling geosynthetics, you must use the Structural Interface option to define the geosynthetics. This will automatically create a joint on either side of the geosynthetic. See the Structural Interface Overview for more information.

Initial Joint Deformation

Initial Joint Deformation refers to how the joint behaves in regard to the far field stresses.

If the Initial Joint Deformation is ON, then the joint will deform based on both the far field stresses and the induced stresses due to any excavations. As a result, the stress field in the vicinity of the joint will be altered from the initial far field distribution. This will be evident if you run an analysis with a joint only (i.e. no excavations).
If Initial Joint Deformation is OFF, then a single joint, with no excavation, has no effect at all (try running this). With no Initial Joint Deformation, only the induced stresses from the presence of an excavation or an external load, can cause joint deformation.

Additional Pressure Inside Joint

The Additional Pressure Inside Joint option allows you to apply a pressure within the joint, which will act normal to the joint boundary. The following options are available.

Groundwater Pore Pressure

If you select this checkbox, then any water pressure within the joint due to groundwater will be accounted for in the joint analysis. This option is applicable if you are using any of the three Groundwater Methods available in Phase2 - Piezometric Lines, Water Pressure Grid or Finite Element Analysis.

Additional Pressure

If you select this checkbox, then you can define a (constant) value of pressure which will be applied within the joint. This additional pressure can be used to simulate the pressure due to hydraulic fracturing for example.

Apply Pressure to Liner Side Only

By default, when you apply the Additional Pressure Inside Joint option, the resulting pressure (load) is applied to BOTH sides of the joint. However, there are circumstances when the additional pressure should only be applied to ONE side of the joint.

This option is applicable under the following circumstances:

The joint must be a component of either a Composite Liner or a Structural Interface.
The material adjacent to the joint/liner must be soil or other porous material with groundwater pore pressure in effect (i.e. effective stress).
You are applying Additional Pressure Inside the Joint using one of the above options (Groundwater Pore Pressure or Additional Pressure).

Under these circumstances, if you select this checkbox, then the load represented by the groundwater pore pressure in the joint, will ONLY be applied to the side of the joint which is adjacent to the liner. The side of the joint adjacent to the solid elements will NOT have the additional pressure load applied, since the groundwater pore pressure is already accounted for in the solid elements. If your model fits these criteria, then you should use this option.

The option is NOT applicable and should NOT be used under the following circumstances:

The material adjacent to the joint/liner is hard rock (no pore pressure considered within the solid elements). In this case, the pressure inside the joint should be applied to BOTH sides of the joint (i.e. liner AND solid elements).
Joints which are NOT a component of a Composite Liner or Structural Interface (e.g. a joint boundary).

Stage Joint Properties

The properties of a joint can be modified at different stages of a multi-stage model, by using the Stage Joint Properties option in the Define Joint Properties dialog. Any of the parameters entered in the Define Joint Properties dialog, can be increased or decreased by user-defined factors at different stages. For details about staging joint properties, see the Stage Material Properties topic, as the procedure for staging properties is the same.

Allow slip starting from stage #

The Allow slip starting from stage # option allows the user to specify that joint slip (i.e. inelastic yield) cannot occur before a specified stage. To use this option, select the checkbox, and enter the stage at which the joint is allowed to slip. Note that elastic displacements may still occur before this stage.

Apply SSR

The Apply SSR option for joints, will ONLY be available if you are carrying out a Shear Strength Reduction (SSR) analysis (i.e. the Determine Strength Reduction Factor checkbox is selected in the Project Settings dialog).

Furthermore, the Apply SSR checkbox will only be enabled if the joint Slip Criterion = Mohr-Coulomb. (It cannot be applied for Barton-Bandis or Geosynthetic Hyperbolic slip criteria).

The Apply SSR option for joints has the following purpose:

By default, if you are carrying out an SSR analysis with Phase2, the strength reduction is ONLY applied to the material strength parameters (i.e. rock or soil types which have plastic material strength properties).
The strength reduction is NOT applied to joint strength properties (i.e. joints will retain their original strength properties during the SSR analysis).
If you want the SSR analysis to be applied to joints, then select the Apply SSR checkbox for the desired joint type(s). The SSR analysis will then be applied to material AND joint properties. By default, the Apply SSR option for joints is always OFF.