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# Boundaries Overview

The first step in creating a model (after defining the Project Settings) is to draw the boundaries of the model.

The following is a brief summary of the boundary types defined in Slide2.

## External Boundary

Closed polyline, defining the extents of the soil region you wish to analyze.

• The upper segments of the External Boundary represent the slope surface you are analyzing.
• The left, right and lower extents of the External Boundary are arbitrary, and can be extended as far out as the user deems necessary for a complete analysis of the problem.
• Only one External Boundary can be defined for a model, and it must be defined before any other boundaries can be added.
• An External Boundary must be defined for every Slide2 model. All other boundary types are optional and are defined as required.

## Material Boundary

Arbitrary polylines are used to define zones of different material properties, within the External Boundary.

• In order to properly define a material zone, a Material Boundary must form one or more closed regions, by virtue of intersection with the External Boundary, and/or other Material Boundaries.
• Any number of Material Boundaries can be defined for a model.

## Water Table

An arbitrary polyline is used to define a Water Table (phreatic surface) for pore pressure calculations.

• Also used to define ponded water, if the Water Table boundary is drawn above the External Boundary.
• Only one Water Table can be defined for a model.

## Piezometric Line

An arbitrary polyline is used to define a Piezometric Line for pore pressure calculations.

• A maximum of 20 Piezometric Lines can be defined for a model, each line representing the pore pressure conditions in one or more soil regions.

## Tension Crack

An arbitrary polyline is used to define the lower limits of a Tension Crack zone.

• The Tension Crack zone can be defined as dry, water-filled or partially water-filled, using the Define Tension Crack option in the Properties menu.
• Only one Tension Crack boundary can be defined for a model.

## Drawdown Line

An arbitrary polyline is used to define the lower limit of the Water Table for a Rapid Drawdown analysis.

• Can only be defined for a Rapid Drawdown analysis.
• Only one Drawdown Line can be defined for a model.

## Soil Profile Boundaries

Soil Profile boundaries are only applicable if the Soil Profile option has been enabled in Project Settings. Soil Profile boundaries can represent material boundaries, the ground surface, or other model boundaries.

## Weak Layer

A weak layer is an arbitrary polyline which is used to model a potential sliding interface such as a joint or any other type of discontinuity. A weak layer is assigned strength properties. Failure surfaces which coincide with a weak layer will use the strength parameters assigned to the weak layer.

See the Weak Layer Overview topic for details.

## Anisotropic Surface

An anisotropic surface is used to define the direction of anisotropy for anisotropic material layers such as folded bedding. See the Anisotropic Surface Overview topic for details.