# Design Factor of Safety

A **Design Factor of Safety** can be specified under the **General** tab of the Input Data dialog. The **Design Factor of Safety** is the minimum Factor of Safety required for the excavation (i.e., all wedges should have a **Factor of Safety** greater than or equal to the **Design Factor of Safety**).

**NOTE:** The **Design Factor of Safety** does NOT influence the calculation of Factor of Safety in **UnWedge**. It is used primarily to calculate a required support pressure, as described below. It is also used in a Probabilistic Analysis to determine Probability of Failure.

## Required Support Pressure (for individual wedges)

For each wedge, **UnWedge** automatically calculates the **Required Support Pressure** necessary to achieve the **Design Factor of Safety** for the wedge.

- If the wedge Factor of Safety is already greater than the
**Design Factor of Safety**, then the required support pressure = 0 for that wedge. - If the wedge Factor of Safety is less than the
**Design Factor of Safety**, then the required support pressure will be a positive value. - The required support pressure can be displayed in the Wedge Information panel, by selecting the
**Support Pressure**checkbox in the**Wedge Information Filter**dialog. - The
is assumed to be a uniform pressure applied normal to the excavation boundary.**Required Support Pressure**

The required support pressure can be used as a starting point for the design of support (e.g., bolt capacity, shotcrete thickness, etc.).

Required Support Pressure (Joint Combination Analyzer)

If you are analyzing more than three joint orientations with the **Joint Combination Analyzer** option, the **Required Support Pressure** can be used as a criterion for determining which joint combinations are the most critical (e.g., which wedges require the maximum support pressure to achieve the **Design Factor of Safety**).

Required Support Pressure (Tunnel Axis Plot)

For the **Tunnel Axis Plot**, in the drop-list of **Data to Plot** you will notice the **Pressure for FS=##** option. The Factor of Safety is the Design Factor of Safety. This option allows you to determine the support pressure necessary to ensure that the Factor of Safety of all wedges is greater than or equal to the Design Factor of Safety, for a given tunnel orientation. For more information see the Tunnel Axis Plot topic.

Wedge Visibility Filtering

For the **Wedge Visibility** option (available in the **Sidebar** for the **3D Wedge View** and the **Perimeter Support Designer** view), in the drop-list you will notice the "**FOS < Design**" option; this refers to the **Design Factor of Safety**. If you select this option, only wedges with a Factor of Safety less than the **Design Factor of Safety** will be displayed.

Probabilistic Analysis

In a Probabilistic Analysis, the **Design Factor of Safety** is used to calculate the Probability of Failure. The number of failed wedges required in the calculation of Probability of Failure is defined as the number of valid wedges with **Factor of Safety < Design Factor of Safety.**