# Water Pressure

**Water Pressure** can be accounted for on the joints, by defining the Water Pressure as a Joint Property in the **Input Data** dialog.

Generally a **Water Pressure** of zero is assumed, since underground excavations are usually free-draining. However, the influence of water pressure on the stability of a wedge may be important in some cases, for example, when the wedge has been supported with shotcrete and no allowance has been made for drainage.

There are four options available for defining water pressure in **UnWedge**:

- Constant
- Elevation
- Ground Surface
- Water Pressure Grid

## Constant

The **Constant** Water Pressure option assumes a constant (i.e., uniform) water pressure, over the entire joint plane. The units of pressure are determined by the Units selected in Project Settings.

## Elevation

The **Elevation** Water Pressure option allows you to specify the y-coordinate of a water table. The value of **Elevation **must be consistent with the y-coordinates of the Opening Section. For example: if the uppermost vertex of the Opening Section boundary has a y-coordinate = 50 meters, and you want to define a water table which is 10 meters above this point, then enter an Elevation = 60 meters.

The water pressure will be calculated on each wedge face, based on the vertical distance from the wedge to the Elevation.

The "zero pressure on the boundary" box means that the boundary is free draining and there is no pore pressure allowed to develop on the tunnel surface. If this is off, it is as if drainage is prevented due to blockage of the drainage system and pore pressure is allowed to build up to the hydrostatic condition at all points on the joint surface. It's the worst case situation and you'll find that, in general, it gives a lower Factor of Safety.

**NOTE:** The **Elevation** Water Pressure option is only strictly applicable for horizontal excavations (i.e., Plunge = 0). If the excavation is nearly horizontal (e.g., within a few degrees), then you may use the **Elevation **option, and the error should not be too significant. If the excavation is more than a few degrees from horizontal, or is vertical, then you should NOT use the **Elevation **option at all, because the water pressure will not be calculated correctly.

## Ground/Truncation Surface

The **Ground/Truncation Surface** Water Pressure option assumes that the user-defined ground surface is the piezometric line. As such, the water pressure will be calculated on each wedge face, based on the vertical distance from the wedge to the ground surface.

The "zero pressure on the boundary" box means that the boundary is free draining and there is no pore pressure allowed to develop on the tunnel surface. If this is off, it is as if drainage is prevented due to blockage of the drainage system and pore pressure is allowed to build up to the hydrostatic condition at all points on the joint surface. It's the worst case situation and you'll find that, in general, it gives a lower Factor of Safety.

## Water Pressure Grid

The **Water Pressure Grid** option applies the user-defined water pressure grid to the model. The Water Pressure Grid is defined from the **Analysis** menu.