Setting the Water Regime

LimitState:GEO

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Setting the Water Regime

Introduction

At a global level, water pressures may be specified either by the use of a water table, or by the specification of an $ru$ value. Water pressures are determined according to the theory described in Water Table Theory.

Drawing the Water Table

To draw a water table, click on the water table toolbar button , and then with the mouse click on a series of points across the geometry to define the water table. To finish, right click the mouse to bring up the context menu and select Finish. The water table will then be displayed as shown below. It is displayed in light blue if the Water table enabled setting in the project Property Editor is set to true. If not enabled, it is displayed in faint grey.

Figure 87:

Example water table added to a simple footing problem

To select the water table, click on any part of the water table. The entire water table or individual vertices may be selected and moved using the mouse. When selected, the water table coordinates also appear in the Geometry Editor where they may be edited. Vertices for the water table must be specified with monotonically increasing x-coordinates.

Clearing the Water Table

To clear the water table, right click anywhere on a blank area of the Viewer Pane to display the context menu and select Clear Water Table.

Enabling the Water Table

The setting Water table enabled in the project Property Editor is used to define whether the water table is used in an analysis.

If set to true, for drained behaviour the water pressure at any point is determined by the following:


Position	Pressure (kN/m $2$ )

Above water table	0
Below water table	$γwd$

Table 6:

Determination of water pressure (where $γw$ is the unit weight of water, and $d$ is the vertical depth below the piezometric surface)

If the setting Water table enabled is set to false then all materials are assumed to lie above the water table (i.e. zero pore pressure conditions for drained materials). The water table is then displayed in a faint grey.

Use of an $ru$ Value

An $ru$ value may be utilised in place of a water table. In this case the water pressure $u$ at any point is given by

u = ruγh

(20)

where $γh$ is the overburden pressure which may be represented by the defining $h$ as the vertical depth of the point in question below the ground surface.

To give additional flexibility in LimitState:GEO, the nominal ground surface is defined using the water table. For conventional use, the user will draw the water table along the soil surface.

To enable the use of $ru$ , click the next to the Water table entry in the Property Editor. Three new values will appear as shown in $ru$ values. Set Use $ru$ coefficient to true.