Eqn.
1By default, Water Pressure is NOT applied to an Swedge model, and the analysis is therefore applicable to a DRY slope.
To include Water Pressure in the analysis, select the Water Pressure checkbox in the Input Data dialog. Three different options are available for defining the Water Pressure, by selecting from the Type list. These options are described below.
Filled Fissures
The Filled Fissures option assumes that extreme conditions of very heavy rainfall have occurred, and that in consequence the fissures (Joint Set 1, Joint Set 2, and Tension Crack) are completely full of water. Further, it is assumed that the pressure varies from zero at the free faces to a maximum value at some point on the line of intersection of the two failure planes (Joint Set 1 and Joint Set 2).
For the Filled Fissures option, Swedge calculates average values of water pressure on the failure planes as follows [Hoek & Bray (1981)]:
With NO Tension Crack
Eqn.
1
where: 
and 
are the average values of water pressure on failure planes
1 and 2
= unit weight of water
= total height of the wedge
With Tension Crack
Eqn.
2
where: , and
are the average values of water pressure on the failure
planes 1 and 2, and the tension crack, respectively
= unit weight of water
= depth of bottom vertex
of the tension crack below the upper ground surface
The above formulae are simple estimations which are useful in the absence of more precise information.
Note:
To simulate intermediate water pressures, the user can effectively vary the water pressure (as can be seen in Equations 1 and 2 above) by varying the unit weight of water between zero and the actual unit weight. This allows the user to perform a sensitivity analysis on the effect of water pressure on the safety factor of the wedge. However, the % Filled Fissures option can also be used, as described below.
To simulate a "dry" slope, the user can de-select the Water Pressure checkbox in the Input Data dialog, or enter a Unit Weight of zero.
% Filled Fissures
The % Filled Fissures option allows the user to specify the average height of water in the fissures, as a percentage of the completely water filled state. NOTE: due to the wedge geometry, and the derivation of the water pressure equations, the relationship between the average water pressure and the % Filled, is NOT linear, and is in fact a cubic relationship.
For example, if % Filled = 50 % :
Custom Pressure
The Custom Water Pressure option allows the user to specify the actual average water pressure on each plane, independently. This is the most flexible Water Pressure option, and may be useful if actual water pressure data is available on the failure planes of the wedge. However, remember that these are still average values, and only one value may be specified for each plane (Joint 1, Joint 2 and Tension Crack).
Display of Water Pressure Forces
If Water Pressure has been applied to a model, using any of the options described above, the Water Pressure forces will be represented on the model, with blue arrows displayed at the center of each wedge failure plane, directed normal to the plane. These arrows are symbolic and indicate that Water Pressure has been applied. They do NOT indicate the magnitude or method of Water Pressure calculation. The display of these arrows can be turned on or off, and the default arrow colour can be changed, in the Display Options dialog.