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Seismic Force

Seismic Force can be included in the UnWedge analysis by selecting the Seismic Force checkbox, under the General tab of the Input Data dialog, and entering the following data:

Seismic Coefficient

A dimensionless number defines seismic acceleration as a fraction of the acceleration due to gravity. Typically the Seismic Coefficient might be around 0.1 to 0.2. The magnitude of the seismic force applied to a wedge is related to the seismic coefficient and the mass of the wedge, as follows:

  • Seismic Force F = m a g where:
  • a = Seismic Coefficient
  • g = acceleration due to gravity (= 9.81 m/s2)
  • m = mass of the wedge


The following options are available for defining the direction of the seismic force:

  • Sliding Direction: With this option, the seismic force will be applied in the sliding direction or direction of movement (e.g., falling wedge) for each individual wedge. In general, applying the seismic force in the sliding direction for each wedge should give close to a worst-case scenario, although it is not necessarily the worst possible direction, other directions may result in a lower Factor of Safety.
  • Local X/Y/Z: This allows you to define up to 3 separate seismic coefficients, in the directions of the local X, Y, and Z axes. The local X and Y axes correspond to the (local) horizontal and vertical axes of the Opening Section you have defined. The local Z-axis corresponds to the Tunnel Axis Orientation. Note that you may define positive or negative values, to indicate the sense of direction along each axis. The positive X and Y directions are the positive axes of the Opening Section (i.e., X is positive to the right, Y is positive up, in the local coordinate system). The positive Z direction is looking "down" the Tunnel Axis (i.e. facing the Opening Section).
  • North/East/Down: This allows you to define up to 3 separate seismic coefficients, in the global North/East/Down directions. You may use positive or negative values to indicate the sense of direction along each axis.
  • Trend/Plunge: Enter the Trend/Plunge of the seismic force
Remember that when you include seismic force in the UnWedge analysis, only a simple pseudo-static force is being applied to each wedge. In order to fully assess the effect of seismic loading on wedge stability, you should re-run the analysis, using several different directions for the seismic force. The effect of changing the direction and magnitude of the seismic coefficient should be analyzed for each wedge.
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