How is the daylight envelope on a stereonet constructed?

The Daylight Envelope is simply the locus of all pole vectors whose corresponding plane (dip vector) lies exactly on the slope plane.

Dips-daylight_envelope.PNG

Kinematic analysis results for planar sliding (see Tutorial 4 for more information)

 

The envelope is calculated by taking the trend and plunge of a number (say 30 or 45) of sequential points on the slope great circle (starting at one equator and moving to the other).  For each successive point, calculate a daylight pole point at trend+180, 90-plunge and connect the dots to get the daylight envelope.

The daylight envelope represents all planes which can theoretically daylight from a given slope.  In practice, planes which have a similar dip direction to the slope plane are more likely to fail, so angular limits can be placed (typically plus/minus 20 degrees or so) to decrease the number of critical planes.

Note:  to display the dip vector “tic” for a user plane, right-click on the plane after adding, select Edit User Plane from the popup menu, and select the Marker Ticks checkbox.