# Display Daylight Envelope

Daylight envelopes corresponding to planes can be displayed using the check boxes provided in the Edit User Planes or Edit Sets dialogs.

Daylight envelopes are used primarily in slope stability analysis work. It is kinematically feasible for any poles that plot within a daylight envelope to slide. To determine the stability of such poles, you must also consider the frictional strength of the planes. Friction cones can be added to the stereonet using the Add Cone option.

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 option from the popup menu, and select the Marker Ticks checkbox.

Interactive Daylight Envelope

A moving daylight envelope can be displayed while using the Add User Plane option, by selecting Daylight Envelope option from the right-click menu.

Derivation of the Daylight Envelope

The Daylight Envelope used in Dips is derived as follows:

• The area within the Daylight envelope contains the poles to planes which have dip vectors outside the slope (i.e., sliding is kinematically possible for these planes).
• The actual locus of the envelope represents the poles to all of the planes which have their dip vectors exactly on the slope plane (i.e., these planes have dip vectors parallel to the slope plane).

You can verify this graphically as follows:

1. Add a plane representing a slope orientation, and display the daylight envelope.
2. Now select the Add User Plane option again.
3. Place the cursor exactly on the daylight envelope, at any location. Click the mouse and add the plane.
4. Notice that the dip vector of the plane you have just added (i.e., the midpoint of the great circle, marked by a "tick"), is exactly on the original (slope) plane, for which the daylight envelope is displayed.
5. Repeat step 3 and 4, for any number of planes, and you will see that the dip vectors of the added planes, will all lie exactly on the great circle of the slope plane.
6. Also notice, if you add a plane with a pole INSIDE the daylight envelope, the dip vector of the plane will be OUTSIDE of the slope great circle (i.e. the daylighting condition).
7. Conversely, any plane with a pole vector OUTSIDE of the daylight envelope, will have a dip vector which is INSIDE of the slope great circle (i.e. plane does not daylight).

The daylight envelope in Dips represents a theoretical limiting boundary, within which it is physically possible for planes to slide out of a slope.