Daylight Envelope Helper
A quick design helper for planar rock-slope sliding: it returns the steepest slope face that keeps a discontinuity from daylighting, tells you whether the joint can slide at all (dip vs friction), and — if you enter a proposed face angle — whether that face daylights the joint and is kinematically feasible. Used by geotechnical and mining engineers scoping cut-slope angles.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the discontinuity (joint/bedding) dip ψp in degrees.
- Enter the friction angle φ of the discontinuity surface.
- Optionally enter a proposed slope face dip ψf to test whether it daylights the joint.
How it works
For a plane to slide it must both daylight in the slope face (the face dip ψf must exceed the joint dip ψp) and be steep enough to overcome friction (ψp > φ). So the steepest face that prevents daylighting is simply ψf = ψp — cut the face no steeper than the joint's dip and the plane stays buried in the slope.
If ψp ≤ φ the joint is self-stable no matter how steep the face, because gravity cannot overcome friction on it. When a proposed face angle is supplied the tool combines both tests into a single feasibility verdict.
Worked example
Joint dipping 35°, friction 30°, face cut at 45°. Steepest non-daylighting face = ψp = 35°. Can it slide? 35° > 30° → yes, if it daylights. Face at 45° > 35° → it daylights. Combined: planar sliding is kinematically feasible, so a 45° face is unsafe for this joint — flatten it to ≤ 35° or add support.
Common mistakes
- Treating a 'feasible' result as a factor-of-safety failure — this is only a kinematic (geometry + friction) screen, not a stability calculation.
- Ignoring dip direction and lateral limits — a joint only daylights if its dip direction is within about ±20° of the face; this helper checks the dip magnitude only.
- Forgetting other failure modes — wedge and toppling can occur where planar sliding cannot; check them separately.
Frequently asked questions
What does it mean for a joint to 'daylight'?
A discontinuity daylights when it emerges on (dips out of) the slope face — the face is steeper than the joint (ψf > ψp). Only daylighting joints can slide out of the slope.
What is the steepest safe face angle?
To stop the joint daylighting, cut the face no steeper than the joint dip ψp. A face at or below ψp keeps the plane contained within the slope.
Why does ψp have to exceed the friction angle?
If the joint dip is less than or equal to the friction angle (ψp ≤ φ), the gravitational driving force cannot overcome frictional resistance, so the block is self-stable regardless of the face angle.
Does this replace a full kinematic analysis?
No — it only checks daylighting in the joint's dip direction. A full stereonet-based analysis also checks dip direction, the ±20° lateral limits, and the wedge and toppling modes. Use the Plane Failure Kinematic Screen for that.
Is this a stability (factor-of-safety) calculation?
No. It is a kinematic feasibility screen based on geometry and friction. Even a 'feasible' plane may be stable once water pressure, cohesion and reinforcement are accounted for.
Related tools
- Plane Failure Kinematic Screen
- Wedge Failure Kinematic Screen
- Saturated Slope FoS Calculator
- Tension Crack Depth Calculator
- Batter Ratio Calculator
- Toppling Kinematic Screen
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