--- title: "Conditions for plane failure" subject: "Engineering Geology II" chapter: 6 key: "semester_4/engineering_geology_ii:plane-failure-conditions" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/plane-failure-conditions site: BCT Engineering --- # Conditions for plane failure Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data - Asked in: 2073 Bhadra, 2 marks; 2070 Bhadra, 2 marks (asked 2 times) - Video: [Planar Rock Slope Failure Explained | RocPlane Tutorial](https://www.youtube.com/watch?v=Uoi1iKNBqow) — Soil Mechanics and Engineering Geology - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/plane-failure-conditions - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6 ## Question What are the conditions for plane failure of a rock slope? ## Short answer Plane failure occurs when a discontinuity strikes nearly parallel to the slope face (±20°), dips out of the face at an angle less than the slope but greater than the friction angle, and is bounded laterally by release surfaces or a tension crack. On a stereonet, the plane's pole lies inside the daylight envelope and outside the friction circle. ## Answer **Plane failure** is the sliding of a rock block along a single plane of weakness (joint, bedding or foliation) that daylights in the slope face. According to Hoek and Bray, it occurs when all these conditions are satisfied: 1. **Strike:** the plane strikes parallel, or nearly parallel (within about $\pm 20^\circ$), to the slope face. That is, the dip directions of the plane and the slope differ by no more than about $20^\circ$. 2. **Daylighting:** the plane dips less steeply than the slope face, $\psi_p < \psi_f$, so that the plane is exposed in the face. 3. **Friction:** the dip of the plane is greater than its friction angle, $\psi_p > \phi$, so that the block can slide under gravity. 4. **Release surfaces:** the sliding block must be free at its sides, either through lateral release joints of negligible resistance or because the plane passes through a convex nose of the slope; a tension crack usually bounds it at the top. ```text crest tension crack \ | \ |__ face \ \ \ sliding plane \ \ \ (dip psi_p) \_____\_\___ ``` **Stereonet test:** on a great-circle plot, the dip vector of the plane lies between the slope great circle and the friction circle, within about $\pm 20^\circ$ of the slope dip direction; equivalently, on a pole plot, the pole lies inside the daylight envelope and outside the friction circle. If $\psi_p \leq \phi$, the slope is stable against plane sliding. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What strike condition is needed for plane failure? A. Plane strikes perpendicular to the face B. Plane strikes parallel or nearly parallel (within about 20°) to the slope face C. Plane dips into the slope D. Any strike Answer: B. Dip directions of plane and slope must be within about 20°. (Key point: Strike) 2. What does daylighting require? A. Plane dip greater than the slope angle B. Plane dip equal to friction angle C. Plane dip less than the slope face dip so the plane is exposed D. Plane horizontal Answer: C. psi_p < psi_f means the plane appears in the face. (Key point: Daylighting) 3. What is the friction condition? A. psi_p < phi B. psi_p > phi, so the block can slide under gravity C. psi_p = 90° D. phi > psi_f Answer: B. If dip is less than or equal to the friction angle the block stays stable. (Key point: Friction) 4. Why are release surfaces needed? A. To raise cohesion B. To drain water C. To make the plane steeper D. The block must be free to move laterally Answer: D. Lateral joints or the end of the face free the block. (Key point: Release surfaces) ## Related in Engineering Geology II - [Conditions for wedge failure](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Awedge-failure-conditions) — Engineering Geology II, chapter 6 - [Cut slope design: N41W/56 hillslope](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acut-slope-n41w-56) — Engineering Geology II, chapter 6 - [Failure mode: HS 75/230, BP 68/045](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Afailure-mode-hs75-230-figure) — Engineering Geology II, chapter 6 - [Cut slope design: quartzite N55W/50](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acut-slope-quartzite-n55w-50) — Engineering Geology II, chapter 6 - [Cut slope design: limestone N48E/52](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acut-slope-limestone-n48e-52) — Engineering Geology II, chapter 6 - [Failure mode: HS N45E/70, bedding N55E/45](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Afailure-mode-hs-n45e-70) — Engineering Geology II, chapter 6 ## Related in other subjects - [Preventive measures for landslide and rock slope](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apreventive-measures-for-landslide-and) — Engineering Geology I, chapter 6, asked 4× - [Causes of slope failure](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aslope-failure-causes) — Soil Mechanics, chapter 11, asked 4× - [Dry sand failure plane 25°, find σ3](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afine-dry-sand-failure-plane-25) — Soil Mechanics, chapter 10 - [Plane failure versus wedge failure](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aplane-failure-versus-wedge-failure) — Engineering Geology I, chapter 6