--- title: "Canal slope stability: HS 138/45" subject: "Engineering Geology II" chapter: 6 key: "semester_4/engineering_geology_ii:canal-slope-stability-hs138-45" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/canal-slope-stability-hs138-45 site: BCT Engineering --- # Canal slope stability: HS 138/45 Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data - Asked in: 2074 Bhadra, 5 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/canal-slope-stability-hs138-45 - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6 ## Question Interpret the stability condition of the rock slope where a canal alignment has to pass. The orientations of the discontinuities, hill slope and the internal friction angle are as follows: HS = 138°/45°, J1 = 234°/38°, J2 = 098°/58°, J3 = 315°/60° and ϕ = 25°. [Figure: stereonet] ## Short answer Stereonet analysis with phi=25° shows no planes daylight in a dangerous orientation; J1-J2 intersection plunges 21.3° (\phi$ | Result | |---|---|---|---|---| | J1 (38/234) | 96° | yes | yes | Safe | | J2 (58/098) | 40° | no | yes | Safe | | J3 (60/315) | 177° | no | yes | Safe | #### Intersections | Pair | Intersection plunge/trend | Face apparent dip along trend | Result | |---|---|---|---| | J1 x J2 | 21.3° / 174° | 39.0° | Stable (plunge below $\phi$) | (The other 2 intersections plunge into the hill, away from the face, and cannot slide.) #### Interpretation - **J1** dips toward 234°, $96^\circ$ away from the face direction: it runs across the slope and cannot slide out. - **J2** is steeper ($58^\circ$) than the face ($45^\circ$): it does not daylight. - **J3** dips into the hill (315° against 138° + 180° = 318°) at $60^\circ$; toppling needs $\psi \geq 90^\circ - 45^\circ + 25^\circ = 70^\circ$, so it is below the limit. - The only intersection that points out of the face is **J1 x J2**, plunging $21.3^\circ$ towards 174°. Its plunge is **less than $\phi = 25^\circ$**, so the wedge cannot slide. **Conclusion: the rock slope is kinematically stable against plane, wedge and toppling failure** and the canal alignment is acceptable. Provide drainage and lining to prevent seepage water from raising pore pressure, and check local loosening. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. J2 (58/098) differs by 40° from the face. Why is it safe? A. It dips steeper (58°) than the face (45°) and is outside ±20° B. It dips less than phi C. It strikes parallel to canal D. It is horizontal Answer: A. It is steeper than the face, so it does not daylight. (Key point: J2 does not daylight) 2. What toppling limit applies to J3 and the outcome? A. 45° + 25° = 70°; toppling occurs B. 90° − 45° + 25° = 70°; J3 dips 60°, so no toppling C. 25°; toppling occurs D. 90°; no toppling possible Answer: B. Toppling needs dip ≥ 90° − psi_f + phi = 70°; J3 is 60°. (Key point: Toppling limit J3) 3. Why can the J1 x J2 wedge not slide? A. Its plunge exceeds the face apparent dip B. It plunges into the hill C. Its plunge 21.3° is less than phi = 25° D. J1 is vertical Answer: C. If the line plunges less than phi, friction prevents sliding. (Key point: Wedge test) 4. What is the overall conclusion? A. Unstable against plane failure B. Unstable against toppling C. Needs complete excavation D. Kinematically stable; provide drainage and lining Answer: D. No plane, wedge or toppling mode is possible; drainage controls seepage pressure. (Key point: Conclusion and remedy) ## Related in Engineering Geology II - [Stability: HS 202/65, joints J1 to J4](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Astability-hs202-65-j4-52) — Engineering Geology II, chapter 6 - [Stability: HS 175/52, BP 150/48](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Astability-hs175-52-bp150-48) — Engineering Geology II, chapter 6 - [Cut slope design: HS 110/40, B 130/20](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acut-slope-hs110-40) — Engineering Geology II, chapter 6 - [Slope stability: N40W/55 slope, two joints](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aslope-stability-n40w-55) — 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 - [Stability: HS 310/43, BP 306/31](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Astability-hs310-43-bp306-31) — Engineering Geology II, chapter 6 ## Related in other subjects - [Slope orientation and discontinuities for stability](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aslope-orientation-and-discontinuities-for) — Engineering Geology I, chapter 6 - [Rock mass, rock material, discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-rock-material-discontinuities) — Engineering Geology I, chapter 5 - [Factors controlling hill road alignment](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-alignment-factors) — Transportation Engineering I, chapter 5, asked 2× - [Methods of locating hill road alignment](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-alignment-methods) — Transportation Engineering I, chapter 5