--- title: "Geology in road alignment selection" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:road-alignment-geology" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/road-alignment-geology site: BCT Engineering --- # Geology in road alignment selection Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2072 Asoj, 3 marks; 2068 Magh, 3 marks; 2078 Chaitra, 3 marks; 2071 Magh, 5 marks; 2078 Poush, 3 marks (asked 5 times) - Video: [Alignment Selection and Construction Problems of Hill Roads](https://www.youtube.com/watch?v=AlpdKUBZehU) — “Padam Civil Universe” - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/road-alignment-geology - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question Describe the geological considerations (engineering geological parameters) for selecting a road alignment. ## Short answer Key parameters are strong, massive rock (quartzite, granite), dip of bedding into hill (favourable), and avoidance of faults, shear zones, steep slopes, old landslides, wet areas and flood zones. Investigation includes air-photo study, mapping, test pits and geophysics; the alignment with least geological risk is chosen. ## Answer Geology controls the cost, stability and maintenance of a road, especially in the hills. The alignment is chosen after a geological study. #### Engineering geological parameters 1. **Rock and soil type:** strong, massive rocks (quartzite, granite, gneiss) give stable cut slopes; phyllite, schist, shale, Siwalik mudstone and loose colluvium are weak and landslide-prone. 2. **Geological structure:** relation of bedding, foliation and joints to the road cut. - Dip into the hill (against the slope) is favourable. - Dip out of the slope (daylighting) gives planar and wedge failures; avoid it. - Avoid alignments parallel to faults and shear zones. 3. **Slope and topography:** gentle, stable slopes; avoid steep slopes, gullies and scarps; prefer ridges and stable benches. 4. **Landslides and unstable ground:** avoid old landslides, debris slopes and eroding areas. 5. **Weathering and overburden thickness:** deep soil and highly weathered rock mean large cut volumes and frequent failures. 6. **Groundwater and springs:** avoid wet areas, seepage zones and swampy ground. 7. **Streams and floods:** minimise crossings; stay above flood level; choose stable stream banks. 8. **Seismic hazard:** avoid active faults. 9. **Construction materials:** quarry, aggregate and borrow areas along the route. 10. **Foundation of structures:** bridge and culvert sites. #### Investigation Reconnaissance with air photos, geological mapping, test pits, trenches and geophysics at critical sections; slope stability assessment by kinematic (stereonet) analysis. Select the alignment that has the least geological risk, and then design cuts, drainage and protection works. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which rocks give stable road cut slopes? A. Strong massive rocks such as quartzite, granite and gneiss B. Phyllite and shale C. Loose colluvium D. Siwalik mudstone Answer: A. Weak rocks and colluvium are landslide-prone. (Key point: Rock type) 2. Which bedding orientation is favourable for a road cut? A. Dip out of the slope (daylighting) B. Dip into the hill, against the slope C. Dip parallel to the cut facing out D. Alignment parallel to a fault Answer: B. Daylighting dips give planar and wedge failures. (Key point: Dip direction) 3. How should groundwater be treated in alignment selection? A. Prefer springs and swamps B. Ignore water C. Avoid wet areas, seepage zones and swampy ground D. Align along drainage gullies Answer: C. Water weakens slopes and pavement. (Key point: Groundwater) 4. Which tool checks cut slope failure modes using joint data? A. Chemical analysis B. Plate load test C. Aerial survey only D. Kinematic analysis with stereonet Answer: D. Joint sets are compared with slope direction for planar, wedge and toppling failure. (Key point: Investigation) 5. What does deep soil and highly weathered rock mean for alignment? A. Large cut volume and risk of failure B. Stable cuts C. Lower cost D. No need for drainage Answer: A. Thick weak material is hard to cut and unstable. (Key point: Overburden) ## Related in Engineering Geology II - [Geological parameters for canal alignment](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Acanal-alignment-parameters) — Engineering Geology II, chapter 4 - [Geology in tunnel site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-site-selection) — Engineering Geology II, chapter 4, asked 4× - [Geology in bridge site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Abridge-site-selection) — Engineering Geology II, chapter 4, asked 6× - [Road, dam and foundation site geology](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aroad-dam-building-bridge-foundation) — Engineering Geology II, chapter 4 - [Geology in reservoir site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Areservoir-site-selection) — Engineering Geology II, chapter 4 - [Direct surface methods for road alignment](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Adirect-surface-methods-road) — Engineering Geology II, chapter 4 ## Related in other subjects - [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× - [Special considerations in hill road design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-special-considerations) — Transportation Engineering I, chapter 5, asked 5× - [Electrical considerations in conductor selection](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aconductor-electrical-considerations) — Transmission and Distribution System Design, chapter 4, asked 3× - [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