--- title: "Geology in bridge site selection" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:bridge-site-selection" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/bridge-site-selection site: BCT Engineering --- # Geology in bridge site selection Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2081 Chaitra, 1 marks; 2078 Baisakh, 4 marks; 2076 Bhadra, 2 marks; 2071 Bhadra, 3 marks; 2070 Magh, 3 marks; 2076 Baisakh, 2 marks (asked 6 times) - Video: [Bridge Engineering Geology: Building on Solid Ground](https://www.youtube.com/watch?v=ejIE9-D0B4w) — earthscience classes official - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/bridge-site-selection - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question Describe the geological considerations (parameters) for bridge site selection, including investigation of the bridge foundation. ## Short answer A good bridge site has shallow, sound bedrock, stable abutments, and a straight river reach avoiding faults and meanders. Foundation investigation includes geological mapping, boreholes at piers, geophysics, and lab tests to evaluate bearing capacity, scour, and seismic risk. ## Answer A bridge site must have a safe foundation for piers and abutments, stable banks and a stable river channel. #### Geological considerations 1. **Foundation rock:** sound, massive, strong bed rock at shallow depth is the best; avoid deep soft alluvium or weak rock. 2. **River behaviour:** a straight, stable reach with narrow channel; avoid meanders, braided reaches and confluences; assess scour depth and flood level. 3. **Geological structure:** avoid faults, shear zones and highly jointed rock; bedding and joints should not dip out of the abutment slopes toward the river. 4. **Abutment slopes:** stable, no landslides, rock fall or debris; low erosion. 5. **Weathering and overburden thickness.** 6. **Groundwater:** water table and seepage affect excavation of foundations. 7. **Seismicity:** active faults and liquefiable sand near the site. 8. **Construction material** for concrete and approach roads nearby. 9. **Approach alignment** should be stable. #### Investigation of the bridge foundation - Geological mapping of both banks and river bed. - Boreholes at each pier and abutment location, with core drilling in rock to at least 3 m into sound rock. - Test pits; SPT and sampling in soil. - Geophysical profiles (seismic refraction/resistivity) across the river. - Laboratory tests for bearing capacity, scour and shear strength. - Hydrological data for flood and scour assessment. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which foundation is best for a bridge? A. Deep soft alluvium B. Highly weathered shale C. Fault shear zone D. Sound, massive bed rock at shallow depth Answer: D. Strong rock near the surface gives safe pier and abutment foundations. (Key point: Foundation rock) 2. What river reach is preferred for a bridge? A. A straight, stable reach with a narrow channel B. A meandering reach C. A braided reach D. A confluence Answer: A. Straight stable reaches avoid channel shifting and bank erosion. (Key point: River behaviour) 3. How deep should core drilling go in rock at piers and abutments? A. Only to the surface of weathered rock B. At least 3 m into sound rock C. 1 m in soil D. Not required Answer: B. This confirms the rock is sound and not a boulder or weathered layer. (Key point: Borehole depth) 4. Which structural condition is unfavourable at abutments? A. Massive unjointed rock B. Stable abutment slopes C. Bedding dipping steeply downstream toward the river on abutment slopes D. Low erosion Answer: C. Such dips promote sliding of the abutment slopes. 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