--- title: "Road, dam and foundation site geology" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:road-dam-building-bridge-foundation" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/road-dam-building-bridge-foundation site: BCT Engineering --- # Road, dam and foundation site geology Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2074 Bhadra, 2+1.5+1.5 marks - Video: [GEOLOGICAL CONDITIONS NECESSARY FOR CONSTRUCTION OF DAMS, TUNNELS, BUILDINGS, AND ROAD CUTTING](https://www.youtube.com/watch?v=gRZWOjElTpE) — BSCE 2A - 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-dam-building-bridge-foundation - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question What are the geological considerations for selection of road and dam sites? How do you investigate the foundation site for a building and a bridge? Mention in brief. ## Short answer Roads: stable slopes with strong rock, bedding dipping into hill, avoid faults and wet ground. Dams: strong massive rock, narrow valley, watertight, stable abutments. Investigation: buildings – boreholes to at least 1.5-2 times footing width, SPT, plate load tests; bridges – mapping, boreholes at piers with core drilling 3 m into sound rock, geophysics, scour study. ## Answer #### Geological considerations for road site - Stable slopes with strong rock; avoid landslides, steep weak slopes, and old debris. - Favourable structure: beds dipping into the hill; avoid faults, shears and daylighting joints. - Avoid wet, swampy ground and springs; limit stream crossings; stay above flood level. - Materials (aggregate, borrow) along route. #### Geological considerations for dam site - Sound, strong, massive foundation rock with low weathering; narrow valley. - No active faults, shear zones or highly jointed zones. - Watertight foundation and rim; no solution cavities. - Stable abutments and favourable dip (upstream); materials nearby. #### Investigation of foundation site for a building - Desk study and surface mapping of the site. - Test pits and boreholes (to a depth of at least 1.5-2 times the footing width B below foundation level) to find soil/rock type, thickness, water table. - SPT, plate load test, soil samples for laboratory tests (bearing capacity, settlement). - Look for fill, soft clay, expansive soil, liquefiable sand, slope stability. #### Investigation of foundation site for a bridge - Mapping of both banks and the river; scour and flood study. - Boreholes at each pier and abutment, with core drilling 3 m into sound rock; SPT in soil. - Geophysics (seismic refraction/resistivity) across the river. - Lab tests for bearing capacity and foundation depth below scour level. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which foundation is preferred for a dam site? A. Sound, strong, massive rock with low weathering B. Thick soft clay for easy excavation C. Highly jointed shale with shear zones D. Cavernous limestone with solution cavities Answer: A. A dam needs a strong, sound, little-weathered, watertight foundation without faults or cavities. (Key point: Dam foundation rock) 2. For a road cut, which bedding attitude is favourable? A. Beds cut by shear zones along the route B. Beds dipping into the hill C. Beds dipping out of the slope and daylighting D. Beds parallel to a wet swampy valley floor Answer: B. Beds dipping into the hill give stable cuts; daylighting joints and faults cause slides. (Key point: Road bedding dip) 3. How deep should boreholes for a building foundation go? A. Always 3 m into sound rock B. Exactly the same as the footing width C. About 1.5 times footing width, at least 1.5 B below foundation level D. Only 0.5 m below foundation level Answer: C. Test pits and boreholes go to about 1.5 B below foundation level to find soil type, thickness and water table. (Key point: Building borehole depth) 4. What is done at each pier and abutment when investigating a bridge foundation? A. Only surface mapping of the road approach B. A plate load test on the river water C. No drilling, since geophysics alone is enough D. Boreholes with core drilling about 3 m into sound rock, SPT in soil Answer: D. Bridge investigation needs boreholes at each pier and abutment, core drilling into sound rock and SPT in soil, with foundation depth below scour level. 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