--- title: "Geology for dam and bridge sites" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:dam-bridge-site-considerations" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/dam-bridge-site-considerations site: BCT Engineering --- # Geology for dam and bridge sites Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2069 Poush, 5 marks - Video: [Required Geological Consideration for Selecting Dam and Reservoir Site - Geological Investigation](https://www.youtube.com/watch?v=TPg6afvpYx4) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/dam-bridge-site-considerations - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question Write the geological considerations that should be taken in selection of dam site and bridge site. ## Short answer For dams: strong, massive rock, narrow valley, watertight, upstream dipping beds, stable abutments, no active faults, nearby materials. For bridges: sound rock or dense gravel at shallow depth; straight, narrow, stable channel reach; stable banks; no faults; consider scour depth and liquefaction. ## Answer #### Dam site 1. **Foundation rock:** strong, sound, massive, little weathered (granite, gneiss, quartzite); avoid weak shale, phyllite and soft soil. 2. **Structure:** avoid faults, shear zones, closely jointed rock and unfavourable bedding dip; upstream dip is preferable. 3. **Watertightness:** no cavernous limestone, open joints or permeable gravel; plan for grouting. 4. **Abutments:** stable and free from landslides; adequate rock cover. 5. **Valley shape:** narrow with sound rock at shallow depth. 6. **Seismicity:** no active faults near the site. 7. **Reservoir:** watertight, stable rim, low sedimentation. 8. **Materials:** aggregates, clay and sand nearby. #### Bridge site 1. **Foundation:** sound rock or dense gravel at shallow depth for piers and abutments; avoid thick soft clay and loose sand. 2. **River channel:** straight, narrow, stable reach; avoid bends, braided reaches, confluences and fast-shifting channels; estimate scour depth and maximum flood level. 3. **Structure:** avoid faults and weak shear zones; stable bedding. 4. **Banks and abutment slopes:** no landslides, rock fall or active erosion. 5. **Groundwater:** affects excavation of foundations. 6. **Seismic hazard,** liquefaction of sandy soil. 7. **Approach roads** on stable ground; construction materials and access. Investigation by mapping, core drilling at piers and abutments, geophysics and in-situ tests supports both selections. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which condition is a geological problem for a dam site? A. Cavernous limestone or open joints in the foundation B. Massive granite with little weathering C. Narrow valley with sound rock at shallow depth D. Upstream dip of beds Answer: A. Cavities and open joints cause leakage, so watertight foundation and rim are required; grouting is planned. (Key point: Dam watertightness) 2. Which bedding dip is preferable at a dam site? A. Vertical dip only on one abutment B. Upstream dip C. Downstream dip D. Any dip, it does not matter Answer: B. Upstream dip is preferable, along with avoiding faults, shear zones and closely jointed rock. (Key point: Preferred dip for dam) 3. Which river reach is best for a bridge site? A. A braided reach at a confluence B. The widest part of the floodplain C. Straight, narrow, stable reach D. A bend with a shifting channel Answer: C. Bends, braided reaches, confluences and shifting channels are avoided; scour depth and maximum flood level are estimated. (Key point: Bridge river channel) 4. What foundation is preferred for bridge piers and abutments? A. Thick soft clay B. Loose saturated sand C. Fill material D. Sound rock or dense gravel at shallow depth Answer: D. Soft clay and loose sand give settlement and liquefaction problems; sound rock or dense gravel is preferred. 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