--- title: "Site investigation for dam and reservoir" subject: "Engineering Geology I" chapter: 1 key: "semester_3/engineering_geology_i:site-investigation-for-dam-and" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/site-investigation-for-dam-and site: BCT Engineering --- # Site investigation for dam and reservoir Engineering Geology I · Chapter 1: Geology and Civil Engineering - Asked in: 2063 Baisakh (old course), 3 marks; 2059 Chaitra (old course), 8 marks (asked 2 times) - 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_3/engineering_geology_i/site-investigation-for-dam-and - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-1 ## Question Describe the geological investigation methods (direct and indirect) for the site of a large dam and its reservoir. ## Short answer Direct methods include geological mapping, core drilling, test pits, adits and in-situ Lugeon tests for permeability. Indirect methods are seismic refraction for bedrock depth, electrical resistivity for water and weak zones, and aerial photo interpretation. Reservoir investigations check leakage paths, slope stability and siltation. ## Answer A dam is a huge structure that must stand on rock able to carry the load, resist sliding and stay watertight, and its reservoir must hold water without leakage or slope failure. Investigation is therefore done at the dam site and the reservoir area. #### What is to be found out - Rock type, strength and weathering of foundation and abutments. - Structures: dip and strike of beds, joints, faults, shear zones, folds. - Permeability and groundwater conditions. - Depth to sound bedrock and thickness of overburden. - Slope stability of reservoir rim; sedimentation; seismicity. - Availability of construction materials. #### A. Direct methods 1. **Geological mapping** of the dam site (scale 1:500 to 1:2000) and reservoir (1:5000 to 1:25000). 2. **Test pits, trenches** across suspected faults and in overburden. 3. **Core drilling** at the axis, abutments, spillway and stilling basin; core recovery, RQD and logging; permanent record of the rock. 4. **Drifts/adits** in abutments to see rock at depth and carry out in-situ tests. 5. **In-situ tests**: Lugeon (packer) water pressure tests for permeability, plate load test, shear tests and borehole dilatometer tests. 6. **Laboratory tests** on cores: compressive strength, density, porosity, durability. #### B. Indirect methods 1. **Seismic refraction**: depth to bedrock, rock quality and the fractured zones. 2. **Electrical resistivity**: depth of water table, weak and saturated zones, buried channels. 3. **Gravity and magnetic surveys**: large buried features. 4. **Aerial photographs and satellite images**: lineaments, landslides and reservoir rim features. 5. **Geophysical logging** of boreholes. #### Reservoir-specific checks - Leakage paths through permeable beds, solution cavities in limestone, open joints and faults. - Stability of reservoir slopes (old landslides which could be reactivated). - Watertightness of the reservoir rim and divide. - Silting and seismic activity, including reservoir-induced seismicity. The data are combined into a geological report with plans, sections and treatment recommendations such as grouting and cut-off. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A dam foundation must be able to: A. Be highly permeable for drainage B. Consist of soft clay C. Carry the load, resist sliding and stay watertight D. Be fully weathered Answer: C. Foundation must be strong, stable and watertight. (Key point: Dam requirements) 2. Which in-situ test measures rock permeability using packers? A. Plate load test B. Dilatometer test C. SPT D. Lugeon test Answer: D. Lugeon (packer) tests measure water take of rock. (Key point: Permeability testing) 3. Which is an indirect method? A. Seismic refraction B. Core drilling C. Test pits D. Drifts and adits Answer: A. Geophysics infers conditions without direct access. (Key point: Direct vs indirect) 4. Which is a reservoir-specific check? A. Concrete mix design B. Leakage through open joints, faults or solution cavities C. Road alignment D. Bridge scour Answer: B. Reservoir must hold water; leakage paths and rim stability are examined. 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