--- title: "Tunneling and site investigation process" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:tunneling-site-investigation-process" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/tunneling-site-investigation-process site: BCT Engineering --- # Tunneling and site investigation process Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2079 Jestha, 1+6 marks - Video: [Geotechnical Investigation](https://www.youtube.com/watch?v=7Ds9e-5R280) — Tunneling and Hydropower - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/tunneling-site-investigation-process - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question What is tunneling? Describe the process of geological site investigation for roads, bridges and dams. ## Short answer Tunnelling is excavating an underground passage without removing overburden. Investigation stages are desk study, reconnaissance, detailed. Roads: mapping, boreholes at high cuts; bridges: mapping, boreholes at piers with core drilling 3 m into rock, scour study; dams: core drilling with Lugeon, adits, mapping, in-situ deformation tests. ## Answer **Tunnelling** is the excavation of an underground passage through rock or soil, without removing the overlying ground, by drilling and blasting, mechanised excavators or tunnel boring machines, followed by support and lining. It is used for roads, railways, hydropower waterways, water supply and metro. #### Geological site investigation process Common stages for all structures: (1) desk study, (2) reconnaissance, (3) preliminary investigation, (4) detailed investigation, (5) reporting. **Roads** - Study maps, air photos; mark landslides, steep and weak slopes. - Map geology along alternatives: rock type, joints, bedding dip relative to the cut, weathering, springs. - Test pits and trenches, boreholes at high cuts, bridges, and fills; geophysics at doubtful places. - Stereographic analysis of slope failure, materials survey. **Bridges** - Map both banks and river channel; study flood and scour. - Boreholes at piers and abutments, core drilling in rock; SPT in soil; geophysics across the river. - Lab tests for bearing capacity and tests of aggregate. **Dams** - Regional geology, structure, seismicity, and reservoir watertightness. - Mapping of the dam axis, abutments, and borrow areas. - Core drilling with Lugeon tests; adits and trenches; geophysics. - In-situ tests for strength and deformation of foundation rock; grouting design; materials investigation. Outputs: engineering geological maps, sections, logs and recommendations for design. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is tunnelling? A. Cutting an open trench and refilling it B. Building a surface embankment C. Drilling only vertical shafts for water D. Excavating an underground passage without removing the overlying ground Answer: D. Tunnelling excavates through rock or soil underground, followed by support and lining. (Key point: Definition of tunnelling) 2. What is the usual order of stages in geological site investigation? A. Desk study, reconnaissance, preliminary, detailed, reporting B. Detailed, reconnaissance, desk study, reporting C. Reporting, desk study, detailed, preliminary D. Preliminary, reporting, desk study, reconnaissance Answer: A. Investigation proceeds from desk study to reconnaissance, preliminary and detailed investigation and then reporting. (Key point: Investigation stages) 3. Which test is used in core drilling for a dam site to check rock permeability? A. Aggregate abrasion test B. Lugeon test C. SPT in the river D. Plate load test only Answer: B. Core holes at dam sites are tested by Lugeon tests to assess permeability and grouting need. (Key point: Dam permeability test) 4. Why is bedding dip relative to the cut mapped for roads? A. To estimate traffic volume B. To select the bridge type C. To judge slope stability and failure by sliding D. To find the river flow rate Answer: C. Dip relative to the cut decides whether planar or wedge failure may occur; stereographic analysis supports this. 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