--- title: "Indirect sub-surface methods: tunnel and dam" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:indirect-subsurface-tunnel-dam" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/indirect-subsurface-tunnel-dam site: BCT Engineering --- # Indirect sub-surface methods: tunnel and dam Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2081 Chaitra, 2+2 marks - Video: [Electrical Resistivity and Seismic Method of Geological Investigation - Engineering Geology](https://www.youtube.com/watch?v=j1U-u_Tb7I0) — 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/indirect-subsurface-tunnel-dam - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question Describe indirect methods of sub-surface site investigation for tunnel alignment and dam site. ## Short answer For tunnels, indirect subsurface methods include seismic refraction to assess rock quality and faults, electrical resistivity to locate water-bearing fractures and cavities, and borehole geophysical logging. For dams, seismic refraction determines bedrock depth, resistivity identifies seepage paths, gravity/magnetic surveys detect buried faults, complemented by packer permeability tests. ## Answer Indirect sub-surface methods estimate underground conditions from measured physical properties, mainly by geophysics, often in boreholes. #### Tunnel alignment - **Seismic refraction survey** along the alignment: gives rock velocity and so rock quality, depth of weathered layer and bed rock, and fault or shear zone (low velocity zones). - **Electrical resistivity profiling/tomography:** detects water-bearing fractures, clay-filled shear zones, karst cavities (low resistivity for water and clay, high for dry cavity). - **Ground penetrating radar** for shallow cover at portals. - **Borehole geophysical logging** (gamma, resistivity, sonic, caliper) and **cross-hole/down-hole seismic** for stiffness and fractured zones. - **Seismic reflection/tomography** to predict ahead of the face (TSP) during construction. #### Dam site - **Seismic refraction** for depth to sound bed rock, thickness of overburden and rippability on abutments and river bed. - **Resistivity** for sand-gravel thickness, groundwater and seepage paths; cavities in limestone. - **Gravity/magnetic methods** for buried faults and dykes. - **Borehole logging and packer tests** for permeability, with cross-hole seismic for the rock mass between holes. - **Spontaneous potential and tracer methods** for seepage. These are checked with a few cored boreholes. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In tunnel alignment, a low-velocity zone indicates A. A fault or shear zone B. Sound hard rock C. Dry cavity only D. Nothing Answer: A. Low velocity means weak or fractured rock. (Key point: Seismic refraction) 2. Resistivity tomography detects A. Only gold B. Water-bearing fractures, clay-filled shears and karst cavities C. Rock strength in MPa D. Slope angle Answer: B. Water and clay give low resistivity. (Key point: Resistivity) 3. Seismic refraction at a dam site gives A. Bridge scour B. Rainfall C. Depth to sound bed rock, overburden thickness and rippability D. Silt load Answer: C. These guide excavation and foundation. (Key point: Dam site use) 4. How are indirect results confirmed? A. Not required B. By more geophysics only C. By photos D. With a few cored boreholes Answer: D. Boreholes calibrate the interpretation. (Key point: Checking) ## Related in Engineering Geology II - [Indirect methods of surface investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aindirect-surface-methods) — Engineering Geology II, chapter 4 - [Direct methods of sub-surface investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Adirect-subsurface-methods) — Engineering Geology II, chapter 4, asked 3× - [Surface versus sub-surface investigation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Asurface-vs-subsurface-investigation) — Engineering Geology II, chapter 4, asked 3× - [Geology in tunnel site selection](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-site-selection) — Engineering Geology II, chapter 4, asked 4× - [Direct surface methods for road alignment](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Adirect-surface-methods-road) — Engineering Geology II, chapter 4 - [Geological investigation of tunnel and road](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-road-investigation) — Engineering Geology II, chapter 4 ## Related in other subjects - [Site investigation for dam and reservoir](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Asite-investigation-for-dam-and) — Engineering Geology I, chapter 1, asked 2× - [Methods of surface and subsurface site investigation](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Amethods-of-surface-and-subsurface) — Engineering Geology I, chapter 1, asked 4× - [Tunnel alignment survey and ideal bridge location](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnel-alignment-survey-ideal-bridge-location) — Transportation Engineering II, chapter 5 - [Hydraulic design of tunnel; alignment criteria](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-hydraulic-design-alignment-selection) — Hydropower Engineering, chapter 6