--- title: "Geological documentation in tunnelling" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:tunnel-geological-documentation" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/tunnel-geological-documentation site: BCT Engineering --- # Geological documentation in tunnelling Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2080 Chaitra, 5 marks; 2077 Chaitra, 3 marks; 2069 Bhadra, 4 marks; 2071 Magh, 4 marks; 2079 Asoj, 2 marks; 2078 Poush (asked 6 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/tunnel-geological-documentation - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question Describe the engineering geological documentation carried out during underground (tunnel) excavation and explain its importance. ## Short answer During tunnelling, geological documentation records rock type, discontinuities, groundwater, and rock mass classification at the face after each round. This mapping checks design assumptions, guides support selection, provides a contract record, and ensures safety by warning of unstable zones. ## Answer **Geological documentation** in tunnelling is the systematic recording of the geology encountered in the excavated face, walls and roof, during construction, in the form of maps, logs, photos and reports. #### What is documented (face and tunnel mapping) 1. Rock type, colour, grain size and mineral content. 2. Rock strength and weathering grade. 3. Discontinuities: bedding, joints, foliation, faults and shear zones, with strike, dip, spacing, persistence, roughness, aperture and filling. 4. Groundwater: inflow rate, location, seepage, water quality. 5. Rock mass classification (RMR, Q) of each round. 6. Overbreak and instability: falls, squeezing, rock burst, gas. 7. Support installed (bolts, shotcrete, steel ribs) and any problems. 8. Photographs, sketches and plan/section of the tunnel (usually at 1:100-1:200), drawn after each blasting round, with chainage. #### Procedure - Mapping is done at the face right after excavation and before shotcrete covers it. - Data are plotted on the tunnel log (development map of roof and walls) and longitudinal geological section. - Prediction of the geology ahead using probe drilling and seismic methods. #### Importance 1. Checks design assumptions and the pre-construction geology. 2. Choice and modification of support based on the actual rock class. 3. Provides basis for payment, claims and contract disputes. 4. Safety: warns of unstable zones, water and gas. 5. Provides a permanent record for maintenance, future repair and for similar projects. 6. Helps to predict conditions ahead and plan construction. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is recorded in tunnel face mapping? A. Only the colour of rock B. Rock type, discontinuities, groundwater inflow, rock class and support installed C. Only the cost of excavation D. Only the tunnel length Answer: B. Mapping covers geology, water, classification (RMR, Q) and support. (Key point: What is documented) 2. When should the face be mapped? A. After lining is completed B. Before blasting C. Right after excavation and before shotcrete covers it D. Only at the end of the project Answer: C. Once shotcrete covers the face, the geology can no longer be seen. (Key point: Timing) 3. Why is documentation important for support design? A. It removes the need for support B. It increases overbreak C. It is used only for decoration D. Support is chosen or modified based on the actual rock class Answer: D. Actual conditions are compared with design assumptions. (Key point: Importance for support) 4. What other use does documentation have? A. Basis for payment, claims and a permanent record B. Reducing water inflow C. Replacing probe drilling D. Stopping blasting Answer: A. Records settle disputes and help later repair and similar projects. (Key point: Contract and records) 5. How is geology ahead of the face predicted? A. Counting the bolts B. Probe drilling and seismic methods C. Reading the lining D. Surface photography only Answer: B. These help plan construction ahead. (Key point: Prediction) ## Related in Engineering Geology II - [Why tunnel documentation is done](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Awhy-tunnel-documentation) — Engineering Geology II, chapter 4 - [Geological problems in tunnel construction](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Atunnel-construction-problems) — Engineering Geology II, chapter 4, asked 3× - [Short note: Underground excavation process](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Ashort-note-underground-excavation) — 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 - [Overbreak and tunnel geological survey](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aoverbreak-tunnel-survey) — Engineering Geology II, chapter 4 - [Engineering geological problems of Siwalik zone](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Asiwalik-problems) — Engineering Geology II, chapter 2, asked 4× ## Related in other subjects - [Underground water tank: excavation, brick, plaster](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Aug-tank-g-brick-plaster) — Estimating and Costing, chapter 6 - [Methods of tunnel excavation](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atunnel-excavation-methods) — Construction Management, chapter 4 - [Short note: Tunnel excavation by blasting](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atunnel-blasting-note) — Construction Management, chapter 4 - [Engineering geological map and site investigation](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aengineering-geological-map-and-site) — Engineering Geology I, chapter 1, asked 3×