--- title: "Geology in tunnel site selection" subject: "Engineering Geology II" chapter: 4 key: "semester_4/engineering_geology_ii:tunnel-site-selection" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/tunnel-site-selection site: BCT Engineering --- # Geology in tunnel site selection Engineering Geology II · Chapter 4: Engineering geology in site selection, investigation & construction/excavation - Asked in: 2069 Bhadra, 5 marks; 2078 Poush, 3 marks; 2076 Baisakh, 3 marks; 2068 Magh, 3 marks (asked 4 times) - Video: [Geological Investigation For The Site Selection Of Tunnels](https://www.youtube.com/watch?v=UKnqVaR3M5M) — Gaurinanda.A _Official_ - 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-site-selection - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-4 ## Question What engineering geological factors (considerations) should be assessed for tunnel site and alignment selection? ## Short answer Hard, massive, homogeneous rocks like granite are best; tunnel axis should cross strike at a large angle, avoiding faults, thrusts and high groundwater inflow. Adequate overburden for stability, portal sites on stable slopes; assessed by mapping, geophysics and drilling. ## Answer A tunnel passes through the ground, so the rock mass is both the load and the structure. Site and alignment are selected on the basis of the following. #### Geological factors 1. **Rock type and strength:** hard, massive, homogeneous rocks (granite, gneiss, quartzite, limestone) are best; avoid soft, weak or swelling rocks (shale, phyllite, clay). 2. **Geological structure:** the tunnel axis should cross the strike of beds at a large angle; avoid alignment parallel to the strike, folds and fault planes. Avoid faults, shear zones and thrusts (MBT, MCT). 3. **Rock mass quality:** RQD, RMR, Q values; joint spacing and orientation. 4. **Groundwater:** inflow, high pressure and water-bearing fractures or karst increase risk; select alignment above the water table when possible. 5. **Overburden (rock cover):** enough cover for stability but not too deep (squeezing, rock burst); low cover under rivers and valleys is a weak zone. 6. **Portal sites:** stable slopes with good rock, free from landslide, rock fall and flood. 7. **Weathering and alteration** at the portals and along the route. 8. **Special hazards:** gas, hot water, high temperature, rock burst, squeezing, karst cavities. 9. **Seismicity and active faults.** 10. **Construction facilities:** access, muck disposal and water supply. Selection is based on mapping, geophysics (seismic refraction, resistivity), exploratory drill holes and geological sections along the alignment. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which rocks are best for tunnelling? A. Shale and clay B. Phyllite C. Swelling rocks D. Hard, massive, homogeneous rocks such as granite, gneiss, quartzite Answer: D. Weak and swelling rocks cause squeezing and swelling. (Key point: Rock type) 2. How should the tunnel axis be oriented to structure? A. Crossing the strike of beds at a large angle B. Parallel to the strike C. Along a fault plane D. Along fold axes Answer: A. Parallel alignment exposes long weak planes. (Key point: Axis orientation) 3. What is the ideal rock cover? A. As shallow as possible B. Enough for stability but not so deep as to cause squeezing or rock burst C. As deep as possible D. Irrelevant Answer: B. Shallow cover below rivers is weak; very deep cover raises stress. (Key point: Overburden) 4. What makes a good portal site? A. Slide-prone debris slope B. Flood channel C. Stable slopes of good rock free from landslides and floods D. Weathered colluvium Answer: C. Portals are vulnerable to instability. (Key point: Portals) 5. Which are special tunnel hazards? A. Only dust B. Only noise C. Only traffic D. Gas, hot water, rock burst, squeezing and karst cavities Answer: D. These must be assessed early. 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