--- title: "Importance and types of tunnel lining" subject: "Hydropower Engineering" chapter: 6 key: "semester_7/hydropower_engineering:tunnel-lining-importance-types" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnel-lining-importance-types site: BCT Engineering --- # Importance and types of tunnel lining Hydropower Engineering · Chapter 6: Water Conveyance Structures - Asked in: 2080 Baishakh, 2+4 marks; 2069 Chaitra, 3 marks (asked 2 times) - Video: [Exploring Hydraulic Tunnels: Shapes, Design, Methods, and Support Structures ⛏️🌊 #tunnel #design#tbm](https://www.youtube.com/watch?v=pI254zTUVE8) — Hydropower - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnel-lining-importance-types - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6 ## Question Why is lining important in hydropower tunnels? Explain the importance and the different types of lining. ## Short answer Lining reduces friction, provides support, resists internal pressure, prevents leakage and erosion. Types: shotcrete (non‑pressure), plain concrete (20–40 cm, non‑pressure), reinforced concrete (pressure), steel (high‑pressure sections), precast segments (TBM, weak ground), and unlined with bolts in good rock. ## Answer #### Importance of lining in hydropower tunnels 1. **Reduces friction loss**: a smooth concrete surface has $n\approx0.013-0.015$ against $0.03-0.04$ for rough rock, so a smaller section can carry the same discharge with less head loss. 2. **Structural support**: carries the rock load and prevents falls of rock, slaking and collapse, especially in weak or jointed rock. 3. **Resists internal water pressure** in pressure tunnels: the lining (reinforced concrete or steel) transfers the pressure to the rock and prevents hydraulic fracturing. 4. **Prevents leakage** of water into the rock (loss of water, slope instability, damage to nearby structures) and prevents the entry of groundwater when the tunnel is empty. 5. **Protects the rock** from erosion and from deterioration, and prevents damage by high velocity flow. 6. **Smooth flow** reduces cavitation and vibration, and provides a durable surface for maintenance. #### Types of lining | Type | Features and use | |---|---| | **Shotcrete (sprayed concrete) lining** | Thin layer (5-15 cm), with mesh/fibre; quick, used for primary support and for non-pressure tunnels in good rock | | **Plain (mass) concrete lining** | Poured behind steel forms, 20-40 cm thick; used for non-pressure tunnels of medium quality rock | | **Reinforced concrete lining** | Used in pressure tunnels where internal pressure is high and rock cover is small, and in weak rock | | **Steel lining** | Used for high pressure sections (near the penstock, shafts and under low rock cover); filled with backfill concrete and contact grout | | **Precast concrete segments** | Used with a TBM in weak or soil-like ground | | **Unlined** (with bolts and shotcrete only) | In very good rock with low velocity; common in Norway; needs a large area | Grouting (contact and consolidation) is done behind the lining, and drainage holes relieve the external water pressure. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does lining reduce head loss? A. Smooth concrete has n about 0.013-0.015 against 0.03-0.04 for rough rock B. It increases the roughness C. It reduces the discharge D. It makes the tunnel longer Answer: A. Lower Manning n means less friction so a smaller section carries the same flow. (Key point: Friction reduction) 2. What does the lining do in a pressure tunnel? A. Resists internal pressure by transferring it to rock and prevents hydraulic fracture B. Increases internal pressure C. Is unnecessary D. Only reduces noise Answer: A. Reinforced concrete or steel lining transfers pressure to the rock and prevents hydraulic fracturing and leakage. (Key point: Pressure tunnel function) 3. Where is steel lining used? A. Non-pressure tunnels in good rock B. High-pressure sections near penstock, shafts and low rock cover C. Every tunnel D. Only in canals Answer: B. Steel lining with backfill concrete and grout is for the highest pressures or thin rock cover. (Key point: Steel lining use) 4. Which lining is a thin (5-15 cm) layer with mesh or fibre for quick primary support? A. Precast segments B. Steel lining C. Shotcrete D. Reinforced concrete Answer: C. Shotcrete is sprayed concrete, quick to place, used for primary support and non-pressure tunnels in good rock. (Key point: Shotcrete lining) ## Related in Hydropower Engineering - [Optimization of hydraulic tunnel diameter](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-optimization-diameter) — Hydropower Engineering, chapter 6, asked 2× - [Tunnel shapes: advantages and disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-shapes-advantages) — Hydropower Engineering, chapter 6, asked 2× - [Surge numerical: 4 m tunnel, 4000 m, 20 m³/s](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-tunnel-4m-4000m-20cumec) — Hydropower Engineering, chapter 6, asked 2× - [Surge numerical: 4.5 m tunnel, 2500 m, 25 m³/s](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-tunnel-4-5m-2500m-25cumec) — Hydropower Engineering, chapter 6, asked 2× - [RoR layout: pressure tunnel and free surface](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-layout-pressure-tunnel-free-flow) — Hydropower Engineering, chapter 2 - [Types of tunnel supports and necessity](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Atunnel-supports-types-necessity) — Hydropower Engineering, chapter 6 ## Related in other subjects - [Tunnelling in hard rock and lining types](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atunnelling-hard-rock-lining-types) — Transportation Engineering II, chapter 5 - [Criteria for designing and laying out tunnels](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Ahydraulic-tunnel-design-layout) — Hydro Power, chapter 5, asked 2× - [Bridge site factors; importance of tunnel ventilation](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-tunnel-ventilation-importance) — Transportation Engineering II, chapter 5 - [Tunnel advantages and pressure tunnel section](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Atunnel-advantages-pressure-tunnel) — Hydro Power, chapter 5