--- title: "Tunnelling methods; shotcreting" subject: "Hydropower Engineering" chapter: 6 key: "semester_7/hydropower_engineering:tunnelling-methods-shotcrete" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnelling-methods-shotcrete site: BCT Engineering --- # Tunnelling methods; shotcreting Hydropower Engineering · Chapter 6: Water Conveyance Structures - Asked in: 2071 Chaitra, 4+2+2 marks - Video: [Shotcrete Complete Guide | Wet Shotcrete vs Steel Fiber Reinforced (SFRS) | Tunneling & Slope Works](https://www.youtube.com/watch?v=tj7RzFB4EQs) — ACoreConcept - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/tunnelling-methods-shotcrete - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-6 ## Question Discuss various tunnelling methods used in hydropower projects. What is the purpose of shotcreting? Discuss the procedure. ## Short answer Methods include drill-and-blast, TBM, NATM, and mechanical excavation. Shotcrete seals the surface, provides immediate flexible support, fills cracks, bonds rock blocks into an arch, and reduces overbreak. Procedure: scale loose rock, wash surface, install mesh or fibres, spray wet-mix or dry-mix concrete in layers holding nozzle 1-1.5 m away at right angles, then cure and test. ## Answer #### Tunnelling methods in hydropower projects 1. **Drill and blast**: drilling, charging, blasting, ventilation, mucking, scaling and support. It is the method used most in Nepal. Large faces are excavated by top heading and benching. 2. **Tunnel boring machine (TBM)**: full-face rotary excavation with a segmental lining; fast, and gives a smooth tunnel, but costly, and it needs uniform ground and a long drive. 3. **NATM**: excavation with immediate shotcrete, rock bolts and mesh, and monitoring of the deformation, which lets the rock carry part of the load. 4. **Road-headers or hydraulic breakers** in soft rock, and **cut-and-cover** at shallow portals. #### Purpose of shotcreting **Shotcrete** is concrete sprayed at high speed onto the excavated rock surface. - Seals the surface, preventing weathering, slaking and loosening of the joints. - Gives immediate support and carries the rock load together with bolts and mesh. - Fills the cracks and bonds the rock blocks into a ring (arch action). - Smooths the surface and reduces overbreak and hydraulic roughness (as a lining of non-pressure tunnels). - Gives quick, flexible support at low cost. #### Procedure of shotcreting 1. **Scale** the loose rock and wash the surface with an air-water jet to remove dust and loose particles. 2. Install **mesh** or add steel/synthetic **fibres**, and set the thickness gauge pins. 3. Prepare the mix (cement, aggregate up to 10 mm, admixtures, accelerator) by the **dry-mix** process (water added at the nozzle) or the **wet-mix** process (pumped wet concrete). 4. **Spray** with the nozzle held 1-1.5 m from the surface and at right angles to it, moving in small circles. Spray from the walls upwards to the crown, in layers of 5-10 cm; apply the second layer after the first has set. 5. Control the **rebound** (material that falls away) and remove it; do not reuse it. 6. **Cure** the shotcrete by spraying water or a curing compound for several days. 7. **Quality tests**: thickness checks, strength tests on cores or panels, and bond tests. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does shotcrete do for the rock surface? A. Only decorates the surface B. Increases roughness C. Drains the water D. Seals it, gives immediate support and bonds blocks into a ring Answer: D. It seals against weathering and loosening and forms an arch ring with bolts and mesh. (Key point: Shotcrete purpose) 2. What distinguishes dry-mix from wet-mix shotcrete? A. Wet-mix has no cement B. Dry-mix uses steel only C. Dry-mix adds water at the nozzle; wet-mix pumps ready-mixed concrete D. No difference Answer: C. In dry-mix water is added at the nozzle, in wet-mix wet concrete is pumped. (Key point: Dry vs wet mix) 3. Which spraying practice is correct? A. Spray from far away B. Nozzle 1-1.5 m away at right angles, walls up to crown, layers of 5-10 cm C. Spray at a sharp angle in one thick layer D. Nozzle touching the rock, crown first Answer: B. Right angle and proper distance reduce rebound; walls first prevents falling material. (Key point: Spraying technique) 4. What is done with rebound? A. Left to form lining B. Removed and not reused C. Mixed with fibres D. Reused in the next batch Answer: B. Rebound is poor quality material and must be removed. (Key point: Rebound) 5. What comes before spraying and after it? A. Scaling and washing before; curing with water for several days after B. Blasting before; none after C. Painting before and after D. Curing before; scaling after Answer: A. Loose rock is scaled and washed for bond, and curing prevents shrinkage cracks. 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