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Tunnel advantages and pressure tunnel section

Short answer

Tunnelsprovide shorter routes, are safe from landslides/floods, carry pressure flow, require low maintenance, cause minimal land disturbance, and rock can share pressure loads in pressure tunnels. A typical pressure tunnel cross-section includes a circular/horseshoe excavation, rock bolts and shotcrete support, concrete lining, grouting, invert, and drainage holes.

What are the advantages of tunneling in a hydropower project? Draw a typical cross-section of a pressure tunnel, indicating the key components.

A tunnel is an underground water conductor used as headrace or tailrace in hydropower projects, especially in the steep Himalayan terrain of Nepal.

Advantages of tunnelling

  1. Shorter route: a tunnel can cut straight through a ridge or across a river loop, giving a much shorter waterway than a surface canal following contours (e.g. Kali Gandaki A, Middle Marsyangdi).
  2. Safe from surface hazards: protected from landslides, rock falls, floods, debris flows and erosion, which often damage surface canals in Nepal.
  3. Can carry pressure flow: a pressure tunnel allows drawing water from a fluctuating reservoir level and reduces penstock length.
  4. Low maintenance: no damage from weather, cattle or people; little seepage in good rock.
  5. Environment and land: minimal land acquisition and surface disturbance.
  6. Good rock carries load: in sound rock the surrounding rock shares internal water pressure, so lining can be light or omitted.

Typical cross-section of a pressure tunnel

        surrounding rock mass
     .  .  .  .  .  .  .  .  .
   .     __________________    .
  .    /  rock bolt  ^       \   .
  .   / /  shotcrete ^  \  \  \  .
 .   | |   concrete lining | | |  .
 .   | |                    | | |  .
 .   | |   water (under    | | |  .
 .   | |    pressure)      | | |  .
  .   \ \                  / /  .
   .   \_\_______________/_/   .
    .  grout   invert  drain   .
      .  .  .  .  .  .  .  .

Key components:

  • Excavated profile (circular or horseshoe/D-shape) in rock.
  • Rock support: rock bolts, wire mesh and shotcrete to stabilise the opening.
  • Lining: plain or reinforced concrete (steel lining near the powerhouse where cover is low) to reduce friction and leakage.
  • Contact and consolidation grouting between lining and rock to fill voids and strengthen the rock.
  • Invert (floor) for construction traffic, and drainage holes to relieve external water pressure.

✓ Answer checked by Claude Opus 5.5 (high effort)

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