Criteria for designing and laying out tunnels

A hydraulic tunnel is an underground water conductor (headrace, tailrace, diversion or pressure tunnel) used where a surface canal is not feasible because of steep, unstable terrain or to shorten the route. Its layout and design must satisfy geological, hydraulic, structural and economic criteria.

Layout (alignment) criteria

1. Shortest practical route between intake and surge tank/powerhouse, to reduce cost and head loss.
2. Good geology: pass through sound, uniform rock; avoid faults, shear zones, thrusts, weak and squeezing rock, karst and water-bearing zones. Cross weak zones at right angles.
3. Adequate rock cover: for unlined pressure tunnels, the weight of the rock above must exceed the internal water pressure. Norwegian criterion: minimum rock cover $C_{RM} = \dfrac{h\gamma_w F}{\gamma_r\cos\beta}$, where $h$ = static water head at that point, $\gamma_r$ = unit weight of rock, $\beta$ = slope angle of the valley side and $F$ ≈ 1.3–1.5 (factor of safety).
4. Stable portals and adits: portals in sound rock, away from landslides and flood levels; adits to give more working faces and reduce construction time.
5. Straight alignment with gentle curves (radius ≥ 5 times the diameter) to reduce head loss.
6. Gradient: enough slope for drainage during construction (about 1:200–1:500); free-flow tunnels follow the hydraulic gradient.

Design criteria

1. Type of flow: pressure tunnel (full flow, circular or horseshoe) or free-flow tunnel (D-shaped/horseshoe, flowing 70–80% full with free-board area of about 15–25%).
2. Size: economic diameter found by comparing excavation/lining cost with value of head loss; minimum size about 2–2.5 m for construction access.
3. Velocity: unlined 1.5–2.5 m/s; concrete-lined 2–4 m/s; steel-lined higher. Velocities must avoid erosion but limit head loss.
4. Shape: circular best for internal pressure; horseshoe/inverted-D easier to excavate by drill and blast and good for external rock pressure.
5. Lining and support: shotcrete, rock bolts, steel ribs, concrete or steel lining depending on rock class (Q-system, RMR), internal pressure and leakage control. Smooth lining reduces Manning's n.
6. Head loss: friction and form losses kept within economic limits.
7. Water hammer and surge: pressure tunnels need a surge tank; check minimum pressure so the crown never goes under negative pressure.
8. Sediment and rock traps near the end of unlined tunnels.
9. Drainage, grouting and access for inspection and maintenance.