Last reviewed 30 Sept 2026 · 9 min read
Why a lining
The rock and the temporary support may stand for a long time, but a tunnel that will serve for 100 years needs a lining that:
- provides permanent structural support, including against long-term deterioration of the rock or the primary support;
- gives water-tightness (or controlled drainage);
- provides a smooth surface for hydraulic or ventilation efficiency (water tunnels) and for safety (fire, maintenance);
- protects the primary support from corrosion and the interior from falling rock;
- houses services (lighting, ventilation, drainage, signalling cables).
Lining systems
Primary and final (secondary) lining
In the drill-and-blast and NATM tunnels, the primary (initial) lining — shotcrete, bolts and ribs — supports the rock during construction and forms part of the permanent support. The final (secondary) lining is a thicker concrete layer placed inside the primary lining after the deformation has stabilised.
- Cast-in-situ concrete lining: plain or reinforced concrete placed behind a steel form (shutter) carriage moving along the tunnel; the thickness is typically 250–500 mm (thicker for pressure tunnels, in weak rock or where the lining is designed to carry the full load). Pumped concrete is placed through the crown and sides; contact grouting fills the crown gap later.
- Sprayed concrete inner lining (a second layer of shotcrete) — used in some tunnels, with the waterproofing membrane between two shotcrete layers (sandwich system).
- Precast segmental linings — rings of precast reinforced concrete segments (typically 5–7 segments plus a key) erected inside a TBM shield; the ring is gasketed (elastomeric seals between segments), bolted or connected by dowels, and the void behind the ring is filled by tail-void grout pumped immediately. Used for soft ground and long TBM tunnels.
- Steel ribs with lagging, cast-iron segments — older methods.
Single-shell vs double-shell
| System | Description |
|---|---|
| Double-shell | Primary lining + membrane + secondary lining; the primary is treated as temporary or partly permanent; a robust and traditional system |
| Single-shell (permanent shotcrete) | Only one shotcrete lining, designed for the whole life; economical but demanding on the shotcrete quality and water-tightness |
| Segmental single-pass lining | Precast segments as the only lining (TBM tunnels) |
Design of the lining
The lining is designed for:
- Rock or soil load — from the rock-load methods, the convergence–confinement analysis or numerical models; for soft ground the full overburden and water pressure.
- Water pressure — in an undrained (fully waterproofed) tunnel the lining carries the full hydrostatic pressure at the depth below the water table; in a drained tunnel the pressure is relieved.
- Internal pressure in pressure tunnels (hydropower and water supply) — the lining acts with the rock as a thick-walled cylinder.
- Seismic and temperature effects, fire, construction loads (grouting pressure, segment handling), long-term creep and shrinkage.
Simple analysis
A circular lining under a uniform external pressure carries a hoop thrust (thin ring), and the compressive stress is ; the lining is largely in compression, which is why plain or lightly reinforced concrete is adequate. For non-uniform loads (soil with a lateral coefficient ) the ring is analysed for bending moments (with the Muir Wood or Duddeck–Erdmann methods, or with a finite element model of the ring with soil springs).
For pressure tunnels the thick-walled cylinder formulae give the tangential stress at the inside face — the critical value for cracking; the surrounding rock shares the load, and cracking of the lining is accepted if the rock can carry the load (with a check on leakage).