What is a pulled tunnel?
A pulled tunnel (Dutch: trektunnel) is a relatively small tunnel that is drawn horizontally through the ground beneath a road, embankment or railway while traffic continues to run above it. The method is used in the Netherlands mainly for cycle and pedestrian underpasses and short culverts. No open excavation across the road or track is needed. It is related to jacked box tunnelling, where the structure is pushed from a launch pit rather than pulled.
How it works
- Next to the road or railway, precast concrete tunnel sections are lined up on a steel frame and post-tensioned into a single unit.
- A cutting edge is fitted at the front: a steel shield that cuts into the soil and keeps the excavation face stable.
- On the far side of the road an anchorage is installed, for example a sheet pile wall. From there, hydraulic jacks pull the tunnel step by step beneath the road using rods running through the tunnel.
- Bentonite is applied to the outside to reduce friction with the soil.
- Soil entering the tunnel is dug out with small plant and removed.
- After pulling, any voids around the tunnel are filled with grout.
Advantages
- The road or railway stays open; no lengthy diversion or possession.
- Little disruption to the surroundings.
- The ground above the tunnel is largely undisturbed, so the risk of settlement is lower.
- The soil cover above the tunnel can be shallow.
- Pulling can be done in stages, for example at night.
Disadvantages and limits
- Only suitable for short tunnels.
- The work must take place above the water table. High groundwater makes it difficult and the tunnel may float; dewatering is then often needed.
- Space is required on both sides of the road for the tunnel sections and pulling equipment.
- Mainly intended for cyclists and pedestrians; vehicle tunnels usually need a heavier design.
Related terms
- Bentonite
- Sheet pile wall
- Buoyancy uplift
- Absolute settlement
- Grout
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