Underground tunnels and utility conduits: inspecting the city’s hidden corridors without walking every kilometre

This guide covers utility conduits (common ducts) that carry power, telecom, water and gas lines under roads, cable tunnels owned by power and telecom companies, and underground flood channels and storage tunnels. They are reinforced-concrete boxes or circular tunnels, and the root of their deterioration is water.

Inspection today centres on patrols and visual checks on foot. Inspectors shine torches on the roof and walls, look for leaks, cracks and rusted fittings, and tap anything suspicious. The more carefully it is done, the longer it takes — walking kilometres of tunnel out and back.

Underground there is also the risk of oxygen deficiency and toxic gas. Ventilation and gas measurement are needed before entry, and with few exits, getting out from deep inside is hard if something goes wrong. Reducing the distance people walk translates directly into safety.

Underground, the drone’s job is to fly the long distances people would otherwise walk and capture the roof with consistent quality. Flying near the roof shows cracks and leak stains far better than looking up from the floor.

Underground tunnels combine four challenges: no GNSS, darkness, tight space and gas. Surrounded by concrete and soil, radio does not carry far, and both control and video links tend to drop beyond corners.

• GNSS-denied: localise with LiDAR SLAM or visual-inertial odometry. Position drifts on long featureless straights, so re-anchor at entrances, ID plates and equipment. GNSS-denied
• Confined space: cable racks and pipes protrude from both sides, so the clear width is often narrower than the drawings show. Use guarded aircraft and decide in advance how to handle contact. Confined spaces
• Darkness: carry lights on board. Turning on tunnel lighting can make images less consistent through shadows and reflections. Darkness
• Oxygen deficiency & toxic gas: a drone can go in first to look, but ventilation and gas measurement before people enter cannot be skipped. Oxygen deficiency & toxic gas
• Radio links: decide beforehand what happens when the link drops — relay nodes, a tether, or autonomous flight that returns on its own.

In long tunnels, the most important thing is to know for certain where every image was taken: which chainage, which wall, what height. Images without that, however many thousands, cannot be compared at the next inspection.

In Japan, utility conduits are built by road administrators under the Act on Special Measures concerning the Construction of Common Utility Ducts, while the facilities inside are managed by each utility. A key feature is that responsibility is split: the road administrator inspects the structure, each utility inspects its own lines. Frequency and methods are set by each owner’s inspection manual.

For flying, Japan’s Civil Aeronautics Act does not cover flight in enclosed indoor spaces; the official interpretation treats tunnels and underground passages whose inside is clearly separated from the outside as indoor, so flying only inside an underground tunnel usually needs no aviation permit or approval. Entering the tunnel, however, is governed by occupational safety rules such as the Ordinance on Prevention of Anoxia, and entrances on the street need road-use permission. Compare countries in world regulations.

• Sounding & touch: delaminated concrete is found by tapping. Images do not reveal it.
• Behind cables and pipes: walls behind densely packed services are out of reach and out of view.
• Equipment function tests: whether fans and pumps run and alarms sound must be tested by people.
• Inspecting gas pipes and cables: the services themselves are inspected with each utility’s specialist methods.
• Rating and accountability: interpreting defects and deciding who repairs what remains a human job.

What follows is the editors’ outlook. Each tunnel entrance has a small hangar, and drones fly fixed sections every night. Positions are matched to ID plates and point clouds, so images overlay last time’s exactly. When a new leak stain appears, the road owner and the utilities whose lines run past are notified at the same moment.

People no longer walk kilometres every day. Instead they go to confirm what has changed and organise repairs. Underground inspection shifts from walking dark corridors endlessly to watching over the city’s hidden side as a whole.