Tunnel inspection: recording the inside of a tube with no GNSS and no light

Tunnel deterioration falls into lining concrete defects (cracking, delamination and spalling, material degradation), water problems (leaks, icicles and ice on walls in winter), deformation from external loads (ground pressure pushing the lining in, floor heave), and problems with fixtures (corroded or loose bolts and anchors for lighting, jet fans, signs and cables). Older tunnels built with timber lagging often have voids left behind the lining.

After Sasago, periodic inspection of road tunnels in Japan every five years, based on close visual inspection, became mandatory from July 2014. For the lining, hammer sounding to find delamination is as important as looking. As with bridges, each tunnel is rated I (sound) to IV (urgent action). Japan has about 11,000 road tunnels (end of FY2024), many built during the high-growth decades.

The pain is that you cannot get close without shutting the tunnel. Reaching the crown means bringing in aerial lifts under one-lane or full closure. For mountain tunnels with no detour, that affects whole communities. Rail tunnels squeeze the work into the short maintenance window between the last and first trains.

For systematic imaging of an ordinary road tunnel lining, vehicle-mounted scanners are currently faster and more consistent. Drones really pay off where vehicles cannot go and people struggle to: ventilation shafts, inclined adits, hydropower headrace tunnels, abandoned old tunnels, and the inside of a tunnel after a collapse or fire when people cannot be sent in.

The basic method is to carry lighting, lock exposure and advance at a constant stand-off. A tunnel is a tube, so flying separate passes for the crown, shoulders and walls makes it easier to build an unrolled lining image later. Capturing distance markers or luminaire numbers in frame makes registration much easier.

Inside a tunnel the drone loses most of its senses. GNSS disappears a short way past the portal, and it is dark. Shafts and ducts are confined. Add exhaust soot and dust, wind pushed by traffic or trains, and the jet of ventilation fans. Around a curve, radio links fade too.

• GNSS-denied: use aircraft that localise from LiDAR or cameras. Beware that a long, straight, featureless tube looks the same everywhere and causes drift; add landmarks such as distance markers. GNSS-denied
• Darkness: onboard lights are all you have. Match light power, direction and fixed exposure to the previous survey. Darkness
• Confined spaces: shafts, ducts and headraces have close walls. Use caged aircraft that survive contact, and battery plans with margin to get back. Confined spaces
• Dust & smoke: soot fouls lenses and adds noise to LiDAR returns. Stop ventilation or avoid entering downwind. Dust & smoke
• Links: curves and long sections break command and video links. Plan repeaters or tethered aircraft, and set the lost-link behaviour (return or hover).

What the operating side should decide in advance for the moment position or link is lost deep inside a tunnel is covered in how operators prepare for losing position or link (Airffic Flight Institute).

Tunnel images become useful once stitched into an unrolled lining map. Thousands of frames are chained by distance from the portal, cracks, leaks and suspected delamination are mapped, and the result is compared with last time. AI is good at proposing cracks, measuring length and width, and showing change. Whether something is delaminated, whether to knock it down and whether to close the tunnel are the responsible engineer’s calls.

Road tunnels fall under the Road Act’s periodic inspection: every five years, close visual inspection, with sounding for the lining. The February 2019 revision kept close visual inspection as the basis while opening the way to inspection-support technologies (generally described as methods the inspector judges to give an equivalent diagnosis), but sounding is still needed in many cases to confirm delamination. Rail tunnels are inspected periodically by operators under railway structure maintenance standards.

Japan’s official interpretation treats a tunnel whose inside is clearly separated from the outside as indoor, outside the aviation flight rules, but the position near portals or if the drone strays outside depends on the situation — check with the Civil Aviation Bureau and the owner rather than deciding yourself. Flying under a closure also needs agreement with the road operator and a police road-use permit. In the United States the federal National Tunnel Inspection Standards (NTIS) adopted in 2015, require routine inspection of highway tunnels, normally every 24 months; in Europe, after the 1999 Mont Blanc tunnel fire, a 2004 EU directive set safety requirements for tunnels on the trans-European road network. Compare countries in world regulations.

• Sounding: delamination is identified by the sound of tapping. Drone sounding devices are still at research and trial stage, not a general substitute.
• Removing loose pieces: knocking down delaminated concrete on the spot is still a job for people on aerial lifts.
• Voids behind the lining: photos cannot see them; GPR or other surveys are needed.
• Systematic imaging of long road tunnels: vehicle-mounted scanners are faster and more consistent.
• Rating and traffic decisions: interpreting defects and deciding closures remain human jobs.

In the Airffic editors’ view, it goes like this. When tunnel lighting was switched to LEDs, positioning markers went into the ceiling too. During a short night closure, a drone flies the crown and fixture mounts in one pass, and its images are overlaid on the scanner’s lining map. Only what changed since last time lights up red, and inspectors go and tap just there.

Shafts and headraces are no longer places people climb down into. A caged aircraft dives in once a year and brings back point clouds and images. People spend their time on what the changes mean and on the order of repair and renewal. Tunnel inspection turns from going into the dark to bringing the dark into view.