Tunnel Inspections

Peter Simpson has been the Canal & River Trust’s principal engineer for its tunnels and culverts department for the past seven years.

It was called ‘tunnels and mining’ until quite recently, the name change reflecting the trust’s growing awareness that the 2,000 historic culverts under its 2,000 miles of waterways were a hostage to fortune as they aged.

Yet the culverts are hidden and rarely noticed by boaters unless they fail – it’s the tunnels that catch public attention. Peter says there are 57 navigable tunnels, most of them built historically, with only four constructed in the last few years (three on the modern Liverpool Link). These total around 30 miles of the whole of CRT’s network.

CRT grades its assets in two ways: by their public impact were they to fail (on a scale of one to five, with one being minor and five being disastrous), and their physical condition (graded A to E, with A being perfect/new and E in imminent danger of failure).

Harecastle Tunnel

Recently, I was able to join the tunnel team as they explored the Trent & Mersey’s Harecastle Tunnel on one of its two-yearly inspections.

The first thing I noticed was the fat spiral-bound printout of the previous inspection report. Tunnels generally undergo a biannual survey, although the longer, more complex Standedge is checked every year.

The inspectors say Harecastle, being still in grade C condition, has lasted well, despite the occasional unnerving gaps where springs chuckle through the brickwork. Its closure would be a major problem, akin to a breach, but the CRT team thinks it will hold up.

Lacking tunnel ventilation shafts, Harecastle is remarkable in being the only tunnel with forced ventilation. The south portal doors are shut and air is extracted as boats pass through, leading to a marked draught.

Assessing condition

Tunnel condition can vary widely, not simply from construction or age, but also local circumstances. For example, the two tunnels on the Caldon Canal have shown very little deterioration over the years, which it attributes to the water quality. Those on the younger Llangollen need more maintenance, perhaps because of the steady water flow which is likely to wear away soft brickwork.

New technology is making a major contribution – not so much in carrying out the actual work, but in surveying and estimating the condition of tunnels. Two technologies in particular – SONAR below the waterline and LIDAR above it – can give very accurate pictures of the state of a tunnel, and particularly its movement. This includes not only the bore but, just as importantly, the invert (the shape of the channel below). Whereas tunnel headroom can slowly lower over the years, the channel profile can swell upwards.

This is an extract of the 'Tunnels of love' article that appears in Waterways World April 2025