May 30th - I spent the morning at work, then late afternoon, cycled into Birmingham to meet with a couple of colleagues. I headed down to the canal at Tyburn for a decent run, and followed the canal into central Birmingham.
One of the little-known features of canals that helped keep them open for years after their commercial value declined was the fact that under their embankments and towpaths, utilities found a direct route through cities for their pipelines and cables. By laying services alongside the waterways, many of the issues with traffic and complexity of installation could be avoided.
Along the canals here in Birmingham run gas, high voltage electricity, fibre optic telecommunications links, and petrochemical fuel pipelines. The keen-eyed explorer can often spot the evidence on the surface in the form of markers, bulkheads, valve access chambers and other infrastructure.
These small, concrete gas pipeline markers indicate a high pressure gas pipeline runs below, but also serve another purpose, as can be seen in this example which has had the cover prised off. The pipeline is metal, and to prevent corrosion, a system of cathodic protection is applied; a low voltage DC supply is connected between the pipe and electrodes - called anodes - in the ground nearby. The flow of current between the anodes and the pipe means that the anodes corrode sacrificially instead of the metal they’re protecting.
A similar system is being fitted to metalwork in the supports of Spaghetti Junction to protect it from water damage.
The markers - about 50m apart along the towpath - house junction boxes for the protection wiring and test points, and contain a magnetically operated switch for test purposes. Large brass studs in the side allow connection of test equipment.