A Canal through Limestone?

Limestone is a permeable rock, since it has cracks through which water can pass.

The Sapperton Tunnel, on the Thames and Severn Canal, tunnels through the limestone rock so that the canal can pass through the Cotswold Hills.

The 3½ km tunnel took 5½ years to build. Along the line of the route, 25 shafts were dug down from the ground surface and the tunnel was excavated from the bottom of these, until they were all connected together. Once the alignment had been checked, the tunnel was enlarged to the correct size.

Before water could be allowed in, the channel had to be made watertight. This was done by lining it with clay, an impermeable rock that doesn’t let water through. The channel was then finished with brick and the sections that were not through solid limestone were also given a brick roof.

Water could then be pumped in to this section of canal, enabling it to be used by boats.

The problem was that the rainwater soaking down into the permeable Cotswold Hills was leaking into the tunnel. This provided an additional water supply in winter, but in summer the springs dried up. Where the spring had pushed a hole through the clay canal lining, the canal water could then leak out through the same hole.

More water was needed to make up for leakage than to transport boats through the locks. The wind pump, by the Thames source, was replaced by a steam engine pump, which was used in the drier summer months, typically from June to October, and could deliver 3 million gallons a day into the canal.

There was a constant need for repairs to deal with the holes. Sections were eventually lined with concrete, but that was over 100 years after the canal first opened. Meanwhile loads of water had been taken from the ground near the source of the Thames. It had leaked back into the ground but not at the same place.

The Thames and Severn canal was closed in 1933, due to the need for repairs, and the Sapperton Tunnel is now impassable as the roof has collapsed in a number of places. You can find lots more information, including diagrams and pictures, on this page of the Cotswold Canals website.

Water shortages caused problems for boats both in the canal and in the river. Join me next week to find out more about travel on the Thames.