Building the Canal - Building Methods, their Problems and Solutions

Surveyors

John Rennie and the committee of management appointed different surveyors at various times to survey the possible and chosen routes.

Each surveyor did this either on foot or on horseback, making measurements and calculations for every mile travelled.

At its western end the building route passed through hilly country and the only way the canal could be constructed at this point was to use the same contour level either side of the river Avon, and build aqueducts which twice carried the canal across the river valley from one side to the other.

This pioneering work established a base line from which the later railway builders could further develop arrangements for constructing similar structures.

Aqueducts and bridges

In addition to aqueducts, 111 bridges were built, mostly of stone at the western end and of brick, east of Devizes where locally quarried stone was less plentiful or suitable.

Dundas Aqueduct

Where aqueduct building in particular was concerned some vested interests had to be accommodated.

The original plan was to build the two Avon valley aqueducts of brick.

However the united voice of canal company shareholders who also had interests in stone quarries, forced John Rennie to build these structures of stone.

Whilst the large aqueducts were major engineering projects, the smaller bridges that allowed access over the canal, were simpler to build.

These were often constructed on a timber frame that was removed after completion of the building work.

Cutting the channel

Picks and shovels together with wheelbarrows for removing waste, were the main tools for digging the channel.

However in deep cuttings, arrangements for dragging men with their barrows up the sides of the cut using horse-powered winches were developed.

Railway builders subsequently used these later methods when the railway system was being constructed.

Tunnels

Tunnels posed particular problems that frequently elicited simple but effective solutions.

The route of a tunnel for example, would be surveyed across the top of the ground and at fixed points the rise would be calculated from the measured elevation.

Sinking shafts at these points to a depth of the calculated rise, would then produce a level for the tunnellers.

Direction for the tunneling work would be taken from plumb lines lowered into the shafts, and these vertical lines would be positioned at the surface to correspond with the surveyed direction.

The plumb lines thus gave the direction at the bottom of the shaft and the result would be a straight tunnel in the right direction.

Avoiding water loss

To minimise water loss the canal channel was lined with clay, usually to a depth of 3 feet (approximately 1 metre).

The hard, quarried clay was broken into lumps, spread in the channel and wetted.

Cattle would then be driven over the clay so that it was "puddled" into a malleable consistency, which allowed it to be more readily used.

Keeping a level course

Early canals followed a single contour as much as possible to reduce the number of locks but this led to some strange meandering routes.

By the time that the Kennet and Avon Canal was built a "cut and fill" technique had become the accepted method for maintaining a level.

Using this technique a cutting would be made through high ground and the spoil used to build an embankment across lower lying terrain.

In practice though the canal builders needed to be flexible, and former methods were sometimes still used if considered appropriate at the time.