Viewers of the UK’s ITV programme “Barging (sic) around Britain” recently may have seen the well-known broadcaster (if not boater) John Sargent cruising along the Grand Union Canal over the busy North Circular Road in London. In the same vein, it has been possible to boat along the Basingstoke over the A331 Blackwater Valley bypass (right), since its completion in 1995. These significant feats of canal aqueduct engineering have made it possible for these two (and of course, other) waterways to be suspended over roadways where drivers are generally oblivious to what is over their heads…
The Ash Aqueduct was built to allow the construction of the bypass road under it, and is cut into the original Ash Embankment. The embankment was constructed from the excavation of the cutting at Deepcut, and is itself a significant construction achievement for its time (as can be seen, left), all undertaken in the early 1790’s with horses and carts, wheelbarrows and hand shovels, and many, many navvies – it must have been back-breaking work.
All this comes to mind following the recent closure of the Ash Aqueduct for scheduled inspection and maintenance, which is described in the Spring edition of the Basingstoke Canal News.
The aqueduct too was a significant engineering project, for which those who were key players in its design received due accolades. We should also acknowledge the fact that the Society, through Stan Meller, then our special projects engineer, made a significant contribution to how the crossing should be designed to minimise the impact on the canal itself. The aqueduct construction project is described in detail in an early Society website article.
A number of designs for the canal crossing the road were considered. Taking the road over the canal, either by constructing a high bridge, or lowering the canal with a stair-case of locks were each rejected. Eventually it was agreed to take the road under the canal. One proposal for an aqueduct was something along the lines of the railway bridge that currently crosses the M25 just south of the M3 Junction – that is, all overhead suspension and inelegance (right). It was fortunately rejected.
What was eventually agreed upon was much more pleasing on the eye (left), consisting of a pre-stressed reinforced concrete channel, supported in compression by 4 steel cables along each side, each cable tensioned to 820 tonnes.
This provides adequate support for the canal in water, plus redundancy in the case of failure (the reason for the recent inspection was primarily to check the state of the cables).
Construction of the new A331 itself was not without its challenges. In order to give sufficient headroom below the aqueduct, the road had to be “sunk” below the 100-year flood level, and so is built on a thick concrete raft to prevent it from floating in extreme groundwater conditions. Provision had also to be made to prevent the hollow filling with water.
Like most good construction projects, the Ash Aqueduct has receded into the background. In fact, it is rare for users of the bypass to notice boats cruising overhead, even if you wave at them….