It must have been a fairly straightforward remit. Nothing to trouble late-Victorian engineers. A river crossing. Not that wide. Not too high above the water either. What would be the natural solution for the 1890s? Drop a couple of cast iron caissons near the bank sides and lift in a whacking great warren truss girder bridge. Sounds like a simple job. Just like so many others being built all over the world at that time.

But there was a snag. This was a widening scheme. And the new bridge was to be built parallel to an existing structure. Again, this should not have been a problem. Widenings around this period were common. The railways were expanding. Extra tracks were needed.

Spot the difference

But there really was a snag – at least in some quarters of opinion. The existing bridge had been built by Brunel. And even as the nineteenth century was closing there were those who were still in awe of the great engineer and the traditions that had been built around him. Despite much grinding of teeth, the directors of the Great Western Railway chose to build a replica brick and masonry structure alongside in homage to the great man. The second Moulsford viaduct over the Thames near Maidenhead was to be built as a carbon copy of the original structure.

Except that it wasn’t. Well, not an exact copy but enough to keep everyone happy.

The general configuration was the same. The materials were similar except that there wasn’t the same use of fancy stonework. When the bridge was opened in 1892, the engineers had kept their part of the bargain and life went on regardless.

But there were several other significant differences hidden away that would only come to light well over a century later.

For those who are unfamiliar with the building techniques of brick or masonry structures it may come as a surprise that what you see is not necessarily what you get. There is a similarity to the acronym wysiwyg (what you see is what you get) which surfaced in the early days of computers. But it doesn’t apply here! What you see with a structure like Moulsford is an apparently solid structure.

wk 27518-16cDR [online]

There are no obvious holes anywhere. But, just as in the tradition of medieval cathedral building techniques, many bridges and viaducts are not solid. They are, in fact, full of holes. Not holes exactly, but voids and vaults, intentionally constructed with the primary purpose of saving materials, weight….. and money.

In abutments, for example, some of the voids were backfilled with loose rubble or have gradually filled up over the generations with ‘stuff’ that has washed down from above.

Getting inside

In Moulsford viaduct, the chambers were generally full of fresh(ish) air, were known about, were accessible and were examined as part of the formal inspection regime.

But up to now there has been a problem. Access to the chambers has been via small manholes that are located in the six foot. So, to carry out an inspection, either planned or for an emergency, both tracks on the viaduct have had to be blocked.

With increasing traffic, this has become untenable.

Whilst emergency inspections are rare, it is the nature of a structure that is over 150 years old that bits can fall off or become dislodged. This applies to internal parts of the structure just as it does to the facework. The tracks are supported on thick stone slabs that in turn are supported by diaphragm walls. If there is a problem with the track, then there is a need to check whether the fault has a link with any deterioration below.

So, there was a need to access the voids another way – a way that did not involve taking possession of the tracks.

An obvious way would be to ‘knock a hole’ (form an opening) in a spandrel and link up all the voids. That would be reasonably straightforward, but at this point Network Rail had the same dilemma that their Victorian forebears faced. How can you possibly deface a Brunel structure? Well, there was a way, subject to all sorts of planning hoops, and it involved getting into the structure from the side generally hidden from view – the elevation that was between the old and ‘new’ structures.

Network Rail working with their designer Arup started work on the project and, as Andy Crowley, Amco’s senior contracts manager, described: “An opening was formed into the chambers from the outsid e and we were able to move between the honeycomb of voids through pre- existing arched openings.”

There wasn’t a great deal of room, but at least the whole structure could be examined without any interference to rail traffic above.

Access to the openings is now via purpose-made gantries hidden from view, designed and supplied by Dura Composites.

Similar, but different

The project also involved a similar exercise with the parallel ‘new’ structure. It too had voids and these too were accessed via manholes between the tracks.

Apart from the generally cosmetic differences between the structures it was assumed that they were otherwise structurally similar.wk 27518-25cDR [online]

But, came the time to knock the hole (form an opening) in the spandrel into a void, what was discovered? Brickwork! And more brickwork. Void there was none.

And so this was living proof that those later Victorian builders were given a free hand with what they could do inside the new structure. Make it look the part, but just get the structure up as quickly and as cheaply as possible and, if it means having a chamber configuration that differs from Brunel’s system of voids linked by elegantly formed stone arches, then so be it.

And that was the nature of the beast throughout the second structure. Narrow chambers and no stone arches.

Apart from dealing with a site prone to flooding, just working in the narrow confines of each structure prompted the need to take unusual precautions.

The voids were, of course, confined spaces within the meaning of the legislation and the Fire Brigade was brought in to assist with site training. Some of the voids were so restrictive that rescue issues needed input from mountain rescue experts – although Maidenhead isn’t renowned for its mountainous terrain. They were brought in from somewhere far more hilly.

The project started in October 2014 and is due to be finished in the latter part of 2015 depending on emerging issues in the 1890 structure. In addition to general repairs and the provision of a new waterproofing and drainage system, those voids particularly inaccessible will be stabilised with foam concrete.

All this is out of sight and, whilst it has been carried out on a specific historic piece of infrastructure, many of the techniques are applicable throughout the railway network.

Apart from detailing an intriguing project on a pair of Victorian structures, perhaps this article will prompt engineers to look at brick or masonry structures in a different way. They are not always as solid as they first appear. What you see really is not necessarily what you get.