On Site at Falling Sands Viaduct - by Nick Yarwood
The last scheduled train service before the start of work on the viaduct left Kidderminster for Bridgnorth at 16:40 on 28 January. Aside from stock movements the following day to prepare for running from Bewdley, that would be the last one for 14 weeks when services resume on 4 April.
For the following two days volunteers and the railway’s permanent way team moved in to dismantle the still shining silver rails and track, and the signaling cables were disconnected from cabinets near each end of the viaduct. The scene was set for peeling away layers that had not been seen since 1878.
First to go was ballast that was set aside for reuse at the sides of the new track across the viaduct to suppress vegetation. Then the spent bottom ballast that proved to be useful later for access routes on site. As each layer a third of a metre thick was peeled away and stockpiled nearby, where the Sugarbeet sidings had been, the colours of fill were revealed. Initially ash and stone gave way to red, yellow and orange sands and rock sand, often in distinct patches where they must have been tipped during construction.
With arched viaducts it’s important not to create disproportionately unbalanced loadings. Whilst it would have been much quicker to dig down up to 1.5m to the arches in one operation, that would put the structure at risk. And so the viaduct’s secrets were revealed layer by layer.
There was also the complication of a live gas main, and its ductile iron predecessor. Fortunately the iron one was first to be exposed and was dismantled and cut into manageable sections. The live one was treated with reverential caution requiring hand excavation in close quarters and support at 3m intervals. The presence of the gas main created complications for progress of the works throughout the project.
Over many years water had been seeping through the structure, causing damage and putting it at serious risk. There’s plenty of water seeping through and staining evident on the brickwork below and all down the piers. We could see cast iron drainage pipes protruding from under each arch. But how they were arranged above was anybody’s guess. There’s no particular pattern for this in Victorian viaducts, each engineer decided at the time how it was to be done.
Finally, when the bottom layer of fill to the crown of the eastern most arch and the dip to the next arch was removed, it was revealed. There we no special arrangements. No iron grill, no pit. Just a couple of bricks placed across the end of the 6 inch cast iron pipe to stop sand going down below. That would have been fine initially, and for quite a long time. Until the iron corroded forming a conglomerate of sand and rust. Out of the six drainage pipes, four were completely blocked up. Those that weren’t had a more random arrangement of bricks that allowed water to get through.
Trapped water found other ways to escape by seeking out cracks and imperfections in the original pitch waterproofing. This had been applied in a layer about 8mm thick to the walls, crowns of the arches and onto compacted smoothed sand above the piers, between each arch. This is where the ends of the pipes were located, dipping down at an angle for a length of 4.5m to emerge through the arches.
The original intention had been to diamond drill though the arches to install new pipes. But what if these old pipes could be cleaned out and lined? Tenacious prodding with bars, timber and finally a sacrificial 6m length of 110mm plastic pipe did the trick. They were lined with plastic pipes and a pit with a stainless steel cover detailed to protect them. This saved a week out of the programme, which proved to be invaluable a few weeks later for concreting as the weather turned very wet indeed, just after it was finished and waterproofed.
As the excavations progressed, various artifacts emerged. A navvy’s shovel, rusted and with no handle - it long having decayed – and with the pointed end worn flat from digging the abrasive soils. Perhaps the handle had broken and it was cast aside in disgrace. There was also a ceramic jar, broken, and embedded in a very large blob of pitch. Maybe that had contained some lunch or was used for drinking out of. Both gave a window into the past, 122 years ago.
The site team have happily shown various visitors around the works. For a group of ten apprentice bricklayers from Kidderminster College this was their first visit to a live construction site and they were surprised how heavy solid Victorian bricks can be. Large arm muscles would be needed to handle them all day!
In the early seventies, BR had chosen to dismantle and rebuild the parapets from the west end, getting about half way across before stopping. This explains why the corbelled detail is only present on the eastern half. Paradoxically, brickwork below is in better condition where it still exists. The Victorians had done it for a very good reason!
A few of these large blue brick corbel blocks, 2ft x 1ft x 6”, emerged from the fill during excavation, and have been retained both as a pattern for later works to reinstate the feature using GRP mouldings, and as a potential exhibition item to help illustrate the story of the construction of Falling Sands Viaduct. Unfortunately, damage had also been done to the parapet structure and to the waterproofing which had to be made good with concrete before backfilling could commence.
Restoring the waterproofing consists of four parts. Panels of fibre-reinforced concrete 200mm thick cast over the arches, expanding sealing strips between the panels and against the side walls, concrete fillets along both sides to keep water from seeping down the sides, and two coats of a special bituminous paint, known in civil engineering as ‘black jack’, over the concrete and the walls. Four concrete panels were poured in one day using a concrete pump over a distance of up to 140m and many extra skilled hands. The intervening three panels were poured two days later.
Above the waterproofed concrete there’s a layer of 40-20mm chippings to allow water to migrate to the low points and out through the pipes, protected above by mesh reinforced geotextile.
The heavy rains could have caused more problems with backfilling than it actually did, again placed in progressive layers to even the loadings along the viaduct. Fortunately storms Ciara and Denis did their worst over weekends, and the contractor, Walsh, refused to be beaten. Preventative pits filled with chippings drained away the worst of the storm water. An additional longitudinal drainage trench through the entire depth and filled with chippings provides a permanent drain for water below the trackbed.
Nearly 3,000 tonnes of stockpiled fill have been dumpered back into place and compacted. On top of that is the ballast. On the south side concrete toughs have been laid for signal cables, with recovered ballast along each side. At the time of writing it’s ready for the track and cabling.
The project has progressed ahead of programme due in part to an exceptionally mild winter. The greatest risk was prolonged freezing or snowfall interfering with concreting and waterproofing. Waterproofing needed seven days to complete and was also vulnerable to rain. With good programming and a week of calm dry weather, it all went to plan.
Mostly though, success is due to the diligent, knowledgeable and hard-working team from Walsh Contractors on site, with the programming, and supporting behind the scenes. In the picture are the guys that have been on site all the time. They are, from left to right, Luke Jones, Lester Gilchrist, site foreman Ryan Hemming, Jake Grant and site manager Jason Grant. Rich Howells is also part of the team and wasn’t available on the day this photo was taken. Huge thanks go to them all and to everyone involved.
Nick Yarwood is an SVR volunteer on infrastructure projects, visiting the works every day to liaise with Walsh, SVR’s Infrastructure Manager Chris Bond, the Engineer Jonathan Symonds and the Project Board.