Viaduct Drainage upgrading

Stanway Viaduct

As many of you may recall, in October and November 2018 we had a team of specialist Engineers carry out a detailed examination of this 15-arch structure, which involved some abseiling work.

October 2018 viaduct examination

The approved 228 -page report arrived on 12.02.2019. Generally speaking, the structure is in fair condition, but, as with most brick structures about 110 years old, there are a large number of defects and minor faults. These include – cracks, damp patches, hollow sounding areas, frost damaged brickwork, vast areas needing pointing and water coming through the brickwork, especially above the piers. 

Water emerging above a pier.

Water penetration of the brickwork, due to defective drainage, is believed to be behind many of these problems.

 

Then in May 2019, the user of the field who grazed horses reported that some brickwork had fallen off the corner of one pier! 

A plan was put together to undertake some brick repairs and the investigation of the water penetration problems. Quotations were obtained, but the cheapest was more than double our budget for the work. In addition, two contractors who had expressed interest declined to quote. With winter rapidly approaching, and when lime mortar work is not viable, we decided that the most important thing was to sort out the drainage issues. A more comprehensive scheme was put together, without the brick repairs, which were non-critical.

Eventually, with Christmas looming, we appointed Walsh Construction, who were already busy with scour repair works on the River Isbourne.

The first thing to be done was to remove all rails and sleepers from the 192.5m long structure. Our P-way dept. agreed to do this starting on Saturday 4^th Jan, but fortunately were able to start on the Friday, instead. In a massive effort they lifted the lot and stacked half at each end of the structure.

The contractor started on Monday 6^th Jan, and then had a completely clear trackbed to work on. The drainage of this viaduct comprises waterproofed arch top slopes leading to a brick chamber located above each of the 14 piers. Each chamber has a steel outlet pipe, from the bottom of the chamber, then passing through the soffit of the arch brickwork. Water is supposed to get into the chamber through a hole app. 4” x 3” in each brick face (4 holes per chamber).  

Pit 14 without its cover of sleepers.

Pit 14 with perforated plate in the bottom.

As you might imagine most of the holes were blocked with a build up of muck and sludge from years of atmospheric debris supplemented by stuff discharged by the trains passing over the structure! None of the pits had proper lids on them; they were either some old concrete sleepers, or some thick stone slabs. New reinforced concrete lids have been cast, on site, each with an access pipe, through which a CCTV camera can be inserted to do regular checks, in the future.

Concrete lids cast on site.

New concrete lids for inspection chambers in situ.

 The contract involved digging out an area around each chamber for the full 7.5m width of the structure x app 4m in length. The fill revealed was primarily ash, with accumulated muck which was reducing the drainage capability considerably. Half of this fill will be re-used by our Drainage team on various future projects and the rest will go away to tip. Having exposed the bitumen waterproofing it was clear that this was breaking down and allowing water through in many places. A much larger job to deal with that and completely waterproof the entire structure will be required in the future. The object of the current work is to replace the current poor quality fill around the chambers with free draining stone so that water disposal will be much quicker, and it will not have time to soak into the brickwork.

The viaduct does not have a very good access, so all excavated materials going out and new stone coming in has to go via Stanton yard about 1/2mile north and be transported by dumper.

Material removed and brought to site by dumper.

Excavation started at the Toddington end and progressed northwards. Once the first 7 pits had been dug and the drain holes in the pit walls cleaned out a layer of plastic sheeting was laid in. The hole was then filled with new free draining stone. On top of that new ballast was placed, ready for the sleepers to be dropped straight on top.

Pit 14 with arch concrete exposed.

In this view of excavated pit 14 the mainly ash infill is clearly visible at the top.

 

Pit 8 with plastic sheeting being laid.

First half ballasted and ready for track laying again.

The first 7 pits were completed on Friday 24^th Jan, and, with the haul distance getting shorter all of the time, the remaining 7 will almost be completed by p.m. on 31^st Jan. Track re-laying, starting at the Toddington end, should commence on Saturday 1^st Feb.

 John Balderstone

 30.01.2020