Modelling a Viaduct


The following picture has been widely publicised on this Website and in Model Railways Online Magazine. We have received several enquiries from readers asking how it was built. This article describes its construction.


The Prototype

The model is based on the prototype Clinnick viaduct:


Clinnick viaduct is in the upper Fowey valley in Cornwall between Bodmin Road and Doublebois station. It is not easily accessible today due to forestation.
Modellers' license has been applied to place the viaduct on a 5 foot radius curve.


Basic Construction

Construction of the model started with building a 12mm plywood frame:


Arch and Pier Arrangement

It is important to note that when a viaduct is built on a curve, for structural reasons, the sides of each arch are always in parallel to each other, even for skew arches. The following diagram represents an aerial view of a curved viaduct. The grey areas in the diagram represent the arches. The left and right edges of the grey are parallel to each other:


The supporting piers are tapered such that they are wider on the outside of a curve than on the inside. Many modellers build this incorrectly and opt to build non-tapered piers and tapered arches (like a cone instead of a tube) which in reality, would never stand up.
Although the frame on the model started with non-tapered piers, 3 mm ply was used for facias on them to create a tapered effect which can be seen in all of the later photos on this page.
The side facias of the viaduct were cut as a single sheet from 3mm ply and nailed with copper nails to the framework. Copper nails were used as they are not succeptable to the corrosive nature of plaster.


Plastering

The whole viaduct was covered with a thin layer of Polyfiller and the stonework was scribed using a Dremel and mini counter-sink.
Polyfiller was chosen for two reasons:

  • Due to its properties of allowing minor movement without cracking, although in practice, this has never been an issue
  • It is not possible to buy a sheet of embossed stone plastic sheet at 1M long, therefore, hiding joints, especially on changes of direction would be a problem
  • Brick paper does not look reallistic because is is difficult to hide joins and it gives no effects of texture or light - it is a medium which simply isn't up to the standard I was trying to achive

I have since found that other plasters with wallpaper paste mixed in are just as effective and usually easier to scribe.


Scribing the Stonework


The two picture above show scribing in progress. The left picture shows the outside of the curve and the right picture shows the inside. It will be noted that the piers are different widths on each side of the viaduct.

Once the scribing was completed, the model was painted with several very watery washes of water colour. I have found that this gives a very effective 'stone' finish. It is very important to note that with water colour, you can darken tones easily, but you cannot lighten them again, therefore, a series of very light grey washes must be used until the desired finish is achieved.


Placement on the Layout

The viaduct was then located on the layout, track laid across it and ballasted:


Note that the track across the viaduct is canted with 2mm of cant on the outside of the curve. Due to the crossover, both tracks are canted in the same plane. Canting presented some interesting constructional issues because it highlighted one of the common mistakes made by modellers when building viaducts and bridges. On real viaducts, if the ballast is dug out, the tops of the arches will be exposed - there is not a lot of depth between the top of the arches and the track. Modellers often place significant depth between the top of the arches and the track bed which makes the result look like an aquaduct! In canting the track, I had to be very careful not to create the 'aquaduct effect' on the outside of the curve.


Point Motor Arrangements

The Peco point motor for the turnout on the viaduct is located inside the supporting pier and there is a pull string for pulling wires through should the motor ever need replacing. The pull rods for the working ground signals pass all the way down through the pier out through the base of the viaduct through to the underside of the baseboard.
As can be seen, handrails are still to be fitted. Faller make plastic fencing which is very close in appearance to those on Clinnick viaduct.


Graham Plowman (28/10/2011)


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