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Five Mile Creek Viaduct

Wairapata Viaduct

Working with Hebel

Five Mile Creek Viaduct

The Five Mile creek Vaiduct is based on a Viaduct of the same name on the former Otago Central Branch Line (now the bicycle Rail Trail).
I took this photo of the demolition train in 1991.
It is made from aluminium purchased from Ullrrich Aluminium in Christchurch ( which lists all of their products and the dimensions of all sides.
I settled on 1.6mm thickness (the smallest available), and worked out a range of flat bar, angle, channel and box sections which would complement each other. With the over 50 linear meters of aluminium on the floor of the garage in May I was ready to start! 
Although the channel and angle come in 10mm widths, the smallest flat bar was 12mm wide.  So to make that blend in, I had to cut 2mm off the edge for 10 linear metres.  I did not want to attempt that with a hand saw!  I made a holding attachment for my jig saw and used a course metal cutting jig saw blade to do it.  It became blunt very quick (by 2.5 metres it was useless), and the rough edge still needed to be filed by hand, but it was faster than the hand saw!
The main span is made using 15mm angle around the outside.  The cross sections have one piece of 10 x 10mm channel running one direction, and a piece of 10mm bar on each side running the other direction.  Once they are riveted to the angle at the top and bottom, it becomes very strong.  The small spans at the ends are made from large box section cut down the middle (so what you see was the inside of the box). Small angle section is then cut down ad fastened for the vertical ribs. 
I used over 250 small rivets as decorative rivets.  Each one required a hole to be drilled, and then the end to be hammer over, so that occupied a number of hours! 
There are also about 100 small (3.2mm) pop rivets which hold the structure together.  The rivet gun was really appreciated after the hammer method!
Originally I was considering having it power-coated.  However after spending an enormous amount of time putting in all those small rivets, I was worried that the powder coating would cover them all up.  I also had reservations about how the power would be able to be sprayed into some difficult areas.  So I decided to spray paint sections myself before assembly.  I gave them another touch up after assembly.
To finish off the viaduct, and complete the Otago central style, I made some look-alike stone piers from Hebel concrete. For details click here


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Wairapata Viaduct
- A piece of NZ history -

  First train on the new Wairapata Viaduct, Jan 2006.

M4 and L266 on the RSR
The "Wairapata Viaduct" is made from a mixture of components. 
The truss spans came from the original Wairapata Viaduct, built in 1930 for Frank Robert's Garden Railway.  To the best of my knowlege there is no "Wairapata" river, or viaduct in New Zeland.  It was a fictious name, as were the other names given to the stations on the railway.  The railway known, as the RSR (Roberts-Stewart-Roberts) railway, was a feature of Frank's back yard in Auckland.  Thanks to his friend WW Stewart, a notable railway photographer, many quality photos of the garden railway exist.  Built in 1930, it is credited as being the fore runner to the NZR model building in New Zealalnd.  The railway was dismantled in 1950 when the locos and most of the wagons were sold to the Government.  They are now held by Te Papa, the national Museum of New Zealalnd. 

On the Mt Magdala Tramway
After Frank's railway was dismantled, two of the spans were passed on to Harold Joynt.  It is unknown what happened to the other 4 spans from the original viaduct. 
Harold ended up moving to Kakanui in Otago, and the remaining two spans ultimately reappeared on his new 'Mt Magdala Tramway' in 1995.  You can see in the photo at left, that by 2004 the sun and saltly air had taken their toll.  After Harold died and the railway was dismantelled, the sorry looking truss bridge then sat in the back of Rob Wilson’s garage for two years, until one day I was cheeky enough to ask him if I could have it. 

After a clean and repaint
By now it needed a makeover! I wanted to return it to it's former glory.  To make it fill the gap which had been established on my railway, I added some 'simple' spans at each end.  There are two at the north end and four at the south end.  These spans and the small trestle legs were recycled from a bridge off my first layout.  The three wider legs under the truss section were built new to a similar style. 

Adding missing parts
Some pieces of the original truss bridge had been cut away.  In an attempt to restore it to it's original look, I made new pieces and slotted them back in.  The bridge had warped so each piece had to be custom cut to fit.  I was careful not to remove or change any of the remaining truss section as it is very historic now, being over 75 years old! 

It took me a full week to make all the parts, clean, assemble and paint it.  A piece of New Zealand heritage is now ready for the next chapter in it life, so bring on the trains! 


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Working with Hebel
The article below was written for our club magaizne, NZ G Scale news, Feb 2006. 


When designing the Five mile Creek Viaduct, I had the need for some stone piers. So how do you go about making representative stone? Obviously you can get a real stone and hit it with a hammer to get a small piece, but the results aren’t very controllable and you may not end up with the shape you require. There are also some plastic veneers produced overseas which have a stone pattern stamped into them. These can look ok if you only have a small flat surface, but for bigger inspirations they work out quite pricy and you still have the problem of joining sheets.

Now there is a new technique which has caught on in Australia and I have plagiarised the idea thanks to Greg Hunter of the Sandstone and Termite Railway in Sydney(see the links page).  This involves carving up Hebel Concrete Blocks to form look-alike stone.

So what is Hebel?
Hebel Concrete is a product of the CSR corporation of Australia. It is part of a lightweight building system for building houses. It has been featured in TV DIY shows in Australia and New Zealand. Its light weight is achieved by the use of aerated concrete. This is concrete which has a chemical to produce lots of little air bubbles in it. So it is also easy to cut with a normal wood saw (just don’t use your best one if you want it to stay sharp!). 
In NZ it is distributed by Lightweight Concrete NZ Ltd, who have branches in the Dunedin, Christchurch, Wellington and Auckland, plus some other towns. (see the contacts page of their website for a full listing, Although they probably prefer to sell it by the house lot or pallet, you should be able to buy individual blocks from one of their main depots. The blocks range in size up to 200mm x 550mm and thicknesses up to 200mm thick. You can get bigger panels, but they come with reinforcing rod in them, so are no good for cutting down. A 200 x 550 x 200mm block costs around $20. 
I found that a cheap wood ‘rip’ saw purchased from a range of hardware shops for under $20, is a good tool for cutting it up. With some care you can cut the blocks down to a thickness of 50mm. Under 30mm thickness has the potential to break. I have also used a course wood blade on a jig saw to cut out some round tunnel mouths. You can use a wood rasp and other files to shape it. 
Once you have reduced the block down to the desired shape, it is time to start putting the stone details into it. When you cut it up you may find the odd large air bubble in it, but once the finished stone work pattern is in it, you hardly notice it, so don’t worry. 
I start by drawing out the intended lines of my stonework with a pencil. You can scribe over them with any-thing sharp; an old broken hacksaw blade, the point of a small file, or any metal object with a point on it. 
Then I use a cross-cut wood saw to open them out some more, and finish off with a course file held at 45 degrees to get an angle on the edge of each stone. The trick is to use course objects. A fine file or saw is no good because the teeth become blocked too easy. 
The concrete crumbles away easily, so prepare for a reasonable amount of concrete dust. You will need to keep brushing the surface to remove excess dust while you work. So put down a tarpaulin if you plan on doing this in the lounge (luckily I did)! 
After you have finished carving your masterpiece, take it outside and give it a good brush. If you plan on colouring it, also give it a quick hose down with water. 
There are likely to be small pieces of dust left in the cracks regardless of how carefully you think you have brushed it. Flowing water over it is the easiest way I have found of removing them. 
The concrete blocks are a white colour. Colouring can be done by using cement oxides (which is coloured powder added to cement to produce coloured concrete) I purchased a range of colours in small bottles from Placemakers for $3 each.. 
Because the rocks in my railway are an orange-brown colour, I try to colour the Hebel a similar colour so it blends in with the surrounding rock and looks ‘natural’. Therefore the colours I will be describing are brown, but you can substitute these which whatever colours work for your situation. 
The method I use is to make a runny mix of dark brown (mix water with the brown powder) and brush it in with an old toothbrush. You need to make sure it has gone down into all the gaps to create ‘shadow’ effect. After the first mix is almost dry you can add a range of lighter colours on the top. 
I put the powder in an old margarine container and add just enough water to turn it to a think paste. Then I lightly brush it over top, similar to ‘dry brushing’ a model. You will probably need to add the white oxide to the yellow, orange and red powders, as they are very strong. 
Initially I thought that I could do away with white powder, and just add water to dilute the colour and make it less strong. However this changes the resulting end colour. So by adding lots of water to a dark brown, you think you have made a light brown, but when the water dries you find that you have actually got pink!
Don’t get caught like me, add white oxide to make the colour lighter, not large amounts of water! Water should be only used to turn it to a liquid so you can brush it on. 
Weather Protection
After the colouring has completely dried, I spray on a light coat of clear lacquer to protect it from the rain, which would eventually start washing it off after a period of time. 
I use a spray can which can be found in the paint section at hardware shops for about $12. I have found the clear lacquer makes the colour look slightly darker, so allow for this when you mix your oxides up, or do a small test piece first. 
That is the method I used, but I don’t claim to be an expert, so you may know of something better. Or you can choose not to colour it and let it weather naturally. 
In my limited experience, I have found Hebel is much easier to work with than Oamaru stone. It does not soak up moisture and need on-going protection sealing like Oamaru stone does. 
My set of four tunnel mouths have been outside for over a year now and only one has needed some spot re-colouring where a heavy hail storm had chipped away the surface in a couple of places. The surface does remain reasonably fragile though, so don’t put it where everyone can pick at it with their fingers, or it will crumble away. 
More tips on carving it can be found on Greg Hunters fabulous website. Look for Sandstone and Termite Railway on the links page. 

Tools used


Scribing Lines

Openning out with a file

Colouring with oxides

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