Balloon loop extension and ground surveying

For the second phase of the line, I wanted a balloon loop at one end. This extension proved to be a lot easier to build than the original line since the Ruston was used to haul the track panels and other loads to where they were needed. I decided to follow the same process as phase one and lay the panels on the grass to determine exactly where the trench should be dug.
I laid the point and straight track in position. In order to have a smooth circle, I needed to find the centre. I did this with two pieces of wire attached to a central rod with a nail at each end. These were 636.6cm (the outer rail radius) and 809.5cm long. By putting the nail attached to the shorter at the end of the outer rail on the straight track, and the nail attached to the longer two track panels (500cm) along I had three sides of a right angled triangle and could locate the centre of the circle with the rod. This was calculated by Pythagoras' equation, i.e. 636.6 x 636.6 x 500.0 x 500.0 = 809.5 x 809.5.
I positioned each curved panel in turn, using the wire a radius long attached to the central rod as a guide to ensure absolute accuracy.
By the time I finished the loop, my precalculated track panels fitted exactly and smoothly. I painted a 50cm wide track bed outline in white gloss paint. To ensure a perfect curve, I attached the paintbrush to the wire that I had used to initially align the track, and lengthened this for the outside, and shortened it for the inside lines.
There was however one small problem. It was clear that there was a quite severe drop in the ground level at one point under the track, and I decided that unlike the phase one straight, I would need to survey the route to try to even out the gradients. I used two techniques. Firstly I fixed two cut plastic water bottles at either end of a 20 metre hose. When filled with water, one end is raised or lowered so that the level is equal in both bottles. These two levels are equal, and the drop down to the ground can be measured. From this I found that the maximum drop from the point was 23cm.
I then carried out a more detailed survey. Taking the ground level at the point as my reference, I drove a wooden stake into the ground every 220cm along the route. A straight piece of wood was laid from the reference to the first stake, and the stake driven into the ground until a spirit level placed on the wood was level. This was then repeated from the first stake to the second and so on. From this I produced a gradient profile of the route, shown on the track plan and loop gradient page.
The maximum gradient was around 1 in 20, in a quite severe dip. However, by taking a constant 1 in 80 gradient down from the point to this, and a 1 in 300 back up, it was possible to even out the dips, and keep the top of the concrete edging that I use at a reasonably constant depth, again as shown on the track plan and loop gradient page. To ensure a constant gradient when I laid the concrete edging, I cut a piece of wood 80cm long, and fixed a piece of metal protruding down by 1cm at one end. When this was laid on the edging, it gave a 1 in 80 gradient when the spirit level laid on the wood was level.
I dug the trench by hand to ensure a very smooth gradient - this was a lot harder than using the mini digger, but at least it was only around 40 metres! The Ruston and wagon were used for hauling all the loads down to the excavation site. The trench was lined with terram and filled with a layer of road core topped with 14mm limestone ballast as per the original track bed.
In order to allow fairly high speed running round the loop, the track bed was given a camber so that the outer rail was higher than the inner. A test train with a spirit level laid on a wagon was used to ensure that the camber was constant throughout the curve. I have shot into the curve at full speed (nearly 8 mph) from the downhill straight without coming off - but it was close! For most running the speed will be limited to 5 mph into the curve.
The completed loop. I constructed a lift off bridge on the far side to allow mower access to the centre.