Gold Coast Water Tanks
I was contracted by a local fabricator to survey 2 of 3 huge water tanks on the Gold Coast. The initial brief was to laser scan survey the 2 tanks so workshop drawings could be produced. I turned up on site and started what was the most challenging project so far.
The largest tank was huge. It was 61m in Diameter, 11m High and full of water. Once I had got all my gear to the top of the staircase I was faced with the first problem. How do I set the RTS up? There was only a grating landing at the top of the staircase and that really was it. I managed to get the tripod erected with its feet on the stair steelwork and it proved just stable enough to set up a control network provided I didn’t move to much. Once we had paddled our way around the tank installing control point stickers and paper targets we were ready.
Next came the scanning which was hard work and nervous work. Trying to stand up in a small boat to position the scanner on top of the steelwork was awful. I was so scared that I would drop a $65K machine in the water and that would be it. Coupled to this the steelwork was covered in duck poo and other delightful things which I was trying to avoid getting all over the scanner. A few hours later we had scanned the tank and as always, the scanner performed and was light and small enough to be able to use it in this kind of situation.
The other smaller tank went better as I had a bit of a system going. You could also walk around a small ledge on the inside so a boat wasn’t required. I was then asked if I could so a quick set up scan on the third tank which had already been surveyed. The client wanted the drilling on the tanks to be set out. They also required a contour map around the base of the tank which I also agreed to do.
The surveys were completed and fabrication was underway. Now it was time for the layout stage of the project.
I firstly had to visit one of the smaller tanks to do the layout and the fun began. I realized very quickly that this was going to be as difficult as the surveys. Firstly, we had to find a way of fixing the RTS to the tank. The initial idea was to mount the RTS on a steel post that would be clamped to the steel or concrete beams towards the center of the tank. Setting up was hard as the boat moved a lot and looking through the RTS to set up on my controls was tricky. However, I soon realized that the steel post was not a good idea as it moves when the sun gets on it. It moves a lot more than you would think. To exacerbate this the tank was moving more that I had ever imagined as well. In the end, we gave up and bolted the mounting post for the RTS to the wall of the tank. This made things much better as the wall moved less but the sun was still a problem. The only way around the sun was to keep checking the level of the RTS and to keep re-setting it up when it had moved. Once set up was done we moved onto the big tank.
The big tank was hard work but the structure of it was different so we could set up in the centre and clamp the mounting post to the steelwork in the centre. We had the same movement issues with the sun but the tank being bigger did not seem to move so much or so I thought until I heard a massive crash which was the tank wall moving slightly and making a beam move and make the bang. It took nearly 2 whole days to mark this tank out. My main problem with the mark out was trying to hold the prism and tablet whilst standing in the boat. It’s hard to keep everything still enough to get a good position mark. The Trimble tablet is very heavy and bulky. It’s hard to use on the ground let alone out of a boat. Trimble… Why can’t we use an ipad? It would be so much lighter and cheaper. However, with all the technical issues we had the steelwork for both these tanks is installed and complete.
The last tank went the best as I could mount the RTS on its tripod. This was where I realized just how much these tanks were moving as they warmed up. I think it must have been about 10mm movement in a couple of hours which was made slightly worse by the small movement of the water in the tank caused by the wind.
All in all, this was an important learning experience. I found out that environmental factors and boats can really throw a spanner in the works at both the survey stage and layout stage of a project. I also learned that it doesn’t matter how good your control network is if the structure moves. However, it is still possible to complete the job successfully so long as you have these factors allowed for.
Most importantly check yourself. Don’t just rely on the technology to tell you things are correct as sometimes even if all your expensive software and gear give you the answers you want it might still be wrong. When I set up I now always perform check measurements between known points just to make sure my set up is good before getting on with the layout job.