Thursday, April 28, 2011

The bridge project

Track building can be a pretty monotonous process. You basically move dirt all day. Lots and lots of dirt. If you’re really lucky there’ll be some tree stumps and rock mixed in. While I enjoy doing it and it’s great to be outside and active every day in a remarkable landscape, sometimes it’s nice to not just move dirt.

Bridge site
Recently, we needed to build a bridge over a particularly rough bit of terrain. This was a great opportunity to not dig for a few days. I was also excited to get involved as I have no experience at this sort of thing and I’m surely no structural engineer or builder. The concept of a bridge is simple but to build one is a different story, so this was a perfect chance to learn some new tricks. Three of us were designated the bridge committee due to our unique skills and ferocious work ethic (Read: luck of the draw).

Now a Chilean bridge is not like what we are used to in good old DOC-spec NZ. This bridge was to be made from whatever we could find or mill with the chainsaw from nearby. Treated timber doesn’t enter the equation anywhere. We didn’t have all of the tools you’d expect for a project of this sort; simple things like a tape measure were notably absent from the process. This bridge was to be rough and ready, but had to get the job done.

The architectural blueprints, which I drew just now in paint
The design we went with was simple. First, posts go into the ground. The posts are cross-braced together widthways. Bearers or runners sit on top of the posts, running in the direction of the bridge. Finally, slats are nailed onto the bearers. It isn’t a design that’s running for architectural breakthrough of the year, but it does get the job done.

Simple enough right? Well it’s not so easy to actually build. For example, where were we going to stick the posts? Uneven ground and rocks limited our options. Once we had decided where we were going to put them and how many we needed we had to find wood for the posts. We opted to rip out some old fence posts further down the track. This led to carrying the things a couple of kilometres uphill back to the site. When the post goes on your shoulder it seems alright; five minutes later is a completely different scene. I have huge respect for anyone who carries serious weight any real distances, because it’s bloody hard work. My muscles felt like they were melting by the end.

Posts in

Next up was post hole digging. We went with the old ‘hole a third the depth of the post’ approximation as the gold standard in sturdiness and safety. Posts were placed about 600mm apart widthways and at a varied spacing depending on terrain.  I forget how many posts we used exactly; I think it was six as we also used a large fallen tree to support a bearer.

Ian uses the stick level

We very carefully and scientifically decided the height to which the posts needed to be cut and marked them clearly. By this I mean we guessed by eye using a stick as a guide, and used charcoal from the lunch fire to put thick black marks onto the posts. This type of bridge building is more of an art than a science anyway. Our initial guesses were cautious as it’s far easier to cut more post off than it is to replace some.

Runners on
A tricky part of the process was sourcing wood for the bridge. Sure we were surrounded by trees, but take a look at a tree and you’ll see it tends to disappointingly wiggly and of uneven thicknesses (Disclaimer: this does depend on the type of tree, don’t find your exemplar in a pine plantation or I’ll look like a fool). So in practice we had some trouble finding appropriate lumber from nearby. Eventually three good-sized trees growing in a row were felled and their trunks were split with the chainsaw to give us the bearers we needed.

The first of the slats
The posts were notched with a chainsaw and bearers nailed in with 5 or 6 inch nails. At the same time, cross bracing between posts went in widthways. The bridge was starting to take shape. All that was left to do was nail slats onto the bearers with 3 or 4 inch nails. The bridge was quite long so it required a whole lot of slats, and as we were using branches as slats it was quite interesting to fit them together in a way that produced a relatively even surface. Spacing was particularly important in the sections of bridge which were curving slightly.

My mad hammer skills

The finished product
And then with the hammering of a final 5 inch nail (we ran out of everything smaller) we were done. A majestic wooden structure spanned what was once a miserable piece of uneven and unpleasant terrain. In terms of time it did take us a few days to build and there were a few frustrating moments (I’m still barely par with a hammer), but I learnt a whole lot and we left something cool on the track for times to come.

The first deflowering of the bridge

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