Polytunnel Part 2 - Prep Work Starts
Even before the 'Purchase' button was pushed on the First Tunnel shopping basket, I had already started doing some ground works on the plot, to make ready some space for the new structure. For what seemed like ages, I was torn between a 6 foot or 8 foot wide tunnel. With it came the decision of a 12 foot long structure which was only available at 6 foot wide, or 10 foot long but only available in an 8 foot width. My ideal size would have been 8 foot wide by 12 foot long. Alas though, that couldn't be. Eventually, I decided the length of the tunnel was probably more important than the width as in reality, growing space wouldn't really be impacted, just the width of the path down the middle of the tunnel.
The intended space for the tunnel had been cleared some weeks ago and had remained under cover so far. The biggest challenge was to make the site as level as possible. At first glance, it was already pretty flat. But, it was only when I went to the plot with a spirit level and a plan did it become evident how much of a difference there was from one side of the ground to the other. Below, the blue pieces of string tied to the metal stakes in each corner are level with one another according to the spirit level.
It's not especially clear, but the blue knot closest in the picture is about 6 or 8 inches higher that the knot in the opposite corner, furthest away from the camera and while this doesn't seem much, when you convert that into the volume of soil and disturbance to the soil structure that would cause across the planned growing bed, that's a lot of effort and disturbance. Now, this would have presented a big problem if I had decided to go down the route of having a greenhouse. Everything would have needed to be perfectly square and level in order for the metal frame to sit correctly, and be able to set the square pieces of glass in place. But, with a polytunnel, that issue goes away. As long as certain parts of the frame are level (which can be achieved by sinking the feet into the soil into variable depths), then it just becomes a question of setting the base rails to be level, but not necessarily the same height as each other. More on that in a later post when I take a look at installing the base rails more closely. For now though, it was time to measure out and put the ground anchors into place.
When spec'ing the build, you have the options of using foundation tubes for soil, screw anchors for soil or a combination foundation tubes and anchor plates. It all depends on the surface you have to build the tunnel on. I could have opted for screw anchors, which as the name suggests, look like giant screws. Using them just involves screwing the anchor down into the ground until the desired depth is reached. A great option if you know how deep the good soil goes before you hit stones or bed rock. The only problem with that option is that if you hit a stone or rock, you then have to dig down, remove the obstruction or break through the rock, back fill and then try again. To me, if you run the risk of having to dig down to reset the anchor point, you might as well commit to digging in each of the anchor points anyway. For that reason, I went with the foundation tube with anchor plate option which means digging a hole, attaching a flat metal plate to the foundation tube and then burying the anchor plate in the bottom of the hole, leaving only the top of the pole poking out the soil.
There is of course a little bit more to it than that as everything has to be perfectly square and in line, and the tops of each of the foundation tubes stick out of the ground need to be perfectly level too. To achieve this, I went to the plot with a full afternoon spare and with no specific deadline in mind. After all, digging 8 holes of a suitable size is only a 15 to 20 minute job. Digging them in the right location and the right depth is another thing. It's probably the most important part of the build as it determines how every other part of the structure lines up and so I was happy to just take as look as was needed to get things done right.
Concentrating on getting everything square and level, I kind of forgot to picture the process of getting to the end goal. Needless to say though, there was a lot of digging, hitting with a hammer, tape measure action (sometimes with two tape measures at the same time!), spirit levelling and running bits of string around the place. Finally though, the days mission was accomplished as can be seen above. By the end of the shift, I'd set all 8 ground anchors in place, all to the same depth and all in line. The tops of each of the metal foundation tubes are all the same height, so it further goes to illustrates the difference in level between opposite corners of the structure.
Part 3 in this series should be up soon enough, but for the time being, thanks once again for reading and joining me in this stage of the build of our polytunnel.
