Win The Future with Mobile High TunnelsJanuary 18, 2012
I talked about bending the pipe in a previous post(here) and now I’ll go over how we did the rest of the tunnel. I designed the tunnel based on the professional manufactured tunnels from a few different companies. SketchUp helped a lot and I would recommend folks who are designing building projects to check it out. Its not to hard to pick up an there are lots of tutorials on the web and classes offered through community colleges and other institutions like Yestermorrow. So I designed it and bent the pipe now what?
Here are a few terms that I’ll be using.
Tube or Pipe – Technically what we are using is tube not pipe. Generally piping is using in moving a fluid and tubes are used for building. When you are giving a diameter of a pipe it is the internal diameter. Tube’s diameter are given in the Outside diameter. That is why a Pipe bender doesn’t quite work with a tube because they maybe both 1” diameter they are measured in different places. I use them interchangeably because its the general concept thats important. Just remember if your looking to build a greenhouse, high tunnel or anything else with metal cylinders its tubes you are looking for.
Bows – These are the two side pipes plus the top connection pipe all put together. In total we have 13 bows made up of 26 pipes + the ridge connector.
Purlin- These are the pipe that run the length of the tunnel that connect to each bow stabilizing the structure.
Cross-braces- Each bow will have a pipe that runs from one side to the other parallel to the ground which will help keep the bow’s shape.
Ground posts an runner pipe- In a traditional tunnel ground posts would be driven into the ground to hold the tunnel in place. In our case our ground posts are attached to a runner pipe that is the length of the tunnel. The runner pipe is what our wheels will attach to.
Lucky for us my dad decided to come spend a week vacation helping us build the tunnel. With him he brought 4 drill presses, a new hand held band saw and some handy tools to make the work move quickly. We had a spot next to the tunnel location where we created a level work area using a few small custom built plywood and 2×3 tables. On the tables we built a jig where the bent pipes would be placed so we could drill holes accurately with each bow. The two sides are connected to a ridge pipe that is the same diameter as they are. A small section of 1.99 pipe will slip over the pipes and then bolted on. This means there will be four holes at the top, two on each side. We initially had 4 drills set up, one for each hole. It worked well for a bit but then one of our drill presses broke. We were able to pivot the other press that was next to the broken one to drill both holes on one side.
While we had the bow in the jig we fitted a cross-brace using the smaller pipe that is also used for the purlins. When the bow left the jig it was ready to be put up. We didn’t have the runner pipe and ground posts ready yet so we just stacked them up for later. To make the runner pipe we had to connect two full length 1.66” pipe by sliding them into a short piece of 1.99” pipe. The toughest part of making the running pipe is creating a straight series of holes that run the whole length. If you have two drill presses life is much easier. You set up the drill press at the distance you want. In our case it was 4 feet on center. After you drill the first hole you slide it down to the next press. You then slide the drill bit of second press through the hole you just made. Now when you drill the next hole with the first press it will be in line and at the right distance. This went very fast and after trying to do some other holes on our greenhouse with out the second press I would say its the best way. You could build a jig that works the same way with out the need to have a second press.
The ground posts are made of a short section of 1.99” tube. We drilled one set of holes to connect to the runner pipe and a second set of holes 90 degrees around the tube and a bit higher. The end of the bows end up sitting on the bolt that goes into the runner pipe and then the second hole set bolts them together. We drilled the bow holes free hand after they had been set into the ground posts. Its tough to make two separate drill holes line up so doing it after the fact made it go faster. If you are really good you could have a jig or skill to predrill everything.
We didn’t quite know how we were going to set up the wheels. We went back and forth on a few ideas so we put all the bows up next. With a normal tunnel this part is pretty simple. You just go along sticking the bows into the ground posts into the ground and they stay put for the most part. We had two independent rails that had ground posts that could swivel. So if you just stuck the bow in it would quickly fall down. We were also building the tunnel over growing crops so that add extra headaches. We had previously squared up the ends of rows where our runner pipes would sit so that part was easy. We laid out the pipes at the right distance and lined them up with our square marks. Each end of the tunnel was going to have diagonal supports on the first and second bows. This would do two things. It would stabilize the structure while we were building it and also would keep the tunnel from falling one way or another when it was sitting or moving. It would have been straight forward if we had 3 people. Two people could hold the bow and the other could set up the diagonal support. We only had 2 people, which meant we had to build some temporary support which consisted of some 2×4′s and duct tape.
The diagonal supports need two bows to be up to work. So once we had that we could start making sure the bows were plum. Once the third bow was up and there were stabilizers between the second and third bow we attached a temporary purlin. There are clamps that you can get that wrap around the bow pipe then around the purlin and then are bolted closed. These are quick and easy to use. As we added new bows we would attach the purlin to hold it up. We were also attaching the ridge line purlin while we were adding bows. We started running out of light so we stopped doing the ridge line and it still was plenty stable. Then the light faded and we just kept going. We brought down lamps and worked by them and headlamps. Then it starting raining and we had to put the lights away but we kept on going. By 10 or 11 I can’t remember we had the last bow up and almost all the diagonal supports in place. With three people you could have the bows up and all the purlins in place in a day. We had to do an extra half day to finish the purlins.
Our basic skeleton was done, we still needed end walls, wheels and to put the plastic up. The end walls are the same as a normal high tunnel except they do not go all the way to the ground. The wheels that we are using are the one that are normal found on riding lawnmowers. They can handle decent amount of weight and they are cheap. You can find them at northern supplies from 5 to 10 dollars depending on the sales. When the tunnel is in place on the ground the wheels are not on. They are only on for the move. This allows you to have one set of wheels for multiple tunnels. With the rimol greenhouse kit and what Elliot Coleman talks about in his book this is not possible. The rail+wheel system that they use needs to be set up for each tunnel. It also costs more. The wheels they use are around 13 dollars per piece not so bad but the shipping cost get you because they are really heavy. When I looked at ordering them I was going to have to pay an extra 250 dollars just for shipping. The other nice thing about our system is that the pipe sits directly on the ground when not moving. The rail system has space that you have to cover with plastic and some sort of weight. I can not find a single picture of our wheel setup. I will make sure to take photos and a video the next time we move it.
For the axles we used bolts that would go through the runner pipe. We did not want to drill to big of a hole because we were worried about the structure of the pipe so we had to have a filler inside the wheel. We made this from an oak dowel with a hole drilled through its center axis. The runner pipe gets jacked up. We used scissor jacks the first time then just lifted it with 2×4′s for the second move. We are also working on another technique that I’ll update later. We have 7 wheels on each side and next spring we will be adding 1 or 2 more on each side. A few of the first bolts we used ended up bending a little bit. We bought some higher quality boats which have held up much better. The extra wheels will take even more weight off the new bolts. Once the wheels were on we were able to move it very easily. We had two people on each side one pulling one pushing. When it was moved back to its winter spot in the fall is was really muddy and the wheels didn’t roll nearly as well. Plywood strips in the tracks made it so three of us could move it. Im pretty sure we can do it with two people once we have it down.
The last thing that is very important is the anchor system. High tunnels have two big threats being crushed from snow or being blown away. The snow load is managed in our design by the gothic shape and the heavy duty tubing. This is the same as a none mobile tunnel. Blowing away in a normal tunnel is taken care of by sinking ground posts in our area at least 2 feet into the ground. Since we don’t have ground posts in the traditional sense we have to do something else. My dad found some heavy duty anchors that can hold 1300 pounds when sunk into the ground completely. There is a disc at the bottom of a long rod. With our soil we were able to spin them down into the ground the full 3 feet but it was a struggle. Right now we have a straight pipe bolted from the anchor to a bow. There are three on each side. I’ve had some sleepless nights worrying about our tunnel while the wind howled and knocked our log house hard enough to shake it. Last night we had winds gusting over 30 mphs our wind mill was howling and the house was shaking but still our tunnel stands strong. But our anchors have held strong. We also built our tunnel in a sheltered area that helps greatly. If we were in a more exposed area we might add another anchor or two.
I think thats a good overview of how we did our hight tunnel. I’m not an expert builder or engineer but I think we have made a quality tunnel that will last a very long time. I tried to take photos of everything important but some times we were just to into it to remember to grab a shot.
I have posted more pictures in our high tunnel gallery that show more steps in detail.
See you folks next time,
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