Building a Caterpillar Tunnel

This is the second instalment in how we build and use tunnels on our farm. The first part presented our first couple tunnel iterations. Today, let’s look at the latest design we’ve tried: a caterpillar tunnel.

We first read about caterpillar tunnels three years ago in the manual High Tunnels: Using low-cost technology to increase yields, improve quality, and extend the season. You can download it for free here. We were immediately attracted to the caterpillar design because it seemed even more affordable to build then our already inexpensive tunnels, and also promised to be easier and more convenient to move. Our only concern was whether the structure would be sturdy enough. This year we decided to build a caterpillar tunnel and  find out whether it was as great as we thought.

We decided to keep the same 14′ width as previous tunnels but build this tunnel the full length of our 300′ field beds – 3 times the length of our previous tunnel design! If this is the first tunnel you build I would caution against building one this big.

Building a caterpillar tunnel is as simple as

  • make (or purchase) arches
  • install the arches
  • put on the plastic

MAKING THE ARCHES

We bent our tunnel arches ourselves inspired by Paul and Sandy Arnold’s directions. The first arches we’d made a couple of years ago were 14′ wide at the base from 21′ long steel pipe. This time we decided to keep the 14′ width but make a taller tunnel and used 24′ pipe. We bent the pipe a round a  13′-diameter half-circle jig on a hay wagon. We left 2.5′ unbent on each end of the 24′ pipe so it can later slide over rebar ground stakes.


One person holds the pipe in place.


Another walks the pipe around the jig.

This is elegantly illustrated in our first ever Going to Seed YouTube video:

We then use a pipe bender to add a couple features to the arches.

A gothic peak so the structure can better shed snow.

And a straight part on each side to slide over the rebar ground stake. (This corresponds to the length that wasn’t bent around the jig.)


We do a couple of test runs with the pipe bender to get the angles right. This is done with one person working the bender and another measuring the width of the arch until it reaches 14′ – this measurement is taken just above the straight part unbent by the jig. We measure the displacement of the hydraulic tube on the bender that corresponds to desired angles. After a few tries, we can quickly replicate the arch shape.

Though precision is important, these arches have a bit of play and close enough is usually good enough.

PUTTING THE ARCHES UP

The arches slip over 24″-30″ pieces of rebar placed in the ground. We aim to have 12″ above ground.

We set up two parallel rows of stakes 14′ apart though it is more important that the rows follow the growing beds than be perfectly straight. Inrow spacing varies in function of the season:

  • 4′ spacing for a snow bearing tunnel
  • 8′ spacing for summer use (some folks supposedly use 10′ spacing!)

The arch slides over the stake.


We then set up a rope as a ridge, wrapping it around the peak of each arch. On both ends of the tunnel, the rope is anchored to a t-stake sunk in the ground.

PUTTING ON THE PLASTIC

On a caterpillar tunnel, the plastic is held in place by 1/4″ rope.

The diagonal stakes holding the ropes are 33″ long (12″ above ground). They are placed between arches. Prior to placing the plastic, we tied the ropes to the stakes on one side of the tunnel.

We unrolled the plastic beside the tunnel.

Pulled it over the arches.

And anchored it to the T-stakes at each end.

Then, we tossed the ropes over and tied them to the opposite stake and voila!!!

Ideally, the plastic on a caterpillar tunnel can be pushed up and held in place with the tension between the ropes and arches.  None of the caterpillar tunnel owners I’ve spoken to is quite satisfied with this solution. Each grower has different strategies from clips to y-shaped branches on the ground. Emily and Reid thought up a solution for Tourne-Sol farm …

A hook made from a bent piece of wire (the same wire we use as arches for row cover) attached to the arch with a hose clamp. The hook can be rotated into the tunnel when not in use.

And with the tunnel in place, we’d revolutionised tunnel technology on our farm!!!

Or so we thought …

LATER THAT WEEK

What can our heroes do to overcome the wild forces of nature that wreak havoc upon their simple livelihoods???

Find out next on Going to Seed

22 responses to “Building a Caterpillar Tunnel

  1. Pingback: Tunnels « Going to Seed

  2. Nice video! Looks like it needs quite a bit of strength! What was Reid grinding in the background?😉

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  4. Pingback: Caterpillar Tunnel Workshop At Tourne-Sol Farm | Going to Seed

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  7. rebar is not enough – obviously

  8. Hello, my suggestion would be to use a wider pipe instead of rebar, so that the arches can fit tight inside the wider pipe ( it can be about 3′ long, 2′ into the ground, 1′ above the ground ), also putting more arches closer together will add strenght, and to add strenght to the walls, add at least three rows of pipes that run horizontaly along the whole lenght of the tunnel (on the inside ) connected to the arches, one on the very top, and the other two about midway between the top and the ground, to connect them, drill the pipes and attach them together with bolts and nuts. More expensive and labor intensive, but much more stronger.

    • Hi Alex,
      Thanks for the suggestions.

      One of our main goals building this type of tunnel is ease to dismantle and rebuild elsewhere. With each iteration we have removed more parts to find the minimum structure that will still stand up. As you can see in this post, sometimes we’d gone too simple. However the modifications we describe in the following post have been successful. We haven’t lost a tunnel in 3 years!

      Over this period we’ve also moved the arches further apart (to 10ft between arches) to reduce cost and reduce the amount of pieces to move.

      I think your idea of fitting arches into a pipe rather than over rebar might be a good one to try. I also really like your idea about the teepee entrance on the end. In our case getting the ridge ropes on the structure is one of the most laborious parts of the building process but if we moved the tunnel less frequently, this teepee entrance would be convenient.

      Take care, Dan

  9. Hello, my second suggestion which is not as labor and cost intensive. Simply add more rows of rope horizontaly along the walls just like you have done with the very top, add rows of rope 1′ to 2′ feet apart, but make sure they are properly anchored at each end of the tunnel to the ground. For anchors, use heavy gage pipes enclose partially in concrete, sunk into the ground at a 45 degree inclined away from the tunnel structure and at a depth of at least 5′, use several bags of Quickrete 80 lbs Setting Post Concrete Mix ( each bag cost about $3.78 ). In order to have access inside the tunnel, make sure the ropes are divided into two bunches at each end of the tunnel, the ropes starting from the top to the right, bunch them together and anchor them together to the right side of the tunnel, the ropes starting from the top to the left, bunch them together and anchor them together to the left side of the tunnel, that way you will have access to enter into the tunnel, it will look like a teepee type of entrance, but the structure will be much more stronger.

  10. Hello. I’ve just discovered your site and am enjoying it. I have three tunnels of similar configuration, except I have a solid ridge pole (galvanized pipe). I am taking out my ridge because I don’t like the friction points between it and the cover and I also think the cover would be more snug without it. Do you think your rope ridge is necessary? Have you changed the configuration of the hold-down ropes since you have gone with 10 foot spacing? I am considering putting one post at each hoop with two ropes tied to each so that they form an X between each hoop section. Any thoughts? Thanks.

    • Hi Jason,

      With our design, I think that a long structure benefits from the added structure of ridge lines. Last winter I skipped the ridge line on one of our overwintering tunnels. It did well for most of the winter, but in April, we lost the plastic. I think that the lack of ridge line gave the plastic too much give and made it easier to flap and catch in the wind.

      I have seen some growers using the crossed rope variant without ridge line that you’re suggesting. (http://multisheltersolutions.com/applications/hanley-caterpillar-greenhouses/) They seem to think it works but I have yet try it so can’t vouch.

      On 10ft spacing we have used the same hold down ropes. Though we generally keep our tunnels on 8ft spacing.

      Whatever you do, I would suggest starting with one structure and seeing what works or doesn’t works before replicating!

      Dan

      • Since my original post, I tried using a rope in place of pipe for the ridge on a smaller structure. I think the rope is better than the solid ridge because of how it is tied to the t-post at the ends. Its cheaper too so after I retrofit my three tunnels, I’ll have enough pipe to bend another tunnel. We live on a high plain at the edge of the Ozark Plateau and it is windy here most of the time. I’m using u-post as my rope anchors and so far they seem to work great. I make the “u” open towards the tunnels so if the rope is pulled on the u creates extra drag in the soil.

        Jason

  11. I am very grateful for the detailed info, links, and updates on how to build these high tunnel greenhouse. I want to build 4 of these which means I need to find a reasonable supplier of pipes.

    I am located in Nova Scotia and the only supplier I found is a fencing company that they would charge me an arm and a leg for it.

    Could you share with me where you buy your pipes from?

    Blessings, Marie-Mai

  12. Hi Dan,
    Jay Gray here. Thank you for your informative, detailed account of your experimentation with caterpillar tunnels. I’m glad to say that my program now has a greenhouse budget. In reading your posts and given the information that Fred has already provided to me over the phone – regarding your success with these tunnels and their flexibility in terms of accommodating a rotation plan – I’m leaning toward the incorporation of a couple at our growing site. This is as opposed to investing a more expensive high-tunnel or the like.

    However, I have a question about weed management in and around your tunnels. What I’ve gathered from the photos above is that you don’t put down and landscaping fabric between your beds or at the edge of the tunnel where the plastic meets the ground. So, I’m wondering how you manage your weeding in those spaces. Do you mow and/or rototiller and/or cover crop?

    Fred said that there may be an opportunity for me to volunteer with you guys during the installation of your tunnels this spring. It would be a great learning experience for me and good to see everyone again.

    Wishing you another bountiful year!

    cheers,

    Jason

    • Hey Jay,

      Well … the weeds along the tunnel edges can be a nightmare!

      For many years we relied on timely hoe interventions and hand pulling teams when things get out of control.

      Last year we put down some landscaping fabric with pretty good success. This year we will repeat that and also try some straw mulch.

      If you find a miracel solution, please share!

      We’d love for you to join us at tunnel time.

      Later
      Dan

  13. the link you posted earlier has been updated to http://multisheltersolutions.com/applications/hanley-caterpillar-greenhouses/ in case you want to purchase one, we ship out east🙂

    • I’ve updated the previous link. Thanks for keeping me up to date!

    • Thanks again Dan. I had the same idea – a combination of both materials. I just ordered my straw today. Miracles have been known to occur : ) We’ll see I guess…

      I’ll be back in touch in few weeks to see if you have tentative cat-tunnel construction schedule at that point.

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