Gabion baskets for post anchors

This will be yet another uber-niche blog post that maybe three people in the universe will find interesting. For the rest of you, the “x” in this browser tab is beckoning for your attention.

For the last year I’ve been using gabion baskets, primarily as retaining walls. They’ve worked really well and I’ve enjoyed the benefits. But this project called for post anchors and I wanted to try out an approach using gabions again.

Mixing concrete is dirty, dangerous, and not something that little kids can help with in a meaningful way. Cement has a lot of embodied energy and while it’s fairly cheap you actually need quite a bit for anchoring. Hauling that much material is a 2 hour one way trip for me. Running the mixer, cleaning things out, and using up the water is also low on my fun list. I have a backhoe so digging holes isn’t hard but if you don’t have a mechanized digging solution it can be very difficult to get deep enough.

Now there are times that I used and will again use concrete: it’s a fabulous material in a lot of ways. But if I can avoid it, I will. And since I’m in a place with zero building codes, I can experiment at my own risk.

The first thing I did was fabricate the metal brackets above. Angle steel, bar stock, and my flux core welder got the job done. I put on a few coats of paint which will eventually chip from the rocks coming in contact but should help a lot for a while. I used a step bit and some cutting oil to drill the holes in the brackets and made those holes a few inches above the top of the gabion cages. A long wood bit, and a second set of eyes telling me to level the drill, got me dead-on accurate for drilling through the wooden posts once they were in place.

I actually put cardboard down on top of the horizontal steel members to try to protect the paint a bit from the rocks that will be sitting on them. Eventually the thin cardboard will break down but again, it will buy me some time especially in the beginning before the rocks become nearly impossible to move.

With the cages “open” I put some very large rocks onto the horizontal steel members. It’s a lot easier to roll them into position than to try to drop 150 pounds of granite from 3 feet and not destroy what it hits. You can see the cardboard sticking out there a bit. Once all the horizontal members had sufficient weight on them the sides went up and the cages got filled like normal.

Midway through the basket filling, above. One of them is getting pretty high, the others got a bit further before we stopped for the day.

This structure is going to be supporting a 20×10 foot solar array in a very windy area with substantial uplift concerns. Having very secure anchors was a big goal we wanted to achieve and I’m hopeful that we accomplished it.

Some back of the envelope uplift calculations showed me that at 130mph each anchor could receive 1,040 pounds of lift. I believe the weight of the structure itself reduces that to whatever degree but each gabion cage should weigh roughly 4,000 pounds. Some of the rocks in the cage adjacent to but not directly over the horizontal members will not provide uplift support, but certainly more than 1/4 of the rocks will be providing ballast.

Stacking rocks may not be the most intellectual of pursuits but it’s relatively low-risk work that even the smallest child can help with. Plus, from clearing land we have a lot of nuisance rocks left over and gabions provide a great way to consume those. They can also be filled over time. Without the panels off, several hundred pounds of rocks on each anchor is sufficient for the time being. Once the panels are on the additional two-tons sitting on each anchor point can be added.

So far we’ve seen no corrosion on the Galfan finished gabion wire that we’ve had in service for over one year in our retaining walls. In a wet or salty environment this may not be the case.

One other comment I’d make is that this is not the fastest way to get things done. I probably invested 10 hours into fabricating the brackets between cutting, grinding, and welding. Painting was just a rattle can every day so I’m not counting that. And the steel isn’t free, but neither are strong ties and sono tubes. I was luckily able to use up some scrap angle and flat stock to get this done. When you can use up material that otherwise would go to waste it’s a great feeling.

If you’re getting paid by the hour there’s no way this project would make sense. But if you like metal fabrication, have a lot of rocks at your disposal, and don’t need to get building permits, you may want to give it a go.