Backyard Ninja Course

My kid wanted a ninja warrior course in the backyard. He'd been training at a local ninja gym and watching American Ninja Warrior, and his birthday party was the deadline. We bought a ninja obstacle course kit. It came with a slackline, among other things, and the whole thing hangs between two steel poles about seven feet high so kids can climb and swing on the obstacles.

Setting the poles permanently meant anchoring them properly. I drilled two holes about four feet deep with a one-man hole digger while my wife filmed. Each hole got a layer of gravel, tamped down, then the pole, then concrete poured around it. I poked the concrete down with rebar to work out the air pockets and left it to dry. The part I'm glad I did was the sleeve. Instead of cementing the course poles straight into the ground, I cemented in a larger-diameter pole as the anchor. The thinner course pole slides inside it and lifts back out, and the anchor gets capped when it's empty. The footings are permanent (buried, safe to mow over, easy to forget about) but the poles aren't. We can pull them for adult gatherings when you don't want steel poles in the middle of the yard.

The first set of poles bent almost immediately once kids climbed on. That story, including the ChatGPT conversations that got me there, is in How ChatGPT Helped Me Bend Steel Poles. I won't retell it here.

The sleeve turned out to be the real payoff. Being able to break the course down is nice, but the bigger thing is that the socket in the ground now works as a general-purpose anchor for whatever pole the yard needs. One buried footing, several completely different uses.

What's in the ground now

• The ninja course and slackline, when it's up. Used for the party in summer 2023 and again the following summer. By summer 2025 I'd added guy wires (twist-in ground anchors with cables pulling against the slackline tension), so multiple kids can bounce on it without sway. Harder to mow around, though.
• String lights, strung from the same pole.
• Right now, a tetherball-style baseball rig for my son. The ball is on a line. He hits it, it wraps around the pole, and it comes back for the next swing.

What I learned

• Sleeve the pole. Cement in a larger-diameter anchor and let the real pole slide in and lift out. The footing is permanent and the pole isn't, and the socket ends up holding whatever the yard needs next.
• Gravel first, tamped down, then the pole, then concrete. Poke the wet concrete with rebar to work the air pockets out.
• The poles were the weak point. The first set bent almost as soon as kids climbed on. How I fixed that is in the linked article.
• Guy wires (twist-in ground anchors with cables pulling against the slackline) stop the sway when several kids bounce at once. They also make mowing harder.

Photos and video

The install: poles and sleeves, the sleeve in the hole, a pole standing in it, and the capped sleeve once the hole was filled.

The pole flexing with each swing, before the fix.

The bent pole after removal.

Close-up of the stress fractures from compression and bending.

Success.