We did a job with Andy Lewis and this is how we set up temporary slacklines without trees. The bolts we used can be bought at https://amzn.to/2VpKaTJ and the chains I used were the cheap 400lb rated ones from my local hardware store.

A more permanent slackline can be rigged without trees by building "dead man anchors", where you bury something and wrap a chain or rope around it and that becomes your anchor on either side.

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