Preventing pallets rotting

I'm going a bit pallet-mad here. Already made a fence out of them:



That's going to have old bricks underneath it to keep them from rotting, or at least slow it down (none of it is permanent except the posts so I can lift the pallets off to fix/replace/add the bricks).

Now I'm starting a new project, and need some suggestions. I have some smaller pallets left over which I've going to use to build a dog tunnel, as the dogs are a bit miffed that they've suddenly lost most of the garden! It'll be something like this:



But a bit sturdier once I've screwed it all together! Now because it's going to be stood in the lawn, rotting is going to be an issue at the bottom and as it's going to have potted plants on top and dogs underneath, I want to preserve it for as loing as possible.

I'm going to dig trenches for the base to sink it a little and backfill to secure it. What's a good way to help stop the bottom edges from rotting (or at least slowing it down considerably)?

 

You could staple some thick polythene to the pallet that's going into the trench.