Tyre foundations

New to this. Not a full earthship query. Looking to build a small (6m*3m) shower block for some tents at our place. We are trying to do everything as sustainably as possible. Looking at using packed tyres as foundations for the wooden structure. I was looking for some advice on the best way to secure the wooden floor structure to the tyre foundations. Any help gratefully received!!

blots n nuts or maybe make peg like spikes (cleats for the floor)?

I use tyres as garden walls about 4 feet high. I fill them with empty beer bottles and I employ people to drink beer, since I could not drink all the bottles I need!!!walls,
I found it best to screw the tyres together with some hex head roof screws. They dont move then.
I think you could screw the timber directly to the tyres, based on my experience with screwing the tyres together, the tyre walls grip the screws well.

If you are affixing wood to tires,  I would suggest torx headed decking screws.
They are pricey, but they last and they drive like a dream.

John,the hex headed screws seem like great idea,  do you get the ones with it without the  washers?
Are they self tapping?

I love your use of beer bottles,  I will have to try that.
Do you cut out one or both of the sidewalls?
I have some tires with both sidewalls cut off,  and one with only one sidewall cut off,the rest of the sidewalls are stacked inside of it, because, they have to go somewhere...
I have been planning to use the tires to geo-stabilize a gravel driveway, an idea known as mechanical concrete(http://www.mechanicalconcrete.com/) , but I'm just collecting/prepping them right now.

It occurs to me that in the future  I will leave a few tabs of sidewall to drive stakes through.

William, you can purchase most of the screws without washers, but if they have them they are easily removed. I keep them for later use..
They are self tapping into softwood and tyres.
I dont cut anything, I stack them in piles of the same tyres, and screw them at each tyre to the adjacent ones.
I put a small zigzag of say 6 inches in the wall to give it lateral strength.IE, each stack is set 6 inches in front or behind the adjacent one, also if I want a 'straight' wall I put a small curve as well as the offsets
to give aded strength to the wall.
I cover the top lay of bottles with clay to shed the water.
They work great on my farm as a low windbreak.
I have filled single layered tyres with rocks as a low step or retaining wall, and gone one or two layers higher by stepping back each layer.
They make great steps down a slope.
I have filled them with soil and planted into them as well.

I collect the bottles from roadsides, hotels and restuarants and individuals who save them.