Concrete base on sloping Garden

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Hi all,

I am wanting to put a concrete base in for a hot tub in my garden. The garden slopes from the rear to the front. If the base is circa 2.5m x 2.5m the drop is approx 30cm. The base will have to be shallow at the rear and deeper at the front as I am building decking around it.
A couple of questions surrounding it, (no pun intended). Is it ok to have a concrete approx 450mm thick at the front? Thin end, rear of the garden will be about 150mm thick. Should I use rebar to strengthen it?
Any suggestions in framing as I will have a flat area where I have dug down, going into a slope.

Let me know if I need to post more detail
Thanks in advance.
Al
 
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Assuming the decking will cover the face of the 'deep end', I'd think about a wall of a couple of courses of blocks (a couple of courses out of the ground) along the deep end, with a return into the slope.
Fill the void with hardcore to allow 150mm concrete slab.
The wall would need a foundation but that would be minimal amount of concrete to mix by hand, say about less than ½³ metre of concrete.
Then the hardcore would only be about .5³ metre of hardcore.
Then the 150mm slab would be a bit less than 1³metre, again could be mixed by hand.
The slab could even go over the top of the blocks wall, by screwing a piece of timber to the face of the blocks, protruding sufficiently above the blocks.
 
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I have now dug out the earth from the area. The aggregate and concrete will be above the ground at the lower end of the slope which is where I will need the supporting wall. (See pic) The aggregate will be starting at ground level and going up, supported by the retaining wall.
I just need to know how deep to dig the foundations for the retaining wall and how thick the foundation will be.
IMG_1018.jpg
 
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Unless you particularly want a wall for aesthetic reasons a row of 900x600 concrete paving slabs laid vertically and lengthways on a bit of semi dry and backed up with concrete on either side would do that - it forms the shuttering, contains the dpm and leaves a reasonable finish compared with the exposed concrete slab edges. Fill in with hardcore to 150mm below FFL and don't forget fall on the slab.
 
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FFL = Finished Floor Level, saved me having to type "top of the concrete", or not!
 

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