Best approach for a retaining wall?

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I'm building my first retaining wall to level the slope in our garden for a garden office. I've been quoted £1,500 to build a 9m x 60cm retaining wall and foundation by two separate builders. Labour only.

Suffice to say I can't afford that right now. I thought it would be more like £500.

I've got quite a bit of DIY experience, but I'm not a builder. I'd really appreciate some help on a few basic questions:

1. How deep below ground do I need to go for a 60cm above ground wall?
2. If I build it in block does it matter what kind of block I buy?
3. How do I build the bottom row? I've seen things like this that recommend 2-3 thick (the bit at the bottom on retaining walls):
4. How much spacing do I need between blocks (so I can calculate how many I need)?
5. If I put a drainage pipe behind and some holes through the block at the front, do I need to do much more for drainage? It will be filled with mostly hardcore.
6. I've read that the bottom row of block needs to be laid into the concrete foundation before it sets, but I've also read you need set steel bars into the concrete to anchor the blocks in place. What is the cheapest option to secure a 60cm above ground wall?

Thanks very much for your help!
 
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I had to build a 900mm wall so I could level off the slope in our garden for a shed to go in.
I put long I beam in sloping into the hill so that top of the wall would be set back about 10-15cm.I had to use a soil auger to make the hole for each I beam, sunk about 800mm into the ground, slid sleepers between each I-beam, so that the I stopped the sleepers from falling out. It worked out cheaper than block/brick.
I also made sure the steel had plenty of primer, allowed to dry properly, ditto for the undercoat and topcoat, about 6 years later, no rust visible.
Also poured in dry mix concrete around each beam, then wet it after all the sleepers were in place. that allowed the spacing to be correct.
 
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I've been told I need a concrete block or brick wall as it will be supporting the shed / office. Is that incorrect?

Would it be better and cheaper using wood?
 
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what sort of soil do you have? That can make quite a difference to the best answer. A timber wall will not last as long as block and brick but can be done quite a lot cheaper. At only 2 feet high it does not have to support a lot of weight. Install the wall with a decent slope and you could replace sections at a time without it collapsing on during any repair. My soil is very free draining and quite stony just below the surface, so timber makes sense for my garden.
 
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The soil is loose and soft at the surface and then clay about 8-12 inches down. I was told by one of the builders who quotes me £1,500 that after the wall was built I'd need to fill it with several tonnes of hardcore. Apparently I need a lot of hardcore (and the guy was saying preferably concrete on top) to ensure there is no movement when the shed sits on top. The shed will be quite big (6m x 3m).
 
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Is it a thin bed/lens of clay, or is the clay really thick? The load of hardcore is a red herring, you just need the concrete raft thick enough not to break when the soil moves. You already have 8-12" of movable stuff.
If the clay is thin then you could drill a bunch of holes under the line of the "shed to come" about 6" diameter breaking through below the clay and fill with dry concrete mix, it will go off nicely later...
 
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Thanks. I'm not sure how deep the clay is. I can have a look when I dig down for the foundations of the wall. How deep should I be looking to go for the black wall foundations?
 
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the town or county you are in will give clues.

within the London basin, clay is hundreds of feet deep.
 
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whereas in bagshot Surrey you usually just have thin lenses of clay to contend with, used to live near there, so had a few clay patches in the small garden we had.
 

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