Thread: New build - uneven base question

Hello team
Long time no see.

Due to damage to my old pond I had to accelerate my planned "one day" rebuild, and that day is here.

I've got a question about a problem with my excavation and how to remedy it please.

The digger man I hired didn't create a very even base and one end had a dip that's quite noticeable, especially after rain when the hole gets water in. I don't want to get him back - significant cost for him and for a new grab lorry, and he was not very pleasant to work with. I don't want to manually dig out the soil to flatten it - lots of effort and would need another grab lorry.

I've got diagrams below in which the dip is exaggerated for clarity, but in reality it's about 7cm difference. My question is about how to remedy that dip.

I have a few options, each with pros and cons. Grateful for your advice, with a particular focus on the resulting structural integrity.
I plan to lay ~5cm (when compacted) of type 1 MOT base and the pour a ~30cm reinforced slab (drawing inspiration from Feline's "over-engineered" build, but going further due to my clay soil).


Option A - fill with soil spoil. Whack it down with a whacker. Then lay a uniform MOT base, and a uniform concrete slab.
Pro: cheap, easy (soil is still on hand)
Con: strength? gooiness of whacking wet clay.

Option B - don't fill, lay uniform wavy 5cm MOT, and let the concrete fill
Pro - easiest of all
(The additional cost of extra concrete is not of concern in the big scheme.)

Option C - fill with MOT, then pour flat concrete
Con - I probably don't have enough MOT and am reluctant to buy another bag only to use part of it and have the other half of the bag sat on my lawn for years while I figure out what to do with it

Option D - fill with broken bricks and soil (both spoil), whack, then lay even MOT and even concrete.
Pro - easy, cheap
Con - as per A?



What are the strength (dis)advantages of each approach please?



Thanks
Pond base problem.png

If it's out by around 7cm, I personally wouldn't worry too much about it.

I'd just try and put a bit more mot down in that area if you can.

Also, even if it's still not quite level, once the MOT is in, it will level itself out when you pour your concrete slab. You just need to ensure any shuttering you put in (if you plan to use any) is level.

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I'm on heavy clay and the base of my slab is 1.2m below the surface. I think a subbase on heavy clay is not required-if I was you I would just use a nice wet mix and it will practically self level. Its the walls on clay ground that are important-clay shrinkage can leave a wall thats been backfilled with dirt unsupported. Only backfill with washed stone. Trying to compact type 1 on soggy clay with a wacker plate might just liquify it. Ideally the clay wants to be disturbed as little as possible.

Thank you
Some useful things to consider there.

Originally Posted by SteveW1

Only backfill with washed stone.

I'm planning to backfill with more poured concrete. Expensive, but I agree that it's the walls that need most support (that's where my last pond failed).
If money were no object, would you still recommend to backfill with stone rather than poured concrete? Could you explain why please?
Thank you.