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Filling (then building on) a pond.

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BearTricks

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Hi all.

Our new house has a small, fairly shallow pond at the bottom of the garden. The previous owner seems to have put it in themselves - it's definitely not a pro job. He will be taking his fish with him, and I want to fill it in at the earliest opportunity because I don't want to take care of a pond and I'm hoping I can build my new workshop on that part of the garden.

The pond is between a metre and two metres across and takes up about a 1/3 of the width of the lawn. It's a relatively narrow victorian semi with a long garden and I was hoping the workshop could take up the full width which means building on top of where the pond was. From a guess it's not much deeper than a foot (mixing my units of measurement here) but I might be wrong. We haven't actually moved in yet but I'm just planning ahead.

The conventional wisdom online says you should fill it with gravel and rocks, and compact it if you can, but you should expect some subsidence. I'm assuming this is for bigger ponds and that my workshop isn't going to tip on its end but while there's a decent amount of info on filling the pond online, there's surprisingly little on building on top of a filled pond.

Any ideas? We're a few miles from the coast so the soil is sandier than usual if that makes a difference.

The other option is to build alongside the fence and avoid the pond altogether which I would prefer to avoid as it would then be on a shared boundary and I don't want to cause any issues.
 

fenhayman

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Is it watertight and is it lined with plastic or puddled clay?
 

Garden Shed Projects

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Tricky to answer with out knowing what you are looking to build. Best to remove all the soft ground at the bottom of the pond down to something solid and then replace with compacted stone, 6F2 or type one. Alternatively, depending on the size of your workshop, you could put your foundations in virgin ground either side and span the floor across it.
 

Jameshow

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What are you intending to build?

Timber I'd just span across.

If brick I'd fill it with rubble / ballast compact and lay a concrete slab. The water will have done a good job of compacting the soil below. What you don't want to do is fill with soil and lay a concrete slab as the soil will contract.

Cheers James
 
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Stigmorgan

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As already stated, first you need to know if the pond is watertight, if it is then you just need to drain it then fill with a compactable material such as 6F2 or Type 1, if it's under a foot deep you should be able to compact it in one layer, if it's over a foot deep then it would be best to do it in layers of a foot in depth. If it's not water tight then you need to begin by removing as much of the saturated material as you can so that you are starting on dry stable ground.
 

Terry - Somerset

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Does the pond stay filled with water naturally or need filling.

The former may suggest a risk of continued soggy soil and damp which would affect any building over the top. If artificial I assume filling it in is no great issue.
 

BearTricks

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It's watertight as far as I am aware but I'll find out on Friday.

The workshop will probably be log cabin style unless can get a deal on a nicer garden workshop. Thinking of buying a prefab one and insulating it myself but I haven't priced materials up yet. Not above building it myself if I can figure out how to.

As for a base, whatever is cheapest and easiest (and looks the tidiest). I'd been thinking of going down the concrete pour route but again it will all depend on price.
 

ColinH2O

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If the pond is still water tight, depending on construction you need to :
 

ColinH2O

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Oops - wrong key

If plastic or rubber liner . . remove
If concrete . . smash it up
If moulded fibreglass or hard plastic . . remove

Otherwise it will remain a swamp, no matter what you fill it with
 

Jameshow

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Oops - wrong key

If plastic or rubber liner . . remove
If concrete . . smash it up
If moulded fibreglass or hard plastic . . remove

Otherwise it will remain a swamp, no matter what you fill it with
You could just stab it a multitude if times . .

Cheers James
 

RobinBHM

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I would recommend digging holes with a fence hole spade, deep enough to reach hard ground, then cast in some 25mm stainless steel threaded bar (With a 100mm square plate say 150mm up from bottom).

once you’ve done that you can bolt a sole plate in and joist in between.

you wouldn’t have to deal with compacting any backfill or anything.

if it’s a big volume of material, get the cheapest crushed concrete or sub soil to backfill with.
 

Stanleymonkey

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I've just filled a pond in for a friend. Small off the shelf liner type. Compacted it don as I filled with soil each time I added more I jumped up and down and walked it over. Filled it proud and six months later it is still proud of the ground! I help them out once a week, so I suppose there was plenty of time to compact each small layer.
 

okeydokey

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When I did similar a few years ago I did as ColinH2O suggested after bucketing the water out and throwing it on the grass (I don't have a pump) the water just soaked into the grass. My pond was filled by rainwater and sometimes a top up with hose pipe when its evaporated/hot weather.
Then I removed the fibreglass liner and dug a few half-hearted holes to break the compacted earth a little bit.
I had taken down a small wall the bricks went into the hole and a few bags of large gravel from Travis Perkins and it ended up sort of level, I then overfilled the hole a small amount with some soil, left it a few weeks with occasional jumping on the soil (with hindsight I should have hired a compacter machine for a day) and job done. Then some used large paving slabs and I put the shed on top, the floor bearers bridged some of the hole anyway. A few year later no problems that I'm aware of.
 

BearTricks

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Paving slabs might be the way. Our seller laid his own patio by the house which we'll be tearing up once the weather warms up and possibly putting some decking in so reusing those is looking like a good idea. I'm sure this board will be full of photos of me bodging my workshop once I get started on it.
 

Keith 66

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A small pond like that i wouldnt worry about too much, if its got a liner rip it out & fill it up.
What you dont want to do is fill a large pond, My wife & i used to have a flat in a close, 6 blocks of 4, 24 in total. The land about 1 acre had originally had one tiny bungalow lots of trees & a very large pond, The builders stripped all the oversite off & bulldozed it into the pond, the trees went on top then got burnt. Where the pond had been was where two of the blocks of 4 flats went.
Of course some years down the line they were the ones that needed huge underpinning work!
 
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