Underwater walling

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Underwater walling

Postby grab » Fri May 22, 2009 4:45 pm

Don't know whether you'd call this a "wet-and-dry stone wall", but anyway. Onto the question.

We've got a blank-canvas garden as the result of much clearing of the previous people's mess. The garden slopes slightly towards the house, so the plan is to split the garden into two levels and build a 2ft dry-stone retaining wall partway up the garden. The lower level nearest the house will be a patio, and the upper (larger) level will have a lawn, a bog/water garden and various other stuff. The patio will also have a pond, with water pumped up to the water-garden bit on the upper level and flowing over a small channel/sluice/whatever in the wall into the pond below. OK so far.

The tricky bit now, and where I could really use some help from the expects. I'd like to use dry-stone wall for the sides of the pond (underwater). This will be about 2-3ft deep at the deepest point. And the 2ft retaining wall will run along the back of the pond, so that at the back (the deep part) the wall will drop straight down into the water. So we're looking at a 4-5ft wall, of which half will be underwater. Why do this? Well, in a large part because we think it'll look great! But we also have two cocker spaniels who like being in the water, and we need to have this pond absolutely dog-proof. An exposed pond liner just isn't an option. And although it's possible to plaster sand and soil over the liner to cover it up, my parents' experience with their pond is that this just slips off over time.

I've looked into what guidance is available on building dry-stone retaining walls. I'm planning for the actual stonework to be about a foot wide, maybe a bit more at the base, at an angle of about 10deg off vertical. I'm also planning on using 2ft-ish deadman slabs back into soil behind (all within the pond liner to make things waterproof) to anchor the wall, at recommended intervals. And I'll be putting in a foot deep of foundations to make sure it's not going to sink under the weight. All this will be contained within a larger-than-usual pond liner, so that everything is waterproof, and I'll be using a layer of sand and some loft insulation or carpet on either side of the liner, to make sure that bits of stone don't touch it.

So. I'm well aware (from trawling Google and hitting gardening forums and finding no answers) that this is an unusual idea. I'm not worried about that - I don't mind trying something new, so long as I've done as much research as possible and really thought it through. What worries me is trying something that isn't new, and which people aren't doing for a very good practical reason! ;-) Hence asking the experts here...

Am I making a mistake I'm going to regret here? Has anyone done anything like this before, and if so, what would you recommend I look out for? And if there really isn't anyone who's done this before, then working from first principles, do any of you dry-stone walling experts have any thoughts on what to do (or not to do)?

Cheers,

Graham.

PS. To pre-empt answers... "Mistakes I'm going to regret" don't include starting a really big project - the entire house and garden is one big project which'll keep us busy for a number of years, and our priority is getting it right, not doing it the easy way. They also don't include serious spadework and general heavy lifting, because I'm used to that. Nor the requirement for careful attention to detail, because I'm a safety-critical engineer by trade (and vocation).
grab
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Re: Underwater walling

Postby stonewaller » Sat May 23, 2009 5:59 pm

Interesting project.
Of course I've walled underwater.....I livein North wales after all, never stops raining. Your basic theory is sound.

Not quite sure I understand all your specifications re tying back etc.
Likely to be too thin given height. Much will depend on stone type but because of the liner you cannot tie the wall back. Ideally you need regular flat stone with (imho) an absolute minimum length into the wall of 12". Depends actually how you do it.

That said the biggest constraint on stability is going to be your ability and experience. As we all know anyone can build a wall, its oh so simple. :lol:

That will be £100 please, 250 if you want an actual specification :D

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Re: Underwater walling

Postby grab » Thu May 28, 2009 11:12 am

Tried replying before, but work internet connection is dodgy. :x

Thanks Sean. I've walked in Wales, so I sympathise with those who have to put up with that weather every day! :wink:

I'll be making the wall thicker around the pond then - more like 2ft at the base and tapering to 18" at the top. Re the tying back, I was planning on digging the hole significantly oversize, so that I *can* backfill with spoil behind the wall (still inside the liner) and tie the wall back properly.

Another option of course is going the concrete route for the pond and cementing in a "dry-stone-style" facing. Then just use proper drystone for the 2ft retaining wall where there's less wall and less risk. One worry there is the cost of getting someone to do the concreting, but then again there's the extra cost of a whole load of dry-stone rock compared to the cost of concrete - swings and roundabouts. And there's the risk of doing it myself either way. Hmm...
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