Wildlife Pond

I think you need to need to get your basic pool sorted with butyl liner, ledges, etc. first. Filled, and allowed to settle. I guess you are thinking to use the feature as something like an "island" in the middle. You say it is 600 mm high, so its "end" must be about 50 square centimetres. The problem you are going to have is to prevent your "engineering" structures from cutting into the butyl liner. Also, you have to decide what materials to use that will be compatible with a wildlife pool. Stainless steel is probably an obvious choice, but IIRC fully galvanised steel will acquire a biofilm that will prevent excessive corrosion and leaching of zinc.

So, as a starter for 10, how about a stainless steel disk say 5-6mm thick and 50cm diameter (for strength and stability), with suitable upstands forming a socket for the "monolith". Then, on the bottom of this for compliance and to avoid a cutting edge, a 10 cm larger disk of (say) 18mm solid rubber sheet (or perhaps EVA foam matting). I think you will need to engineer a side entry for your water supply, keeping the supply at the bottom sounds like asking for trouble. To install it, three strong blokes with a length of scaffolding bar or suitable floor joist, suspending it with rope or nylon webbing like a gantry crane. (Easy enough lift for two guys resting it on their shoulders, with the third to help with adjustments). Lifting on the monolith, not the stainless, for stability so the base will need to be tight on the monolith.

The base has four sides. 200 mm and 250 mm approximately perpendicular, with 350 mm and 150 mm angled joining them. So, I reckon somewhere nearer 500 sq cm.

I wonder if some form of heavy duty plastic construction, re purposed from I know not what, might suffice?

I'm not at all sure that I have such a resource to hand.

Chris

OK, I don't really understand your "monolith" then. I was assuming it was something like the monolith in the film 2001, solid and 60cm high, the base area following from the density of slate.

Yes, heavy duty plastic should be OK to protect the liner, if you can get it in a suitable shape.

I have grown up children. Are you digging the hole yourself? If so, you might only need one other. If not, a pair of labourer / gardner types could do it.

I suppose another approach might be to dig your pond with a roughly spherical depression in the middle, put in the liner, then cast a sand-cement "footing" in the middle and bed your monolith in that, allowing it to set for a week before adding water. I'd still worry about any sharp edges from the monolith damaging the liner while you placed it. You would presumably need a few water changes to remove alkali leaching from the cement.

Are you sure you didn't drop a zero?

Here are a couple of shots immediately after installation, before the planting around it matured.

Chris

Assuming you do mean 600mm / 60cm deep - that's not very deep.

Ours is about that. 2 feet or so.

A few years down the line and it's got a few inches of silt / sludge in the bottom, which is full of life.

When the weather is dry the level drops quite a bit, not helped by a couple of punctures near the edge.

By the time evaporation has dropped the top of the water by 30cm, and the bottom has come up by 15cm, there's only 15cm of water left :(

I've been advised we ought to have a couple of meters depth, and re-digging it is on the list of things to do one day. It'll be better for the four species of amphibian that breed in it.

There's been a pond in our garden since at least 1855 according to maps.nls.uk so they've had time to find it ;)

Andy

My conversion shows 2 feet as 609.6 mm, so not really different?

I'll need to consider that, but it would make it a significantly greater task to excavate.

Sorry. When I read your posting properly, I see you were agreeing with me. My mistake.

Chris

I'm fairly new to ponds and have a small one on the rockery which is probably 2' deep at the most. The past two years the pond has become rancid and I've ended up emptying it, cleaning and refilling from the water butt.

The sludge at the bottom also has much life and I despair that this is going out with the wash. I assume there are such things as damselfly larvae in amongst it. Not seen backswimmers now for two seasons. Yet a frog seems to be happy in there.

I wonder how best to fix the issue, or at least preserve some life. Maybe I'm not clearing the bottom of the pond early enough in the season and this summer has been very dry. Overhanging walnut tree so there are walnuts rotting. I don't have fish and only a small solar pump/fountain.

The water butt allows me to keep the pond from getting too low and I try to avoid tap water. The water from the butt seems clean.

This shows the sort of thing I had considered for support:

Though perhaps over-engineered for my needs.

Upon reflection in a pool about the size of this one:

(interesting details about plants breaching his liner)

it seems pointless to fill much of the small deepest area with a pillar.

I am now thinking that from an engineering point of view, if I create a shallow area, with a small retaining bump before sloping down to the main pool (much as that guy did to create his bog section), I can mount the monolith there, and then provide the gentle slope into the main pond that is advised for wildlife, and I can cover it with some of the slate paddle stones that I already have.

Chris.

Yes sorry, 500 sq cm. The "floating slabs" idea is essentially the same as my second suggestion. Quite a lot of engineering in that, as you say. I think if I was doing it I would go for my original suggestion with a stainless steel "inverted mushroom" as a base. That way the whole lot can come out easily if you do develop a leak somewhere.

A black net over the top to prevent the walnuts and leaves falling in? Maybe add some more oxygenating plants too.

A certain amount of gunge accumulated at the bottom is natural. Plant roots and various invertebrate larvae thrive in it.