Pond building

[DrPhill]

We have made plenty of ponds in previous gardens. This time I am contemplating a departure from the tradition of using old carpet as an underlay for the liner. I am contemplating using a sand/cement mix to stabilise/smooth the hole before using commercial pond underlay and then liner.

Has anyone got opinions on the relative merits of the two methodI?

If I use sand/cement, then what mix? The cement would only be in the mix to hold the sand in position if I cannot get the job done in a day, to prevent sand being displaced while putting the liners in. How much sand/cement mix should I aim for? Hallf an inch seems about right - it is just to cover up stones.

I would appreciate any observations or advice.

 

[iNewbie]

does this help?

http://www.concrete.org.uk/fingertips-n ... lay&id=809

 

[DrPhill]

does this help?

http://www.concrete.org.uk/fingertips-n ... lay&id=809

Thanks, but not really. The link describes a pond where the concrete provides the waterproofing, and needs to be quite thick - 100mm is four inches I reckon. What I am aiming for is a thinnish layer of sand to protect against stones, with waterproofing provided by a flexible plastic liner (with a commercial underlay for guarantee purposes).

It was interesting to read, but just strengthened my prejudice against concrete ponds. What a work-up.

But thank you for taking the time to post.

 

[blackrodd]

If you just want to form a "crust" under you're liner, then use 9-1, and keep it semi dry, as a screed mix would be. It's a common mix we would use in laying paving bricks and slabs etc,
Especially in sloping areas where the sand under moves and you can see dips in the traffic areas after a while.
I would use a couple of inches before the liner and the crust formed would serve the same purpose as above.
As long as underneath is well compacted, of course.
HTH Regards Rodders

 

[DrPhill]

Some useful figures there, thanks Rodders. At 2" (50mm) the pond (2.3m * 2.9m) would require about half a tonne of sand. That is a lot of mixing. I could be content with a thinner layer - an inch starts to look feasible (quarter ton, 10 jewsons handybags?). I was planning on using a 'proper' pond underlay too (helps with the guarantee) but they are little better than old blankets.

The ground underneath is fairly well compacted. It has been exposed (rather than built up) and was previously very sub soil (up to a couple of foot down) or the footings of an old path that has been removed. Since the liner will flexible a little movement would not be a problem. The risk is that changes (soil movement, sand erosion, carpet rotting) introduces a sharp object to the liner.

I am not sure what you mean by 'semi dry'. I would like it to keep its shape while we get the underlay and liner in. The jobs may have to be done on consecutive days. Would that mix be strong enough?

 

[RobinBHM]

If you squeeze a handful of mix, it should stay together without any water coming out.

I just wonder if movement of soil might cause the weak crust to crack and move? I suppose once the pond is filled the water pressure will keep the thin layer of set lean mix in place? It might be tricky to spread out the layer under an inch thick?

 

[blackrodd]

Some useful figures there, thanks Rodders. At 2" (50mm) the pond (2.3m * 2.9m) would require about half a tonne of sand. That is a lot of mixing. I could be content with a thinner layer - an inch starts to look feasible (quarter ton, 10 jewsons handybags?). I was planning on using a 'proper' pond underlay too (helps with the guarantee) but they are little better than old blankets.

The ground underneath is fairly well compacted. It has been exposed (rather than built up) and was previously very sub soil (up to a couple of foot down) or the footings of an old path that has been removed. Since the liner will flexible a little movement would not be a problem. The risk is that changes (soil movement, sand erosion, carpet rotting) introduces a sharp object to the liner.

I am not sure what you mean by 'semi dry'. I would like it to keep its shape while we get the underlay and liner in. The jobs may have to be done on consecutive days. Would that mix be strong enough?

At 2"or 50mm thick, you're area will need less than half a cubic metre,= or 15 cwt,=3/4 of a ton
An average wheel barrow will carry 2 cwt (hundred weight), =10 barrows to the ton,
16 average shovels to the barrow load,= 8 shovels to 1 cwt, (hundredweight), = 50.8 kilos.
The average electric barrowmix will just about take 12 shovels of sand/cement mix,=1.1/2 cwt, (hundred weight).
on level ground.
When you are a plasterer, you need to know stuff like this!
As long as the and cement mix is compacted as a floor screed is, it should not pierce the liner.
I would rather do it all in a single day for peace of mind.
HTH Regards Rodders

 

[John15]

I agree a weak mix, 9 or 10 to one, with a minimum of added water, just enough to hydrate the cement and probably 25 to 35mm thick would do the job. You aren't looking for anything structural, just a barrier between the stony ground and the blanket. If you go for the sand cement option you will have to leave it for a couple of days to harden.

I used a one inch layer of soft sand plus 2 layers of blanket for my pond. It worked well.

John

 

[MMUK]

The last pond I built, over ten years ago, I just used damp building sand for a blind. It held itself well enough to get the liner in place.

 

[DrPhill]

Useful rules-of-thumb Rodders. I will file those away.

The material costs and effort of mixing may put me off, though done without cement (no mixing) might be more attractive. Access to our garden is awkward, so large amounts of heavy materials give me pause. The delivery lorry stops a fair few yards away, then it is barrow the bags down steps. I would likely bring the bags by car so several trips would be needed.

I will think this through - I have time as the weather is still too warm to fill a pond.

MMUK, John15 - thanks for confirming it works. I prefer the idea of of a weak cement mix in as much as I can do the job over several days (less pressure to finish), but pure sand with no mixing has its attractions too. I have not ruled out the 'old carpet' strategy either yet.

 

[blackrodd]

THB, Carpet and sand in the usual way as the liner manufacturers recommend is fairly tried and tested, my landscape architect mate always lays them out in that way and they do seem to stand the test of time!
Regards Rodders

 

[DrPhill]

Carpet and sand?

I may try the sand/cement mix route for the lower parts of the pond. There used to be a path and a wall - some parts of which remain to give the pond 'structure'**, but will need careful covering to reduce risk to the liner. If I use a slightly stronger mix (6 to 1?) I should be able to do the job in stages and not worry about the weather. The shallower parts of the pond are less steeply sloped and could be done with sand or a weak sand/cement mix just be the liner goes in.

**I like shelves with edges that stop the plants/silt/etc sliding all the way down to the deepest part. The shelves then support their own selection of plants and wildlife distinct from the deeper reaches.

This pond is right outside the house - by the front door and outside our bedroom window - so I am trying to do the best job I can within reason. All my other ponds were less prominent and so I worried less.

Should I use sharp sand or builders sand? I have it in my head that sharp sand would be better, but I am not sure where I picked up that idea.

 

[whiskywill]

I will think this through - I have time as the weather is still too warm to fill a pond.

Why does too warm matter?

 

[DrPhill]

Warm matters because the water evapourates, exposing the liner and concentrating the tap water that we will need to use for the initial fill. To protect the liner we will keep the pond topped up with water - necessarily from the tap, further concentrating dissolved solids. In this part of the world with soft water that may not be a disaster, but it goes against the grain.

I decided to 'render' the lower reaches of the pond last Sunday to prevent the soil falling in as well as to protect the liner against rocks. I am glad that I did, as the rain on Monday would have undone some of my hard work. I am also glad that I decided to leave the very bottom unrendered, as the rainwater could drain away. I will use a weaker sand/cement mixture for the shallower parts (maybe even just sand) just before putting the liner in.

 

[MMUK]

Using tap water in a new pond is fine. Any chlorine and other additives (thanks water board) will evaporate in no time in the current weather.

 

[DrPhill]

It is the non volatile nitrates, sulphates and phosphates that we are more worried about. They will promote an algal bloom before the local flora and fauna get established. Add to that our tap water is amongst the most expensive in the country as our water company has the responsibility of maintaining lots of beaches.