New Pond Construction Query

[VanDiesil]

Hi, new poster here needing a little bit of advice.

I have just excavated a new pond in a trapezoid shape.

Dimensions are;

Longest Width - 3.6m
Shortest Width - 2.8m
Sides - 3.6m
Deepest - 70cm d x 2m l approx
Shallowest - 45cm x x 2m l approx

Liner is in situ on a 25mm bed of sand, old bedroom carpet on top of sand with dedicated pond liner over the base and sides. I have left approximately 50cm minimum of liner around the edges (some sides have more due to buying a 6mx6m liner (was going to have the depths of 1m and 50cm but decided against it due to back killing me!!!!).

My dilemma is the edging of the pond. I originally was going to just lay slabs around the edge to trap the liner and underlay in place, but was thinking of making the pond deeper by standing the 900mm x 600mm slabs on the long edge around the pond. My initial thought was to trench around the pond one spade width wide & 30cm deep, leaving a soil edge of 15cm wide from the edge of the hole to the trench, then to place the slabs on edge around the pond and to concrete in place. Would this be sufficient enough concrete to hold the slabs in situ, with 30cm of slab below ground level and 30cm above (giving another 30cm depth to the pond)?

The liner & underlay would then be pulled over the top of the slab and then tucked down the back side of the slab where it would be held in place by the excess soil that has been dug out of the pond (a 75cm high "fence" would be built around the outer edge of the upright slabs with a soil width & depth of 30cm and finished with a capping stone).

My other thought was to dig a trench, a spade deep & wide, around the pond leaving a 15cm gap from the edge, tucking the liner into this and covering the liner excess with concrete, and then laying the slabs on top of this concrete trench. This way would be easier, but would make the pond shallower.

What are your thoughts on which way I should go? I am erring towards the upright slabs, but an a little concerned that they may topple over time. The latter I am a little concerned that the slabs may come loose or the concrete break and end up with a slab in the pond tearing the liner (as I would like to have an overhang of the slab to hide the liner edge).

 

[VanDiesil]

15 views & no suggestions? Do I take it as my thoughts are right? I want to get it finished this weekend, piping & filters in situ and filled & obviously I will have to go for the latter as that is the safe option.

 

[sissy]

Thing is you don't want your pond level with the ground or stuff will wash in with rains and to be honest the only place I put the under liner was on the bottom usually no rocks will go through the sides .Plus the carpet could trap water on the sides and bubble your liner out .As for edging I know there was cement rocks made that were an L shape on here and that covered the liner but the sides of the pond were higher than the surrounding ground

 

[addy1]

I didn't concrete my edging rocks in. Made a ledge around 5 inches lower than the edge of the pond. The edge I dug tilted back so the rocks would not slide off into the pond. Then stacked rocks on the ledge and up and out of the pond. Some over hang, some come right to the edge. They have been pretty stable for quite a few years now. You do want the edge of the pond higher than the surrounding land to keep any run off from getting into your pond.

To stand the rocks on edge looks unnatural, but might still look fine if you like the look. I like my rocks laying more like they would in the wild.

In reading your post again, i think you are saying you will use the upright slabs to make the pond edge higher, tuck the liner over the slab and bury behind it. That would make a solid edge and lock the liner in place. I would think the way you are describing it the slabs would stay in place very well. The cap stone on the dirt fence would look nice. Also create a barrier to keep water from flowing into your pond.

So in reading your thoughts a few times to get the gist of it all i would go with #1 way.

What is you weather like? freezing thawing cycles?

 

[sissy]

glad you got it I reread and still am lost a little .Hard to wrap your brain around an explaination

 

[addy1]

Pictures really do help a lot.

My take is the slabs are going to be covered with the liner, planted in concrete, a dirt berm build up behind the slabs to make a nice berm for a cap stone and to anchor the liner. I like that way since it will keep any rain water from flowing into the pond and make the pond deeper.

 

[VanDiesil]

Thanks, yes Addy, you did read it correct. After rereading it myself, it does sound a little confusing. I was concerned about the weight of the water pulling on the slabs and as the earth dries out then causing the slabs to lean over or the concrete cracking (as a builder friend of mine I spoke to yesterday said that he would have 12" of concrete either side of the slab to make it secure.

We get strange weather over here where it can go cold and stay cold for a long time, but on the other hand, it can go cold for a few days then go mild again and continue this way for the whole winter. The last Winter was very harsh compared to others in recent history, and the preformed pond which has a maximum depth of 24" (I will work in imperial as I know that metric isn't really used in America) and never had more than cat ice on it the whole winter even though temps dropped to -8C and didn't get above 0C for a couple of weeks straight (I did keep the filters running though the winter though so the water flow was constant).

 

[Waterbug]

My dilemma is the edging of the pond. I originally was going to just lay slabs around the edge to trap the liner and underlay in place, but was thinking of making the pond deeper by standing the 900mm x 600mm slabs on the long edge around the pond.

Liners make building very forgiving. Structures can move a lot and still not totally fail. So I kind of hate to say "no this won't work" or "yes, that will be fine". We're talking about risks of failure and what you personally are willing to accept. I'll just voice things that would concern me and options.

My initial thought was to trench around the pond one spade width wide & 30cm deep, leaving a soil edge of 15cm wide from the edge of the hole to the trench, then to place the slabs on edge around the pond and to concrete in place.

You don't say how thick these "slabs" are or what material they are so I have no guess to strength. The strength of "concrete in place" is related to the concrete thickness and kind of reinforcement used which you don't give, so I have no idea.

The 15 cm set back from the pond edge really isn't anything at all. Over a short time that edge would collapse under the liner. The set back is good for supporting the wall vertically, but if you're counting on it to stop the wall from tipping over outward...it wouldn't help any.

Would this be sufficient enough concrete to hold the slabs in situ, with 30cm of slab below ground level and 30cm above (giving another 30cm depth to the pond)?

A 30 cm wall above grade doesn't have to hold a lot of water back, so I'd be inclined to guess whatever you do would hold for some period of time. When you were talking 60 and 90 cm above grade you were talking some serious water pressure and you'd have to be really careful about construction or face total failure.

The liner & underlay would then be pulled over the top of the slab and then tucked down the back side of the slab where it would be held in place by the excess soil that has been dug out of the pond

The liner needs almost no structure to hold it up. The water pressing against the wall pins the liner to the wall like glue. The top of the liner only needs to be stopped from flopping over into the pond. So a little soil is fine.

(a 75cm high "fence" would be built around the outer edge of the upright slabs with a soil width & depth of 30cm and finished with a capping stone).

Not sure I understand this. We're back to a 75 cm high wall? A 30 cm wide soil berm 75 cm high would erode. Do you mean 30 cm at the top and sloped away so maybe 100 cm wide at the base?

My other thought was to dig a trench, a spade deep & wide, around the pond leaving a 15cm gap from the edge, tucking the liner into this and covering the liner excess with concrete, and then laying the slabs on top of this concrete trench. This way would be easier, but would make the pond shallower.

Again, a 15 cm set back would collapse (round over) as time passed.

What you are describing is called a "concrete collar" if you want to Google. Normally the liner is laid over the top of the concrete, not under. Then cap stones laid on top of the liner to hide and kept from flopping into the pond.

There are variations to this. For example after you lay one layer cap stones you can flop the liner back over the top of these stones and lay another course of cap stones. Then back fill to cover the exposed liner. That gives you a higher pond edge, deeper pond. How high you go using this scheme is your call. 15 cm is probably no problem but as you go higher the risks increase. Depends on the sizes and shape of the stones used, how much back fill is used and how well it is compacted. Mortaring the stones can greatly increase strength allowing for a higher wall. But in general I'd say 30 cm would be about the limit for dry stacked, 60 cm with mortar..

What are your thoughts on which way I should go? I am erring towards the upright slabs, but an a little concerned that they may topple over time. The latter I am a little concerned that the slabs may come loose or the concrete break and end up with a slab in the pond tearing the liner (as I would like to have an overhang of the slab to hide the liner edge).

I wouldn't do anything you're saying to do. I'd build a standard concrete wall using bond beam block, rebar, concrete filled, dry stacked. I doubt it would be any more expensive, it would be a lot easier, you can get the 90 cm height without any problem and it would last 100+ years.

However, the biggest joy I get from this hobby is using new building schemes just to see how they perform, so I hate to dissuade you if you'd like to give any of your schemes a go. Worst case it could always be done over again later.

 

[VanDiesil]

Thanks for the detailed input Waterbug, much appreciated.

I have decided to go for the upright slabs. I have a mixture of 24"x24"x2" and 36"x24"x2" concrete slabs and enough of them to go around the pond. My plan was to set these 12" deep in to the ground leaving a 8" or so gap between the edge of the hole and the face of the slab itself. The 8" gap would act as a shelf and would have boulders and plants on top of it to try to disguise the liner.

The fence 30" high, would act as an opposite edge to the slab to keep the soil berm in place.

I have attached a rough (and I mean rough but you can see what I mean) diagram to this post.

The grey is the slab, capping stone & the external paving slabs around it, brown is the soil, black is the liner, green is the fence & the blue is the water LOL

I hope that it makes sense,

 

[Dave's pond Indiana]

I would not use any concrete because with time you will make changes. We all have made changes to our ponds.

 

[dieselplower]

Listen to Waterbug he has built many ponds and I'm sure fixed many poorly built ones as well. Gotta get to work so that is all haha. Have fun building!

 

[Waterbug]

I'd route the liner up behind the cap stone. It gives you some insurance and can also raise the water level if you wish.

[sharedmedia=gallery:images:3524]

 

[Waterbug]

Here's what I would expect to happen over time.
[sharedmedia=gallery:images:3525]

Whether or not this happens, to what extent it happens depends on a lot of things. type of soil, amount of rain, how construction is implemented. People or large pets standing or sitting on the structure could be a serious problem.

I've drawn what happens to many soil shelves...they kind of round over as time goes by.

I'd be pretty sure the cap is going to settle back because there's only soil hold it up. If you compact the soil properly the cap will move less, but compacting the soil depends on the 2 verticals not moving as the soil is compacted. I'd assume they would move.

Not sure what material the vertical green piece is but that could break instead of lean over.

Keeping soil from washing out around cracks in the vertical green material would be expected unless there's some water proofing there, like liner.

I'm not clear on the height. The key dimension imo is from the top of the ground (the horizontal grey) to the top of the water. If that were say 6" the water pressure isn't that much for this type of structure. I'd expect some movement, but not total failure. At 12" I'd expect a lot of movement and probably a re-build in maybe 3-5 years. At 24" I'd start expecting total failure (water flooding out of the pond). At 36" I'd expect total failure soon after the pond was filled. But, no way to know for sure.

 

[Waterbug]

[sharedmedia=gallery:images:3526]
The primary issue is the amount of soil (in red) holding back the water outlined in red. The vertical grey either breaks or wants to lean back at the top and kick in at the bottom. Adding the dirt wall above grade does nothing to hold back anything. If anything it pushes down and would help kick in the bottom of the vertical.

Just from eyeballing it you can see the amount of water outlined in red is more than the amount of soil in red. So just from a mass perspective the weight of that soil is not going to hold back that amount of water. Soil has virtually no sheer strength, meaning the pile of dirt in red doesn't mind moving at all.

 

[VanDiesil]

Those were the exact things I was concerned about Waterbug.

The red soil is the same height as the amount of slab above it (12"). The width of the capping stone would be between 8-12" depending on the ones I purchase. The green section is a wooden closeboard fence to give the outside a view that is pleasant to look at (as well as to stop people from falling over a 12" high berm). The fence would be held in place using 3"x3" pressured treated timber. If I say went down 24" instead of 12" on these in to the ground, would they help with the structural side of things? I am guessing that they wouldn't do much difference to it?

Another thought was to use concrete between the upright slab and the fence (with a small amount down the front of the slab between the liner and the slab to fill in any gaps) which would obviously be stronger than the soil, but very little on the front edge.

The soil is quite sticky clay but obviously it has dried out during the excavation process.

I am limited financially to what I can do, so was looking for the cheapest option that would give me the structural strength I require.