Concrete Pond Construction

Frequently asked questions about Pond Walls

A. The water level in the pond has to be set at the same height as the water level in the pond when the pump is not running. As it is a gravity fed system then the water will find it's own level.
When the pump is working the level in the filter will drop slightly, but will find it's own level when the pump is stopped.

Q. As I am building a pond which will be of max depth of 6 ft and with rendered walls and G4 paint I thought it would also be neccesary to use 9" thick blocks, but my local koi dealer who used to build koi ponds (he tells me) says that 6" blocks with about two inches of concrete behind is sufficient and as I am a bit limited for space I have decided to accept his advice. I also discussed this with another koi hobbist who built a similar size pond last year only used 4" wide blocks with 3 inches of concrete behind.

A.You can do it the way you say and this is recommended in some books. You have to be careful to keep the space behind the blocks clear of all debris including cement that falls down when building the walls so that the concrete binds together and is in full contact with all the blocks. That is why we always recommend 9 inches and most members also pour concrete down the back especially at the bottom of the wall as this helps to prevent it moving outwards under the pressure of the water. It is very difficult to keep only a space of two inches, and in many cases I have seen the sides of the excavation partly falling in due to heavy rain, and then you have a big space to fill in unless you use shuttering. Everyone you talk to has different way of doing things so in the end the choice is down to you.

Q. Does a built up wall i.e.not being able to back fill with concrete create any difficulties,or doesn't it need to be as strong as the bottom due to less pressure being exerted?

A. This is not a problem if the wall is nine inches wide. It will hold the weight of water for the two or three feet above ground. You could backfill below ground with stone dust or similar, but not ordinary soil as this will not compact down sufficiently.

Q. The 2ft above ground will be an inner wall of 12x9x4 high density block and an outer of brick (actually the decorative blocks now available) with a cavity of 2" filled with concrete. Do the blocks need to go 9" wide or will 4" do?

A. Pond edging - 1 shows the wall built up using 4" wide bricks making a total width of one brick. It can be as wide as you like.
Pond edging - 2 shows the 12x9x4 block on it's side, but this was only done as the owner wanted to sit on the wall. If you are using a 2" cavity between the two bricks you will have to ensure that the two are tied in well together at regular intervals.

Q. I am building a pond approx. 10ft x 6ft x 5ft deep. The site is quite heavily sloped with the difference in height between the 2 sides being compensated by a 2.5ft raise. I have planned to build with standard 18in x 9in x 4in concrete blocks but forming a wall 4in thick. For the 2.5ft above the ground I was planning to re-inforce with a second layer of ordinary bricks, backfilling the 3.5ft or so below the ground with about 4in of concrete. Does this sound reasonable to you ?

A. Many koi-keepers in Britain have built their ponds as you have stated below, and seem happy with the results. In South Wales we have solid clay, and as there is always a chance of subsidence especially with the dry weather we are having recently, it is usual to recommend a nine inch wall. When it rises above ground then the wall remains at nine inches but can consist of a facing brick on the outside, and concrete bricks on the inside. Some koi-keepers around the country use six inch blocks, but these can be heavy to put into place. You are thinking of backfilling the part below ground with 4 ins of concrete. This is a good idea, but it is difficult to keep the gap to this width and the amount of concrete actually used might be a lot more than you expect. It is also necessary to keep this space clean before you pour the concrete, but this is difficult as the mortar drops down as you build up the walls.

Q. I have built a 2000 gallon ornamental pond and introduced half a dozen goldfish , orfe and two koi, all of which are healthy and growing rapidly.I now wish to build alongside the existing pond a much larger raised pond in which to keep Koi.I would like the new pond to flow into the existing one. I was thinking of building out of blocks and rendering with cement, facing the outside with granite which is plentiful here in Cornwall.I would like to know firstly , what size and density of blocks, and secondly you mention drainage, does this permenently flow through or is it there to drain the pond . Any information would be much appreciated , perhaps you could recommend a decent publication on a raised pond.

A. Many koi-keepers have their ponds raised about three feet above the ground and the total thickness of the walls is only nine inches. The lower part of the walls are supported by the earth and the back filling that is carried out. If your base is almost level with the ground then I would certainly use steel reinforcing bars to connect the walls to the base, and I would use 18" x 18" x 9" hollow blocks at least for the first two rows. You can then continue with 18" x 9" x 4" high density blocks laid on their side so that they are nine inches wide. You say that the new pond will be for keeping koi. The ponds described in my website are dedicated koi ponds and are built mainly to cater for these fish which will in a few years grow to 20 - 24 inches. They will not tolerate big water temperature changes and for that reason the pond should be at least 4ft 6ins deep over some part even in Cornwall, and preferably five to six feet deep. The drains mentioned in my pages are for removing sediment, waste etc, from the filter system, vortex chamber etc. As large koi produces a lot of waste it is also necessary to have bottom drains in the pond for removal of this waste. These drains are pulled at least once a day, or permanently connected to a vortex chamber where the waste can be removed with the minimum loss of pond water. Unfortunately I do not know of any publication that deals specifically with a raised pond.
You can use the 9" wide blocks below the ground level, and then when you reach the ground level you can then use 4" concrete blocks on the inside and 4" facing bricks on the outside. This will still be a nine inch wall, but will now consist of two 4 inch blocks with one inch of mortar sandwiched between. Some members do build up the wall in 9" blocks and add the facing bricks on the outside of this giving them a 14" thick wall, or in fact slightly less. In your case you can do this, or as granite is usually quite thick then the best solution will be to use the 4" concrete blocks for the inner wall and the granite for the outer part. I was born and brought up in Jersey, and I am a great admirer of the Jersey pink granite. I know a koi keeper in Jersey that has built a pond using granite for the outer wall, and it is very pleasing to the eye. He also has a raised flower bed and steps made of granite. Unfortunately my budget will not stretch to that.

Q. I'm going to try to buy things over a period to spread the cost.I wonder what my wife will say when I've got 800 concrete blocks on the lawn!

A. Probably the same as most other koi keepers wives, but it is not printable here. You also have to worry about the sand and dust which seems to get right through the house no matter how careful you are when building the pond.

Q. Please advise if normal cavity wall blocks can be used for a filter system that is gravity fed. Should I use engineering blocks?

A. The blocks to use are the normal 18 x 9 x 4 concrete blocks used for buildings garage walls etc. Any type are OK except for the insulated ones that are sometimes used for the internal cavity walls.

Q. The concrete floor has set surprisingly well (although it has been warmer here than normal) so I've managed to lay around 75% of the below-ground blockwork today.

A. The building industry says you should wait one week for the concrete base to cure before you build the walls, but I have never seen anyone, including builders, do that.

Q. I'm building a pond on a steep slope. Will the 9 inch block be enough to hold back 6 feet of water without back fill. I plan on a 6 inch slab bottom as you suggest but one entire side of the pond will basically be a 6 foot high retaining wall. I don't know if it matters but assume the pond is 24 feet round by 6 feet deep.

A. This reply was written by someone in the building industry. In answer to the question of the six inch wall, I would say definitely no. What I would suggest is the wall on the retaining end be either turn outward to form an eighteen inch wall using the standard concrete blocks, shuttered to form a 9 inch solid concrete wall with reinforcing reed bars installed vertical and tied horizontally. It would be suggested that the vertical be placed into the foundation when pored, or drilled using a chemical fixing. The water pressure = weight in tons placed against this wall would be sufficient to pull the cement bond between the lap joints, forming a stress crack that would eventually lead to a complete structural failure. Using the standard block I would suggest using a stretcher bond two pointing outward two length ways so that no two joints are in line.

Q. The pond has a the filter area directly adjacent to it and will house 2x3 plastic filter units fed from 3 bottom drains. I propose to stand these on a concrete plynth.
The questions are
a) Should the plinth be free floating ie independant of the pond wall or should the pond and filter housing be attached together - if so - what is the recommended way of attaching?
b) What ever the design in (a) - what should the vertical sections of the drain feed pipes be incased in eg concrete or say gravel to allow for small movement ?
I am concerned that if this section is not constructed correctly, the feed pipes from the bottom drains may be stressed and eventually crack

A. The base (plinth) for the filter should be six inches deep with reinforcing.
It is usually free standing, but like in your case will butt up against the wall of the pond. No attachment is necessary.
In this case you will probably have a 4 ins wall around the filter unit and this will also butt up against the wall of the pond.
Although the bottom drain pipes are bedded in concrete under the base of the pond, when they raise up vertically to the filters it is better to use something like gravel in case you ever need to remove or alter them. In this area we use stone dust for this.
If everything is constructed correctly with a good base then there should not be any movement to cause you problems.

Q. I am building a pond and have excavated a hole 12ft x 9ft x 3ft.
1. I plan to put in a nine inch reinforced base, and build with concrete blocks on top of this. Is this base sufficient?
2. I would like the overall depth of the pond to be 5 feet this would mean coming 3 feet above ground with blocks.
3. I have purchased a four chamber fibreglass filter with vortex capable of coping with up to 3500 gallons. But this does not have a discharge chamber. Do I have to connect this to a main drain?

A. 1. If you look at this page you will see a similar layout to what you propose. It does say the thickness of the base is 12 inches, but looking at the photos it would seem to be only about 9 inches around the bottom drains. I assume you are fitting at least one bottom drain, and that the base will be benched.
2. If you look here you will see a similar layout as you mention here. The height is approx 32 inches. Another view of this can be seen here. It had been intended to have the pond deeper in the ground and a lower wall, but he could not dig any deeper as he came across the main sewage pipe for the house. 3ft is rather high to see the fish, but is useful for keeping away small children. Another wall can be seen on this page. It is assumed that you are using 9 inch thick walls. Many builders think this is too high due to the weight of the water on the walls, especially in the last example where they use a double wall of blocks and bricks. These walls were done by professional bricklayers.
3. You will need some type of drain box to discharge regularly (at least once a day) from the drains of the vortex, and each filter chamber. It is ideal if you can discharge directly to the main drain. If this is too high in relation to the pond, or too far away, then you can have a submersible pump in the discharge chamber and pump to the main drain.

Q. I was talking to a builder friend of mine this morning and he suggested a possible alternative, that of putting in shuttering and reinforcing mesh and pouring concrete behind it, that would be an easier alternative as several companies have these big lorries with hoses that could get over the side of the house and pour it in, any comment!!

A. The builder's idea would be fine, but due to the high cost of the shuttering and the pump truck I do not usually suggest this.
Several of our members have used these trucks for the base of the pond and they are about £160 in Cardiff plus the cost of the other truck with the concrete.