Pond Construction

Courtesy of Chiltern Club Koi

How to plan and build a concrete pond and suitable filtration. How to build a waterfall. Do it once - and do it right!

First, planning.  Size and siting. Budget and construction methods

Building a 'proper'  koi pond IS expensive, and will cost between £1 and £3 per gallon  to build correctly. Yes, you can cut corners. Yes, you can do it much more cheaply. But remember whatever you build will probably be there for many years. It has to last, has to work properly over the years, should need minimal maintenance, and will be holding some valuable fish.  If you have made up your mind to build a pond using a plastic liner and little or no filtration this article is not for you - so read no further. If you want to do it properly - and do it right - then read on!

Plan your pond site as near to the house as sensibly possible so that you can see the fish from the house.  Do not site your pond under overhanging trees or in permanent shade, your pond will be a trap for all those leaves and will not warm up properly in summer. Check for utilities/services running across the site of your new pond. Sewerage and foul water pipes can be very expensive to move. Gas and electric pipes even more so.

What kind of soil do you have, - sandy, chalky or clay?  If you are on sandy/chalky soil you will be able to build your pond at any time of the year as your soil will be free draining.  If you are on clay, it's spring and summer only, as any excavation will rapidly fill with water making construction a nightmare.

What size should you build it?  Koi need roughly 10 gallons for every fish inch. In addition you should aim for a pond with a minimum depth of around 3 - 4 feet. 2500 - 3000 gallons is probably the minimum size you should plan for, bearing in mind that you will inevitably overstock your pond! We all do it!

Do you have good access for a mini digger and how easily will it be for you to dispose of the spoil?  6 cu. metre skips cost well over £100 each these days, so can you keep the spoil on site and distribute this over the rest of the garden?

When planning, allow adequate space for your filtration, which should have at least 25% of the surface area of the pond, preferably 33%.

Have we put you off yet?

Building a koi pond is a substantial engineering project, so you will appreciate why planning is so important.

Having decided 'where' to site your pond, and equally decided what size to build, next consider construction methods and alternatives.

Types of Construction

It is generally accepted that concrete ponds are the way to go. These can either be rendered and fibreglassed, or rendered and painted. Gone are the days when concrete ponds crack and need continual maintenance. Once built, they will last a lifetime if constructed properly.

Filters come in many shapes and sizes, and can be built in situ or bought pre-made and simply 'slotted' into your pond set-up. The choice is yours. Home-made filters have the advantage that they are generally cheaper to construct and can be tailor made to suit your requirements and budget. Pre-made filters are more expensive but have the advantage that they are relatively quick to install, and are designed to work!

Also you should consider the equipment which you will need to run your pond; pumps, air pumps, UV's, pipework, filter materials, skimmers etc. etc. These items cost money and should be considered at the planning stage

Time to build !

O.K. Lets suppose we have chosen our site, the construction methods and have come up with a workable plan. Time to start digging. Here we consider a project to build a concrete pond of around 5000 gallons complete with a part home made and part bought in filter. The final pond is to be built into a rockery and will be complete with a waterfall and feeder pond.

Excavation - Photo shows excavation using mechanical digger. The soil is chalk which turns to solid rock chalk at around 18" depth.

Approximately 200 tons of spoil was excavated from this site. over a two day period. Once completed, the concrete base of the pond was laid. This is 6" thick and reinforced with steel mesh.   Note the trench in the centre for the bottom drain pipework. The excavation depth for the pond floor is around 7 feet. As the soil type here is chalk, there was little danger of the walls collapsing, or of water collecting in the excavation. However it is vitally important that with any deep excavation, precautions are taken to shore up walls properly to prevent collapse during construction.

The excavation for the filters has also been dug, and a 4" concrete base laid in preparation for building the walls. Note the semi-circular excavation in front of the filter base. This is where the vortex will be sited. Filter excavation depth is around 4 feet. Vortex excavation depth 6 feet 6 inches. At the rear of the filter chamber, a trench has started to be excavated which will eventually house a the 4 inch drain pipe from the sump chamber to the main drains.

The filter walls are under construction, using 4" solid concrete blocks. The pond walls have also been started. These are constructed using 9" hollow concrete blocks which will be backfilled with a lean concrete mix as construction progresses. The pond shape here is informal kidney in order to match the curvature of the adjacent patio. This shape also helps to achieve a circular water flow through the pond which aids drainage.

Here, the pond walls are 80% complete, a large fibreglass vortex unit has been installed and connected to 4" bottom drain pipe work, which is shown ready for pressure testing. The vortex stands on a 4" concrete base next to the filter assembly. All bottom drain pipework use glued fittings.

This shot shows the pond and vortex from the waterfall end, here the base for the top pool (which will feed the waterfall) has been laid using 4" of concrete. Once completed, the vortex excavation will be backfilled with sharp sand or pea shingle to allow it to move.

.Here, the pond walls have been finished, and the bottom drains concreted in position. More concrete has been used to 'dish' the pond bottom in order to direct water flow to the drain area. Note the gap in the pond wall, ready for the skimmer to be fitted. The kidney shape of the pond can now clearly be seen. Final depth of the pond is 6 feet.

With the pond walls complete, the excavation round the perimeter of the pond has been backfilled with a weak concrete mix, and construction started on the waterfall. This is made up of concrete 'steps' cut into the chalk embankment, with 4" concrete block walls. Note that the 'hollow' concrete blocks have now also been backfilled with a weak concrete mix.

Basic construction of the waterfall has now been completed, and pond walls rendered using a 2 to 1 sand/cement mix with plastic fibres added to the mix which makes the render not only completely waterproof but prevents cracking and increases the strength of the structure enormously.

The inside of the waterfall construction will be rendered with the same strong, plastic fibre reinforced mixture as used for the main pond walls. This will then be painted with G4 to seal out lime.

Now a decorative retaining wall has been constructed at one end of the pond to disguise the waterfall construction and form a perimeter wall for a small pathway round one side of the pond. The skimmer has been fitted, the pond fibreglassed and the filter chambers rendered and in the process of being painted with G4, a special varnish which adheres to the cement render and completely seals out lime.   Filter chambers from top left; vortex, brush chamber (middle left), sump chamber (bottom left), Large biological chamber (bottom), small bio. chamber (middle right), and pump chamber (top right).

The waterfall construction is now completely disguised using rocks laid inside the block walls and on the concrete floor and used to form the individual cascades.

Waterfall feeder pond completed and rendered and the rockery under construction. The feeder pond will also be disguised with large rocks in order to hide the formality of the design. In use the feeder pond only holds around 12 inches of water. It is fed via a 2 inch pipe, buried in the embankment which feeds from a small pump chamber quite separate from the filters.

The finished article, wooden decking in place covering the filter areas, and rockery partially planted.

The pond 4 years later, rockery planting just a little more mature.

Different views of the filter chambers under construction Note the weir in the first biological chamber. Filter comprises vortex; this feeds a brush chamber via one 6 inch pipe.   Behind the brush chamber is a sump or drain chamber. This is equipped with an overflow from the filters, and houses the drain valves which connect each filter chamber. Each chamber has a separate drain in the centre of a dished concrete base, just like the main pond.  The drain valves in the sump chamber can be opened to drain accumulated gunge from each chamber on a regular basis. The sump chamber is connected to the main drains using 4 inch sewer piping.

The brush chamber flows into a large biological chamber via 2 X 4 inch pipes. This then feeds over the weir to the final bio. chamber and then the water is pumped back to the pond via a 1.5 inch pipe which enters the pond under the waterfall.

The pump chamber complete with pump, 30 watt UV and

switchgear in place.

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