Harmony House





Encouraging local birdlife and amphibians with a natural pond habitat is part of our landscaping plan. If it can double-up as a water source for garden and orchard irrigation then even better. Read further through this page for more information on our aims, research, decisions and results, as well as our tips and advice summary, for this element of our project - the natural pond habitat.


We'd like to create a natural pond habitat to encourage local birdlife and amphibians, hoping also to use it as an extra water source for garden and orchard irrigation.


Building a natural pond habitat basically involves a dug cavity in the ground that can hold water, which is then planted up with appropriate water plants.

The cavity can either be a self-supporting excavation (which in most soil types means a 3:1 grade for the sloping sides), or a “shelled” cavity (where vertical sides are made stable with the use of a fibre glass shell or concrete blocks or other such method). The “shelled” cavity method is more expensive, but the self-supporting excavation needs more space to achieve (3m horizontally for every 1m of depth).

Waterproofing a self-supporting excavation can be done with your native clay (if good enough), special bentonite clay, or pond liners. All water-proofing methods have some some fractional degree of seepage, and in addition pond liners have limited useful lives of between 5 and 20 years depending on the type of plastic used. Bentonite clay on the other hand is there forever once deployed and is considered one of the most effective longterm sealing methods available. The idea is to sandwich a layer of bentonite (either in powdered form, or in pre-made sheet form stitched within geofabric material) between two layers of your on-site clay: when wet the bentonite swells up to 30 times its original volume, creating an effective seal.


For the size of the pond we wanted (doubling up as a water source for gardening needs), a “shelled” excavation would have been far too costly, hence we opted for a simple self-supporting excavation, taking on the chin the overall area required for the 3:1 grade.

We chose bentonite as the sealing method, being by far the more natural and long lasting solution compared to plastic liners. We opted to lay it in the form of Geosynthetic Clay Liners (rather than the powdered form) as GCL rolls are easier to install in a uniform way, which gives a more reliable sealing result.


After some initial problems with an excavator that was under-sized for the job and held up the timeframe considerably, we found a contractor with a larger machine and good quantity estimating skills who managed to finish the excavation for us before the winter rains set in. The same machine was used to lift and deploy the GCL rolls, which weighed about a tonne each. With the GCL sheets laid out over the entire excavation (the overlapping sheets joined with a special bentonite paste supplied by the company) the next step was a 300mm back-fill of normal clay compacted over the GCL sheets (this provided the necessary compression for the bentonite within the GCL sheets to swell and compress against when wet).

As an extra step we added a layer of normal geofabric over the compacted clay to keep the clay from mixing with our water, and a thick layer of nice pebbles over that. The result should be relatively clear attractive water in our pond (which theory says should be kept relatively clear by the action of the water plants and micro-organisms).

The whole process to this point has taken many days of work, which would be dependant of course on the size of the pond involved.  

At the moment we’re letting a few months of rain thoroughly wash the pebble layer free of the dirt and dust the delivery came with, regularly pumping out that dirty collected water. Once the collected rain water begins to look dirt-free (i.e. the pebbles have been thoroughly washed) we’ll allow the pond to fill and use the spring months to plant up our water plants.


It was quite an achievement to get the excavation finished and lined before unfavourable winter weather set in. We were happy with the final size/capacity/cost ratio of this simple excavation method (self-supporting 3:1 slopes) rather than attempting a “shelled” excavation at this size.


Each rain is gradually washing the thick layer of pebbles clean, and we look forward to spring and planting up the pond with a mix of marginal, submerged and floating plants. More research will be needed to tailor that mix of plants towards achieving the best possible water quality.


     - Bentonite clay powder, or GCLs (Geosynthetic Clay Liners, where the  powedered bentonite is stitched between sheets of geofabric), are a natural and proven long term solution for sealing ponds and dams very effectively.