Underlayment

Even though we have a layer of sand in the pond, it's still a good idea to have some form of underlayment between the soil and the pond liner to protect the liner from punctures. Some recommend carpet remnants or even newspaper, and the web sites of the European natural pool designers call for "fleece" as an underlayment.

I bought the pond liner from an outfit on eBay (which I won't name or recommend here since their customer service was so lousy), who also offered a matching quantity of underlayment at a good price and free shipping, so I got that rather than scrounging remnants from the flooring outlet.  This type of product was also recommended by the liner manufacturer to ensure a full 25 year (or more) liner lifespan.  

As you can see from the photos, the underlayment is light green and very much like polar fleece or a heavy felt -- I assume this is the same material the Europeans refer to as pond fleece.  We also noticed that, except for color, the underlayment is almost identical to heavy duty filter fabric that is used in building roads, septic systems, foundation drainage systems, etc.  So, when we ran short of fleece due to overlapping and the irregular pond shape, I bought a roll filter fabric at the local plumbing and septic wholesaler.  The filter fabric comes in 300' rolls in a variety of widths.  A 6' x 300' roll was $108.

The underlayment is in and has been inspected by my helper.  Now it's either put the pond liner in or finish stacking the 7 cords of firewood that has been laying (or lying -- I can never remember which is correct) in the yard ... maybe I'll have lunch first.

Taking shape, looking cool

We went away for a week to spend some vacation time in Vacationland (i.e., Maine) and returned to find this masterpiece sculpted in soil.  Brian had been busy carving out the final pond shape, and backfilling and tamping the deep zone wall to about 2' of its top.  The deep zone got a fairly thin layer of sand which was also tamped into a nice, solid base.  He also dug a trench around the pond perimeter, which will be used to lock the pond liner in place (the edges of the liner drape into the trench, which is then backfilled with soil and tamped).  It is difficult to see in the photo, but Brian also cut lengths of scrap plastic drain tile and wrapped them around the wall braces that extend above the backfill.  The braces have fairly sharp edges and the drain tile wrapping should protect the pond liner from punctures or tears. 

Almost time to put the liner in -- a prospect that has been keeping me awake at night as the liner weighs 1200 pounds.  When it arrived on a semi a few weeks ago, the driver had no way to even move the liner to the rear of the trailer, not that it would have fit on the trailer's hydraulic drop gate even if he could have moved it.  Fortunately, I have a farm tractor with a front end loader and was able to tow the liner out of the truck with a set of chains and then carry it to the barn with the loader for storage.  How we are going to unroll the liner and stretch it over the pond remains a mystery.

Let's do this thing!

The shallow bowl shape of a traditional pond results in either a small deep zone in a moderate sized pond, or a moderate sized deep zone in a large pond.  To achieve a swimmable deep zone in a relatively small pond it helps to form the deep zone with vertical walls.  Two ways to do this are to (1) dig a deep zone, construct walls in it, excavate the surrounding regeneration zone and then lay a liner over the whole shebang, or (2) dig the entire pond, lay the liner in it and then construct a wood wall on top of the liner to separate the deep zone from the regeneration zone.  Each has its advantages and drawbacks.  For instance, option 1 is faster, but requires more liner and excavator work.  Option 2 would require less liner and excavator work for the same size pond,  but constructing custom walls on-site could be time consuming.

As you can see from the photos, we chose option 1.  I sourced galvanized steel in-ground pool walls from the local pool store.  At $3300 for a 16' x 28' structure with hardware, they were not cheap.  However, option 2 would have cost the same in materials suitable for long term submerged durability (white oak framing and Ipe decking to go over the framing), not to mention the labor involved.

With a nice, level excavated site, the walls were a snap to put together.  The walls are 42" tall in 4' or 8' lengths.  The corner sections are pre-formed in a 2' radius.  The panels are bolted together through pre-drilled holes and provided hardware -- a piece of cake!  Our excavator Brian and I had the wall together in less than three hours.

The next day we assembled and attached the provided steel bracing, one brace per panel seam.  Each brace is an isosceles triangle that extends out from the base of the wall to help withstand backpressure from the deep zone water and preserve the structural integrity of the pool.  The long leg of the braces use a rod and nut to help plumb the wall panels.

Once the wall is together, braced and plumb, it's time to pour a concrete collar around the outside to hold it in place permanently.  Concrete was poured from the bottom lip of the wall out to the ends of the braces (about 2') to a depth of 6" at a cost of $577.

Ephemeral pond design

An "ephemeral" design since, as a battle plan rarely survives first contact with the enemy, I anticipate that the pencil-on-paper pond design will not survive much beyond first contact with the reality of the hole in our side yard.   

Being illiterate in CAD, to the left is a basic design published by Oase on which I superimposed the proposed dimensions of our pond (click on it to see a larger image).  This design is not too far different from what we are planning.

Since you found this blog, I'll assume that you have some familiarity with the concept of a natural swimming pool.  Our pond will utilize a pump to draw water through four suction lines -- one from a skimmer and three from 4" diameter perforated flexible hose ("drain tile") buried  in the gravel of the regeneration zone.  The pump will discharge to two lines -- one to a waterfall and one to a discharge line in the deep zone.  Our dock and skimmer will be at the northeast corner of the pond (to take advantage of the prevailing wind from the southwest), while the waterfall and pump will be at the northwest corner.

Most of the regeneration zone will be about 9' wide and consist of natural cobble and gravel 2" to 4" in size.   The regeneration zone on the south side of the pond will be a large beach area of "micro-mini" pea gravel for kids to play in and to provide comfortable access to the deep zone.  Since the beach is larger in square footage, two lengths of drain tile will draw water through that area while one length of drain tile will be used elsewhere.

How did we size our pond?  Basically, we decided that a 45' pond would look nice in our side yard and that a 16' x 28' swim area would be adequate.  From the reading I have done, I believe the regeneration zone should have a square footage at least as large as the swim zone.  Our regeneration zone will be nearly three times the size of our swim zone so we should have plenty of excess filtering capacity.  Initially I planned on a smaller pond since the maximum EPDM pond liner width is 50', which effectively limits pond width to 30'.  Fortunately our excavator contractor Brian proposed that we cut 10' from my already purchased 50' x 75' liner and splice it to give a 60' width -- a great idea that will give us enough width for the beach area without having to shrink the size of our swim zone!

Coming soon ...

Thanks for checking out my blog!  In the near future I will begin presenting the sequence of construction of our swim pond, hopefully with enough detail to help others to gather some ideas about designing their own natural swimming pool and to make some judgement about what may or may not work.  I'm sure that, once we've finished, there will be lots of "I wish I would have" reflections ... hopefully none will be major.  I'll be giving specifics on materials used, suppliers, costs, and how things are put together.

In the meantime, above is a photo of the initial excavation.  The bare soil is the approximate overall pond size and the excavated hole in the center will be a 16' x 28' swim area.  The design was loosely based upon an image of a Bionova natural swimming pool I found on the web (see below).  To orient you, our house would be just beyond the right edge of the Bionova image, and is just beyond the lower edge of the first excavation photo above.  That photo was taken (and many of the photos on this blog will have been taken) from an attic window on the north side of our house.  More on the pond design in the next post.