Pond rebuild.

Hello there,
Spent last weekend splogging around at a friend's 10yr old (ground level) muddy pond about L9m x W4m x D1m (average depth). It's was never built for showing Koi so is always murky & his stunning fish are best viewed when he chucks the pellets in! His liner was leaking and so we had to empty it, take out the fish - bucket out the mud, fill in a cavernous hole that the leak had washed from the under his lawn making its way under a gravel access road to a dyke nearby, patch the liner -refill it from the dyke and tap --- All in a day. Miraculously we lost no fish!
Whilst doing it I was filled with the notion that we were wasting our time (having found so many more holes that the liner could have doubled for a tea strainer.
I said as much to my pal who was thinking the same thing himself. We both agreed that the patching was a temporary measure & because I am retired & have more time. (My friend has a furniture factory builds leather Chesterfield furniture, so is practical.) I would find out what the best way to proceed would be, hence this Email.
I found your site & having a engineering mind found the information useful.
I have the following preliminary questions: -
1. To rebuild his pond (still at ground level) without using a liner, would you recommend a concrete bottom - block walls rendered & then fibre glassed?
2. What thickness of concrete bottom?
3.Would this design necessitate backfilling with concrete? Thanking you in advance for your help.
Garry - Canterbury, UK.

Dear Garry
Thanks for your e-mail, and pleased that you have found the construction pages useful. I am also retired and was a marine engineer.
1. Your choice is the most expensive option, but certainly the best option. If this is a dedicated koi pond then one metre is not deep enough. It should be at least 5 feet deep and preferably 6 feet at the deepest.
The bottom should be benched so that the outside edge is about nine inches higher than the centre.
The depth is the reason why so many people build up walls above ground to prevent having to dig out and dispose of so much soil.

2. Three methods are described in the construction pages
The minimum depth of the concrete is six inches plus the benching.
If you have bottom drains then you will need to dig deeper where the pipes are so they are supported by the concrete.

3. If you lay the blocks on their sides so the walls are nine inches thick then it is not necessary to backfill with concrete except for the part where the first row of blocks are laid on the concrete base. (This prevents any sideways movement).
When backfilling it is advisable to use stonedust, small chippings or something similar. Do not use sub-soil or clay as this cannot be contacted down properly.

The following are my own comments.
A. 9m x 4m is a very large pond. If you are going to deepen it then I would have thought 7m x 4m would be more practical or even smaller.
B. You have not mentioned a filter system. The larger the pond then the larger the filter, pumps, etc need to be.

Hope this is useful and look forward to hearing from you as the design and thoughts continue.
Eric.