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  1. #1
    Member Rank = Tosai Sterlet's Avatar
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    Pond extension – digging out deeper

    Hi All,

    This is a slightly unusual pond project – digging out deeper rather than starting from scratch.

    The current pond was built with concrete block walls above ground, sitting on a concrete footing, and a profiled cast concrete shell below ground. Here is an old photo of the original pond from when I built it back in 2009:

    P1010249.jpg

    I could not reposition and start a new pond, and was not keen on demolishing the whole pond, pergola etc and starting from scratch, so the only option available was to dig downwards.

    I wanted to make the walls continue straight down to a flat base, and increase volume from current 12m3 to around 20m3.
    So the plan was to carefully underpin the existing walls in stages with mass concrete and form a new base.



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  3. #2
    Extreme Koi Member Rank = Sansai perks842's Avatar
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    Good luck to you Sir/Madam.

    That is one serious project, you have your work cut out lol

    Sent from my ONEPLUS A6013 using Tapatalk

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  5. #3
    Member Rank = Tosai Sterlet's Avatar
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    I started by digging out under the original wall footing, working in sections of approx. 900mm length. Depth of each excavation was approx 1m from ground level, and width to match existing wall thickness. The underpins were cast in concrete, forming the new footings and new lower walls. Each underpin section was profiled and interlocked to the next section using shuttering. A work in progress photo of one of the section digs:

    2.jpg

    Excavations for underpin sections were sequenced as roughly diagonally opposite the previous dig, concrete cast and cured for at least 3 days before digging next to a completed section. I don’t have too many photos of this stage, and not really much to see for my efforts – one of the underpin sections cast in concrete.

    3.jpg

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  7. #4
    Member Rank = Tosai Sterlet's Avatar
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    The ground at a depth of about 900mm has a layer of compacted stones and gravel which is like concrete and almost impossible to dig with a spade. Most of the underpin sections bear on this layer of aggregate. I would not have been happy to underpin in the same manner if the ground were clay or similar.

    It took about 5 weeks doing a bit here and there to complete the underpinning phase, and 3m3 of concrete. This was mixed on site and cast when needed to suit the sections.

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  9. #5
    Member Rank = Tosai Sterlet's Avatar
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    The time came to drain down and start the works inside the pond. I was not keen on housing the fish in a paddling pool for what could be quite a long period of time, so decided to ‘move’ the fish to one end of the pond by building a temporary block partition wall. This was not quite in the place I wanted due to the shape of the base and slope (ideally would have been more central). The wall was rendered and return pipework extended to the remaining pond section.

    4.jpg

    Not the prettiest block laying, but this was built up off a curve and slope!

    5.jpg

    With the water level raised back a bit, it was time to get digging

  10. #6
    Member Rank = Tosai Sterlet's Avatar
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    So I had now become committed to the project! I don’t remember the concrete being this thick. The concrete was 450mm/18” thick in places!

    The start of some serious digging works
    6b.jpg

    More digging...
    6.jpg

    Now you can see the concrete underpins
    7.jpg

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  12. #7
    Member Rank = Tosai Sterlet's Avatar
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    When dug to the right depth, I cut out a trench for the new bottom drain. The active bottom drain from the pond can be seen passing through the original footings.

    8.jpg

    At this point I had to dig down over a meter externally to the pond to take the new bottom drain riser pipework and fit the elbow. Fitting an elbow to the new bottom drain was a difficult job, with limited working space, incoming ground water and shingle.

    9.jpg

    The base for this section was cast, tying into the walls as best as possible and maximising surface area contact. This also provided a fillet to the wall-base junction to avoid a sharp corner. The concrete underpins cast below the original wall footing can be seen.

    10.jpg

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  14. #8
    Member Rank = Tosai Sterlet's Avatar
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    At last – a flat surface to work from! The block partition was built up off the new base to join the existing pond shell, in order to complete the temporary partition wall. This would allow the fish to move over at a later date. The new lower walls were rendered with sharp sand to make level and finished with two final coats.

    11.jpg

    The concrete footings were cut back and tidied up, and the new shell rendered. The minor leaks under the upper section of temporary wall made rendering difficult.

    12.jpg

    Once cured the new wall and base were painted.

    13.jpg

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  16. #9
    Senior Member Rank = Nanasai dbs's Avatar
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    A great one to follow, enjoying the detailed progress , very individual too

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  18. #10
    Member Rank = Tosai Sterlet's Avatar
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    Having filled and tested this section, it was time to move the fish over to the newly completed section. The bottom drain pipe which was passing through was cut and capped, and set up as best as possible to form a temporary bottom drain. Moving fish is one of the more difficult parts of a pond build, the Siberian sturgeon was hard work and smashed the net handle! A sling would have been handy. I felt bad putting him in a relatively small pond, although only a temporary measure.

    14.jpg 15.jpg

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  20. #11
    Member Rank = Tosai Sterlet's Avatar
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    After completing works to the first and much smaller section I had the thought that I could always stop at this point. I was now 11 weeks into the project (working ad-hoc, not full time) and still had the main pond section to work on. How long will this larger section take?

    I decided to press on, and it was another strange feeling of commitment breaking up the pond base of the main section.

    16.jpg 17.jpg

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  22. #12
    Member Rank = Tosai Sterlet's Avatar
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    After much breaking out and digging within the main section of the pond, eventually the rear of the temporary wall was reached and so too the new base level.

    21.jpg

    The concrete shell supporting the temporary wall was left until much later stages. The original concrete shell near the window was also left otherwise the structure would be weakened around this opening.

    22.jpg

    I had to use the fork to dig through the hard layer of stones embedded in clay, which slowed down digging.

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  24. #13
    Senior Member Rank = Nanasai dbs's Avatar
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    This is certainly an original way of doing it, enjoying the progress so far

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  26. #14
    Member Rank = Tosai Sterlet's Avatar
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    Digging continued until the final depth was reached for part of the base.

    23.jpg

    The second part of the base was cast, level with the first section. This also forms part of the wall below the window and in-fills a few defects to the new lower walls.

    24.jpg 25.jpg

    This approach, leaving parts of the existing structure in place until new structure was formed ensured the walls could not move.
    Moving on to the next part, it was back to more breaking out concrete and digging. The amount of spoil produced was unbelievable, and had to be lifted out in rubble bags, over the wall and into the wheel barrow.

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  28. #15
    Member Rank = Tosai Sterlet's Avatar
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    After cutting the old botttom drain pipe out, I thought I would see what the inside of the bottom drain pipe looks like after 10 years service, apart from a dollop of sludge in one spot fairly clean and clear.
    26.jpg

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  30. #16
    Member Rank = Tosai Sterlet's Avatar
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    The third base section was dug out, and concrete cast, again forming part of the wall and bonding to the side walls.

    28.jpg 29.jpg

    The deepest section of the original pond, an L shape in the middle, was slightly too shallow by about 50mm/2” to make level with the new base. This was about 300mm/12” thick and made from seriously hard concrete. I needed to reduce the top of the central section. I was fortunate to be able to prise open between concrete layers for some of this section – the rest was hard going!

    30.jpg

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  32. #17
    Some hard graft going on there, will be worth it though

    Sent from my CLT-L09 using Tapatalk

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  34. #18
    Senior Member Rank = Supreme Champion freddyboy's Avatar
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    Agree hard work. Respect lol
    Fred

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  35. #19
    Member Rank = Tosai Sterlet's Avatar
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    Yes it certainly was hard work! I thought I would make use of the opportunity working from home to break it down into smaller bits. I'm not so sure the Mrs was happy about it though.

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  37. #20
    Member Rank = Tosai Sterlet's Avatar
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    Part of the original shell had been left to help support the walls and also to use as a step to lift out the spoil. By this point either I was getting weaker or the concrete was getting tougher. I cheesed off one of our neighbours with noise at this point!

    31c.jpg

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