Water pressure on side wall

[Rob75]

Hi, new to the forum here and about to commence my first pond construction here in the UK.
My intention is it create a raised pond of about 18" high from (treated) softwood timber. My local timber yard sells 6"x6" x 4.8' treated softwood (hardwood or oak would be far too expensive unfortunately).
So I intend going about three layers high (18") by roughly 7'x 6'. I'll then dig down another 1' to 18" deep inside to give me an overall depth of between 2-3' at the deepest. This, I intended lining with a butyl liner (plus underliner). The timber corners would be interlocked and secured with some hefty brackets and screws.
So, here's my question......what kind of pressure can I expect on the timber at the upper 18" of the pond where the water is now above ground level. Is a 6x6 timber strong enough to resist bowing out wards? I have considered a rigid or semi rigid pre formed liner but am struggling to find the right shape for what I need. Any advice or alternatives would be appreciated. Thanks

 

[Big Lou]

Welcome Rob. I would think you timber side walls would easily stand the pressure. What I would be concerned about is the woods deteriation. Have considered treated pine?

 

[Meyer Jordan]

Assuming that the total water depth will be 3 feet and the horizontal dimensions will be, as stated. 7' x 6' then the total volume of water would be about 942 gallons. Water weighs about 8.34 lbs/per gallon. This then would equate to a total weight of the water column being roughly 7,860 lbs.
This force is at its maximum at the bottom of the 3 foot depth and is gradually reduced as depth is reduced. Without going into complicated formulae, suffice it to say that the top half (18") of any vertical wall in this pond will experience hydrostatic pressure considerably less than half of what is experienced in the lower half. This pressure is proportionately reduced until the surface of the water is reached at which point the pressure equals Zero (0).
Also it must be recognized that this pressure is spread evenly over the total wall area (26' x 1.5') 39 sq/ft. Assuming an estimated total pressure of 1/3 7,860 lbs. or 2,620 lbs, we arrive at a force of about 67 lbs/sq/ft maximum.

6"x6" timbers should be of sufficient strength to support this pressure. Keep in mind that pressure treated timbers have a tendency to 'bow' or warp as they gradually dry out. I would suggest that rebar supports be inserted every 18"-24" to provide added stability.

 

[Big Lou]

Rob, I defer to Meyer's great information.

 

[Rob75]

Thanks for the replies, particularly yours MJ for such a well constructed and comprehensive answer. I'll make a note of those figures for later on when I come to purchase pumps etc. You sound like a very experienced engineer I'm guessing!?
Great idea about the re-bar too. I recently built a raised bed from 6"x10" sleeper 'style' timber .......and as you said, despite it all being aligned nicely when installed, 5 months later the corners are not quite as neat because the timber has dried out and shifted a bit.
My intention was to rest all four corners on concrete padstones and do the same in the middle along the 6&7' lengths to prevent vertical sagging. But like you said, as it dries out there will be movement in unpredictable directions.

Can I just ask, would the re-bar need to protrude out of the bottom of the 'stacked' timber? Would you drive the re-bar through all three layers into .......maybe a concrete (unset) bed to further prevent bowing or warping or just cut the re-bar at 18" down to ground level? Thanks again, very much appreciate you sharing your knowledge.

 

[Meyer Jordan]

You sound like a very experienced engineer I'm guessing!?

Engineer? Only if when young having American Flyer and Lionel electric Choo-Choos counts!

In the course of designing and crafting Water Features for almost 20 years, one by necessity picks up a working knowledge of diverse disciplines and also learns how to quickly locate accurate information.
I am pleased that I have been of some assistance.