Help! A question for the structurally minded

Jon W · Post by **Jon W** » Fri Oct 19, 2007 11:09 am

Hi all

I've a slightly tricky question (well for me anyway) regarding structural integrity of my plans for a growing on pond.

I have just received all the materials. The basic plan is as follows.

Internal dimensions 10' x 5' x 3'6" water level.

The pond will be built on an existing concrete pad in a shed.

I have been advised that I should "tie-in" the first course of blocks to the existing concrete floor otherwise that the whole structure could move with the weight of the water. Would you agree that that is the case?

If this is the case the first course of blocks will be hollows. Holes will be drilled in the existing pad and vertical rebar added to every other hollow block "cell". The blocks will then be filled with concrete. A concrete pad will then be poured inside the wall to create a base for the pond (having fitted the bottom drain first!). The rest of the pond will be built up with 7.3Kn dense concrete blocks.

The big issue is that I can only source 3.5 Kn hollow blocks. Given the rebar and concrete fill, will they have sufficient strength to support the rest of the structure?

Regards

Jon W.

tranquil waters koi · Post by **tranquil waters koi** » Fri Oct 19, 2007 6:03 pm

Okey dokey,
many many koi ponds are over engineered by hobbists due to the fear of collapse or subsidence

so I will try and give you a little reassurance if I can

. What you have described sounds a little complicated,

you would find it far simpler to just build the entire structure out of 9in hollow blocks (which in turn will help your thermal retention) you can back fill the hollows with a runny mix of concrete if you wish but the structure will be secure without this measure. As for the base, for the dimensions that you suggest I would recomend that the blocks sit on a base of a six inch minimum thickness foundation in a clay rich site and rising to 9inch if in a sand rich ground.

NOTE do not build within 3 to 4" of the edge of the foundation or pad as you can just crumble the edge off durring construction. Whatever you chose to pour inside afterward to cover the bottom drain will have no real bearing on the strucure as long as this is supported by the same foundation as the walls, this "pad" and the one you are pouring on to do not need tying in, in this application.

In short the key thing is, check the thickness of this pad you plan to build on first and if it is not thick enough then increase it to a sufficent thickness.

hope this helps

regards
Adam

You might want to look at my other thread on this subject:
http://www.koiquest.co.uk/forum/viewtopic.php?t=3627

Bob Hart · Post by **Bob Hart** » Fri Oct 19, 2007 6:39 pm

Jon,

If I were building this pond, I would only use 4" thick blocks. The forst one's I'd lay flat and then I'd build up with the 4" blocks on edge to the height I'd require. The 4" flat blcoks I'd use a s a guide for the new bace, which I'd lay 2" over the top of those blocks, giving a 6" bace.

In my opinion hollow blocks are a massive over kill and I dont think you need to tie into your original bace at all if you are pouring a new base on top of it.

You'll get a few opinions here, the difficulty is making your mind up from them. Many people build ponds with hollows and concrete poured in them, just to be sure - but they are just not needed. How do I know, experience from building a few this way and trying to demolish a few later on!!!!

Jon W · Post by **Jon W** » Fri Oct 19, 2007 6:41 pm

Hi Adam

Thanks very much for the response. Underneath the existing concrete pad is red sandstone. The existing pad is 12" thick. Given internal dimensions of 10' x 5' with a water depth of 3'6" do you think I can use 4" dense blocks used "upright" or would they need to be layed "flat"?
Also I have already bought the blocks which is having a bit of influence on the decision.

I have seen freestanding ponds using 4" blocks which are larger than mine but having no experience of this I am a little wary.

Regards

Jon W.

Jon W · Post by **Jon W** » Fri Oct 19, 2007 6:56 pm

Hi Bob

The "masterplan" was to pour the new base inside the first course of blocks which would be hollows. 2 courses of 4" blocks would then be laid flat (being vaguely aware that the force of water is greatest where the base meets the wall and thereafter the structure would be built with 4" blocks laid vertically. This differs from your suggestion insofar as no blocks are laid on the new base. Given that the existing pad is about 12" do you think this is feasible.

Cheers

Jon W.

Bob Hart · Post by **Bob Hart** » Fri Oct 19, 2007 7:53 pm

Sounds great Jon, go for it

And dont forget the pictures!

GERRY5 · Post by **GERRY5** » Fri Oct 19, 2007 8:11 pm

JON ..dont see any problems of building on a 12 inch thick base whatsoever , your pond works out at about 1100 galls or just short around about five cubic mtrs or five metric tonnes of water ...the weight will fall firmly on the base , and agree with bob that hollows filled with cement may well be overkill , but offer substantial peace of mind , built two ponds now using each method you are considering both are 1500 gallons and neither have budged at all ...good luck with the build

if ive got me figures wrong chaps ...shout out

Jon W · Post by **Jon W** » Fri Oct 19, 2007 8:42 pm

Hi Bob and Gerry

Thanks very much for the responses. All the materials arrived this morning. Although I've researched this and sort of know that 4" vertical walls should be okay I started to panic. My next door neighbour took one look at the blocks and said that I should build it stronger - he's a farmer not a builder but he's pretty clued in about most things. However I knew that there is a lot of experience on the forum regarding blocks/pond building and would always defer to the concensus presented here.

I will definitely post up pictures as the saga begins.

Thanks very much for the reassurance.

Regards

Jon W.

Davej · Post by **Davej** » Fri Oct 19, 2007 9:44 pm

Hi Jon

You are only talking 1000galls and no great depth, tying in the first layer that is going to be filled with the base concrete is overkill!!

I'd go with totally with Bob on this one, it's how I did mine!

Let's have some Photos....

Regards

Dave

Bob Hart · Post by **Bob Hart** » Fri Oct 19, 2007 9:53 pm

Jon,

If you are concerned about the 10 foot wall length, just build a double width concrete wall in the middle. Bricklayers do this with single width walls at certain lengths, making piers at predetermined positions for strength.

Jon W · Post by **Jon W** » Fri Oct 19, 2007 10:15 pm

Hi Bob and Dave

I think I'm now feeling brave enough to continue. Only problem is that I haven't got any sand for the mortar until Monday. The building suppliers delivered 3 tons of aggregate/sand mix instead 2 aggregate mix and 1 sand. By then I might have managed to carry all the blocks inside and tidied the place up properly.

Inicidently do you agree that class 1 mortar is the right mix for this job (1 part cement, 1/4 lime, 3 parts sand)?

Cheers

Jon

Bob Hart · Post by **Bob Hart** » Fri Oct 19, 2007 10:16 pm

Jon,

I've never used lime, I usually used 4-parts sand, to 1-part cement. What you are prposing is certainly stronger and I cant therefore see anything wrong with that.

Jon W · Post by **Jon W** » Fri Oct 19, 2007 10:21 pm

Hi Bob

Just wondered if the additional strength comes at the price of reduced flexibility. And I got it wrong; it should be 4 parts sand not 3!

Cheers

Jon

Bob Hart · Post by **Bob Hart** » Sat Oct 20, 2007 7:30 am

Jon,

I'm not a builder, I just do what I'm told - LOL

I have a brickie friend and I know Justin as well now. My original brickie friend said 4:1 so I always did that. Mix it wet, with some washing up liquid in it and just lay your blocks. Wahising up liquid was the back up stuff, cant remember the name of the proper stuff to be added, but a bit of cheap Tesco washing up liquid does the job. If you mix it too dry, you just never get anywhere and it looks awful.

Jon W · Post by **Jon W** » Sat Oct 20, 2007 12:22 pm

Hi Bob

I think the word you are looking for instead of washing up liquid is plasticizer - makes it easier to work the mortar. I'm sure I'll manage to make it look awful irrespective of how much plasticizer I use! I've never laid a brick in my life. I'm using this growing on pond as a tester to see if I can develop sufficient ability to self build a new outdoor pond.

By the way I have looked at the job again this morning and decided to build the first course a brick width out from the pond wall and use this as the boundary for the base. That way I can build the walls on the new pad rather than the existing one; just in case it isn't as thick in the middle as it is at the sides.

Cheers

Jon W.