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Observatory Base Problem

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2 hours ago, KyleStoke said:

Hole finally dug, bit of shuttering to do and then concrete pour but that should the last major obstacle


It doesn't look that deep on the picture but once the shuttering is added its just under a meter deep.

Make sure that you have a long piece of wood about 3" round or square during the pour. Keep plunging it into the concrete mix in the hole and vibrating/jiggling it up and down, moving it all around the whole area. It will ensure that the concrete settles right down into position with little or no air pockets inclusions. When the pour is finished use a flat bit of wood about 10-12" square to paddle the top for a good 10 mins. Then level out to your required depth, adding a bit more concrete if necessary to keep from sinking below the required depth. Water will rise and the level will sink slowly. Check again after an hour or so and reskim by float to keep absolutely flat. Don't rely on your eye use a good level. Check it is accurate before the pour.  After it is set you can do the outer area.



If you are pouring the whole area of the base in one go (recommended ). Obviously do the pier bit first then work out wards. You can use hard compressed foam as a shutter for the top few inches to give the vibration insulation to the pier area from the rest of the obsy base, wood will be very difficult to remove and is not an vibration insulator. As you get towards the top of the pier, start filling in around the outside of  the insulation area to give it strength and continue to fill the whole area until at a uniform level.

Doing it this way you can use a long wood beam with handles each end to tamp down the whole area level. Just be careful not to smash the foam insulation. The foam should be capable of being tamped down carefully without bits breaking off. Ordinary very soft foam is of no use. Then it is all set you can if wanted dig out the foam and fill back in with a softer spray in expanding foam such as Soudal Flexifoam, available from Screwfix.

Lastly if no rebar is used to strengthen the obsy floor then you will need a depth of about 4 to 5 inches of concrete for crack resistance. less than that may in time fail and then leak.

Hope it all goes well for you.


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Been at work all day today but as if by magic the observatory fairy's have been and the project is complete.

My advice is to hire an electric demolition breaker. The biggest you can get and the low voltage supply for it from a tool hire shop. Break it all up into manageable pieces and reuse them to sink into

If you do hire a breaker. Don't bother with those giant electric ones. They are utter rubbish. I never hire those electric ones as they are never powerful enough, and are too huge, awkward and heavy t

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Thansk for the advice Ian and Derek.

I will be doing the concrete myself after it is delivered Tuesday. 

Derek I was going to agitate the concrete with a rake as I poor it in then traditional wood and float to finish off. I have done the initial shuttering for the pier section today it is all nice and level and gives me a final depth of 93cm which im pretty happy with.

I will keep you posted as to how i get on.

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If as you say you are doing it yourself, best of British. You can use the float to push down then pull up quickly (vibrtate) to settle the concrete. It is not just to keep a level but to get it to filter down below/ between  the top of the rubble layer underneath, or you may end up with it either sinking down later when you aren't there to float it again flat. It may also get voids and become a sounding board so to speak, (sound hollow when walked on).

Just for info this:



went in there plus a lot more:

IMG_1545.JPGTo get that:084.JPG

Hope you are fit :)


Edited by Physopto
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Derek, I wouldn't describe myself as fit, after i dug the old concrete out I couldn't move for 2 days, but the more Ive done the easier it has got. 

I'm keeping focused on the end goal as soon as i have this finished I can relax and enjoy, well sort of.

Having seen your photo of the round thing I can remember reading the artical when you were doing it.

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Yup all fun and games Eh. Get a good nights sleep I have a feeling you will need it.

Have fun. Plenty pics, gives you  a chance to get your breath anyway :p

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Well after rain stopped play for most of the week, finally got back to it today.

Area prepped and readyIMG_20160514_093459349.jpg

The Pour begins



And 3ish hours later i have a hole filled with concrete


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I use something similar to encase my scope in its box. It is a good shock protector. There are other similar types of closed cell foam out there. Some very good ones are made up of reconstituted foam. It looks like different colours of foam bonded together. It used for for carpet underlay. But it depends upon what you want. You can set it up in place of the shuttering and pour concrete around it if it is about 30mm thick, as it is relatively stiff enough to stand on its own. The concrete will then hold it in position indefinitely.

One important thing is to make sure it is of the closed cell type or you can get moisture ingress.

There is another method. That is to use a box section very similar to your present shuttering, leaving about  a 30 mm gap. But make it stiff enough not to collapse under the weight of the concrete when poured, the gap can be maintained with wooden wedges. After the concrete has set remove the box section and wait for the concrete to set for a day or so more. The concrete will still be internally wet. Then fill the gap with Soudal Flexifoam. ( available from Screwfix ). This is a soft expanding foam that will adhere to the concrete and prevent most if not all capillary action of the water, as it actually bonds to the concrete. After a day just trim off the excess foam from the top with a sharp flexible knife. I have used it around the edges of doorways and around the flooring to wall gaps in houses, before the skirting is fitted. It is especially useful as an insulator for awkward shaped piping under floors, where conventional foam cannot be applied.


Edited by Physopto
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21 hours ago, Pompey Monkey said:

I used exercise mats for my pier base vibration insulation. It took two to do the job: £5 each from TKMaxx :)

You build is looking great, BTW! :)

Thanks Paul

I think the stuff im going to use is similar, it says it is sound dampening therefore should also be vibration dampening. I will make my mind up when i see it.

I was getting some other stuff off amazon so thought for a couple of quid its worth a look.

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Finally the base has been completed, I ended up using 20mm closed cell foam to insulate the pier plug from the rest of the base.

Im glad its all done hopefully its upto spec.


Yesterday I was too tired to even walk back over to it.


An this is it this morning drying nicely, hopefully the conduit is not too far out from the center.

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