Concrete raft for ~3 ton water tank. How?

6 Oct 2010
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United Kingdom
Hi all,

I need to build a raft to support a heat reservoir for solar thermal installation. The cylindrical tank weighs 350kg empty, capacity 2240 litres; height 2m10, diameter 1m10 (plus 20cm thickness insulation).

The tank will be sited beneath the house in the crawl space, where I have excavated down to the footing; there is 2m50 clearance between the upper surface of the footing and the ceiling.

The crawl space can be seen here:

The soil at that point (approx 1m50 below original ground level) is somewhat clayey, most probably laid down through glacial action; I don't believe it has been disturbed since, except immediately adjacent to the footing. A local engineer visited and described the soil as highly cohesive with excellent load-bearing quality. The soil is slightly damp but not moist.

I envisage the tank centred on a rectangular raft, 1m60 x 1m, with the long edge parallel to, but not in contact with, the second-most northerly wall in the image; the southernmost edge of the tank will be approximately at the dimension line marked '250'.

Ignoring the mass of the raft itself, this gives a loading of ~16kN/m^2, which I understand to be modest.

What type/mix/dimensions of base, and concrete should I use, and what form and thickness reinforcements?

How can I ensure the structure doesn't tilt?

Any advice appreciated - let me know if further diagrams/photos/data needed to clarify.

Best wishes,
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So you have to crawl underground to gain access to this vessel?

Can you draw the position of the vessel in your diagram and a wee man crawling on his way to access the vessel?
Hi Norcon,

Thank you for your response.

Access to the crawlspace is through a full height door to the rear of the garage - no need to crawl. The dotted grey line show the approximate original earth level.

Tank emplacement (not to accurate scale, but close-ish) per the image; around the tank (ie not the full depth of the garage) I've excavated as indicated by the blue line. This should allow just enough space for pipework and tank insulation.

View media item 67878
Tank will be passed through a hole in the wall between the garage and the crawlspace. I'll either have a lintel put in permanently, or just props for the duration.

Correction: The diameter of the tank body is 1m20; that of the support (cylindrical steel cylinder welded to the tank) is ~1m05, so raft will have to be at least that width.

Hope this is clearer now.

Best wishes,

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