Ring beam on expansive soil 1

[sdz]

I'm designing a ring beam for a bolted water tank on ground and the site has expansive soil (high shrink/swell).
Are there any extra precautions I need to take or design methods?

 

[EireChch]

Yes, you need to account for shrink and swell.

If it shrinks you could have excessive displacement. Same in reverse if it swells.

Over excavate and replace the expansive soil.

 

[Lomarandil]

Just to play devil's advocate, if the expansive soil has somewhat uniform potential and any connections have the flexibility to accommodate movement -- any other concerns still to consider?

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just call me Lo.

 

[EireChch]

For a tank with a flexible steel plate base, expansive soil could distort the base and change to desired slope, low point of the tank etc.

Banking that it would be uniform swelling / shrinking would be a big call.

 

[sdz]

I'm working on a range of tanks from 3.6m dia to 14.5m dia. These are "farm" tanks and I won't be responsible for supervision of construction.
So far I have considered what might happen if the material inside the ring gets wet and I have allowed for a lateral pressure with Kp=3, compared to Ko=0.5 for at rest lateral pressure. This load doesn't seem to be a problem so replacing it with a non-expansive doesn't seem to be justified.

I'm now considering the case of an extremely expansive soil, Class E, with ys>70mm.(characteristic surface movements)
If the wetting and drying is uniform around the tank it's probably not too bad but I'm concerned that a leak at one point with a localized heave mound could cause bending and torsion in the ring beam.

Does anyone have a reference to design methods for such a case or software that can handle a circular ring beam?