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Sheeny's right. The best way to prevent such movement is to bore through the reactive soil to rock, or to some other material that doesn't move with the change of moisture content of the soil. The weight of the base is of little concern, because concrete at a SG of 2.4 is very little different from wet clay at 2.2, and it will effectively "float", or anyway, sink very slowly. Ask your friend if he knows any soils engineers. If so, the soils engineer would be able to give good advice based on local knowledge, and would be unlikely to charge. (A bottle of half decent red?)
Failing that, get a bobcat with an augur, and drill a 300mm dia pier down about 150mm into non reactive soil. Fill this with concrete reinforced with 3 x 12mm dia reinforcing bars (held together with some stirrups or heavy guage wire) and you should have a pier that will go nowhere.
That's easily said from a desk I know, but if it means removing the observatory to bore the pier, that's a different matter, but it was one that you canvassed when you suggested 2 cm of mass concrete as a base.
Digging the trench is not a bad option, but consider two points.
1 Your trench must go down through the reactive soil, or it won't be isolated.
2 You will need to allow the soil within the trench boundary to reach equilibrium. At the moment it's wet, but it will dry out over time, and you must wait until it's dry before setting up again, otherwise you will have a recurrence of the problem.
Finally, ensure that any rainwater discharge from the roof of the observatory is channelled well away from the building, so that localised flooding does not occur.
Good Luck,
These are thoughts of a retired builder, but the best way to go would be ask a tame soils engineer.
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