The reason is that when soil heaves up due to ingress of moisture is that it will stay up even after it dries out. It seems that in domestic construction the loads are not enough to push soil back down.
In my reports I have recommended partial demolition and reconstruction of slab to get down to designed levels.
It would be interesting if anyone else has different experience.
Building Expert
I agree the loads are very light on these slabs as little as 15kpa for single storeys so I've been told.
The reactive clays will shrink back when they are dried out in normal situations even without a load but with these slab heaves cases
they are not normal situations and I can explain why but it will be a long blog.
In your recommendations if the soil is still wet on the edges and dry in the middle of the slab and the slab partially reconstruction then that poses the risk of long term settlement at the edge as the clay dries out and this could take a long time.
I have heard engineers talk about Uretek the middle of the slab to match the edge but this would disconnect all the plumbing under
the slab and again if the wet clay under the edge beams slowly dries out this will cause differential settlement damage.
It really is a hard complex problem.To date no one has come up with a solution.I am trialling a experiment system in the western suburbs of Melbourne with some limited success so far.
They are trying deal with the same problem in certain parts of America.