I posted on a Homeone forum about slab heave a while ago. I said that slab heave movement could be reversed simply by reinstating the soil moisture regime under the house that was present at the time of construction.
I have re-considered this week whether my statement holds true for waffle slabs – and I’d like your opinion on my theory.
Background info:
Clay soils expand when soil moisture content increases.
Waffle slabs are constructed in top of a level building pad.
The ground is built up around a waffle slab after the slab is poured so that the slab is not so high above ground.
My revised failure mechanism for slab heave of a waffle slab is:
1. Poor surface drainage allows water to drain under the waffle slab.
2. The water under the slab migrates to the lowest part of the natural ground under the slab. The water is trapped under the building by the built up ground around the waffle slab.
3. Where the moisture has drained to, the moisture enters the clay.
4. The clay soil expands, forms a mound and lifts the slab locally.
6. A void forms under the waffle slab ribs adjacent to the wet patch of heaved clay because the stiffness of the waffle slab spans over the dryer area of soil.
7. Where the wet patch of soil is adjacent but just back from the slab edge, the waffle slab cantilevers over the moist clay mound. Recorded differences in slab surface levels exceed the estimated ground movement (ys) for the site because of the see-saw effect.
7. Soil and water migrates and partially fills the void in the lower area. The natural ground level has changed.
8. Removing the moisture from the soil can not reverse the movement of soil that has occurred under the slab.
9. The waffle slab can not re-level itself even with cut-off trenches and a stable soil moisture profile. The movement of soil under the slab and stiffness of the slab system means the waffle slab sits on a changed ground surface.
Under these circumstances, some reversal of slab movement would be achieved but the waffle slab would never completely return to level.
Reversal of slab heave in a raft slab would also be unachievable in a doming slab because of the collapse of soil into the strip footings adjacent to the heaved mound; however sideways migration of soil under a raft slab is less likely because soil moisture accumulates in the deeper perimeter edge beams resulting in edge heave rather than slab doming. Reversal of slab heave is achievable in edge heave raft slabs because there is little sideways movement of the soil under the slab and reversal is quicker to achieve because of the proximity of the edge beam to atmospheric drying.
Do you agree that it is harder to reverse waffle slab doming than waffle slab edge heave? Is demolition the only recourse for a domed waffle slab that has deflected beyond AS2870 limits?