Slab on Grade - Mitigating Heave in Stiff Clay 2

[Caribbean]

We are building a slab on grade in the Caribbean where the Geotech Engineer has specified a potential free heave of 5”, a differential heave of 2.5” and an edge distance of 1.5m. The area is classified as very stiff to hard clays in a warm tropical climate with extreme dry and wet seasons. If I understand correctly the edge distance is the area of influence in the slab on grade that is likely to be exposed to a free heave of 5” if nothing is done to mitigate it. It means that the slab that is beyond this 1.5m edge distance (or internal slab) is not exposed to the heave as outlined above. However, the geotech engineer has suggested we find a way to stop the transfer of surface water into this 1.5m active zone around the edge of the slab on grade in which case we would mitigate the edge of the slab from heaving which ultimately means designing the slab as a simple slab on grade instead of a suspended slab.
Questions:
1. Has anybody seen such a design where the edge or active zone is the only potential area of concern since it is expected that water will move in and out of this zone depending on the season.
2. Any tips on how to mitigate this edge water. One suggestion was to excavate the active zone, which was specified as 3m below the slab on grade, and replace with concrete or some water stop.
3. Is 5” of heave realistic?
4. Should we be concerned about the free heave at the centre of the slab. In other words, it is likely that only this 1.5m edge area has the potential to heave 5” and not the centre of the slab. The water table is 1.5m below slab on grade and 1.5m of backfill at 95% compaction has been placed under the slab on grade.
5. Any good references or guidebooks that I can read up on for slab on grade/heave etc in warm temperate climates.

 

[JAE]

I'll take a stab at your questions:

1. I used to work in South Texas where there was expansive clays prevalent. Yes, the perimeter of a slab-on-grade foundation usually sees more variation in moisture content and thus experiences more heave/shrinkage.

2. We usually used a "stiffened slab-on-grade" design procedure. Refer to these two links here:

thread256-15722

thread256-6848

To mitigate the edge water, using deeper turn-down edges (i.e. turn down beams) is a good approach. Basically, the stiffened slab-on-grade was sort of a waffle slab (two way) where monolithic turn-down beams occurred at about 12 feet (3.6m) o.c. to create a grid of stiffness to help minimize any heave deflections in the slab and to span between different high areas when heave occurs.

3. We would get estimates of heave around 2" to 3" (50mm to 75mm) and I would say your 5" is not unreasonable.

4. In the center of the slab, you could potentially get center heave from leaking water pipes under the slab or from rising water tables with hot drying occuring around the perimeter...but this may be highly unlikely.

5. No references other than the documents in the links above.

Hope this helps a little.

 

[EdGeotech]

Here in Hawaii, the standard of practice is to overexcavate the clays in the upper 1 to 3 feet and replace it with compacted granular structural fill. The intent is to place a relatively rigid section below the slab that will add some overburden pressure to resist the swelling soils, and reduce the potential for moisture changes in the underlying soils. As long as the exposed subgrade is properly moisture conditioned, and not allowed to dry significantly, prior to placement of the granular structural fill, we haven't had to take out more than 3 feet of material below a slab, even in clays that showed a 20% expansion in ring swell tests.

As an alternative, you could look at post-tensioned slabs. We have found that although post-tensioned slabs are great at resisting the anticipated differential movements without excessive cracking, differential movements that can affecct walls, doors, and windows can still occur. So, in highly expansive soils, we make sure to extend the thickened or down-turned edges deep enough to resist movement and, when necessary, still underlay the slab with granular structural fill down to the bottom of thickened edges.