Slab on Grade & Mitigating Heave in Clays

[gsl2007]

We are building an entertainment centre in very expansive clays in a tropical climate with a very wet and very dry season. The centre contains stands that will be supported on auger cast piles with a combination of pile caps and interconnecting beams. The floor slab as designed is not a suspended slab (6” single reinforced slab). The geotech engineer has recommended a number of scenarios to mitigate the heave of 4”-5”at the egde of the slab including removing 3m of clay and replacing with suitable compacted fill, a turn down wall 3m deep, stiffened slabs, 1.5m extended slab beyond the building etc. All of which are very costly.

Any response to the following would be appreciated:
1. Is the 1.5m edge distance the only part of the slab that is likely to experience heave of 4”-5”
2. Has anyone seen other ways to mitigate this heave by way of a plastic water barrier extended beyond the building.
3. If the zone of influence in the horizontal plain is 1.5m, why would the vertical zone of influence be 3m.
4. What have other contractors done to mitigate heave.

 

[dik]

If your geotekkie is good, you shouldn't have to second guess him. Discuss this with him.

It is possible that the soil will desicate under the building and that only the edges will undergo any fluctuating changes in moisture. If greater the plasticity of the clay, the longer time for desication if it occurs. If the plan dimension is small enough it may be that the clay may not desicate. If the clay is impervious enough, movement at the edge should not fluctuate seasonally over a long period of time, the edge soil will remain near the current moisture content and soil within will dry out.

I would have thought that the edge distance would be 3m-5m and not 1.5m, but your geotekkie should be aware of local conditions.

The greater vertical dimension could be influenced by long term desication from beneath the building footprint and the effects of water only affecting the outer 1.5m.

Is it possible to use friction piles neglecting the top 3m or so (maybe include drag or polywrapped sonotube) and structure the slab? Skin friction could be maybe 1/10 the compressive capacity. Structuring only the areas the currently cannot move (and they may end up being 4" higher than the surrounding areas)limits the flexibility of the interior layout... you might want to consider using a structured slab throughout.

Can you use an alternative flooring material... peagravel or maybe concrete pavers?

Dik

 

[dik]

Forgot to add... what is local practice? and since the owner is benefiting financially from whatever is done, he should clearly be on the hook for anything that goes bad. If the perimeter is not likely to move then it may be possible to construct ramps from the entrances to the interior. These can either float (slope increases) or can be removed and reconstructed as required.