Floating slab for basement/parking garage

[TexasGeotech]

Have any geotechs or structurals that frequent this forum ever used a floating slab (ground-supported slab) for the bottom slab of a building with below-grade levels (e.g. a parking garage) with expansive clay subgrade soils? Just want to know if it is practical and desirable. For example a 2-level below-grade parking garage with clay and clay-shale 50 feet deep. The ground-supported slab and vertical height considerations could accomodate several inches of heave, no? The clays are deep enough to not be affected by near surface moisture fluctuations outside the building footprint, and at depth the only water source would be thin seams of flaggy shaley limestone with groundwater, within the clay-shale formation, so to account for some possible heave, i want to be able to suggest a ground-supported floating slab and not a more expensive structurally supported slab (on the pier foundation elements). Any comments are appreciated.

The building would be water-tight i imagine, and any water infiltration along thin backfill zone behind basement walls will be collected by a drain and sump pump system.

 

[dik]

I've done several buildings with underground parking garages. Depth is limited to about 40' due to limestone bedrock. The lowest slabs is invariably a conc slab on grade. Our soils are highly plastic and have a great potential for shrinkage when dessicated. Because our soils are virtually impermeable, once capped, moisture content remains pretty much the same; they don't move very much. A vapour retarder reduces the likelihood that moisture will wick from the underlying clay through the concrete slab.

If you have swelling clays I would think that you would want to maintain the moisture content and I would try to achieve this by using a vapour retarder membrane between the slab and granular fill. In addition, to accommodate movement, I would probably use isolation joints that permit large movement; it may be necessary to undertake a long term maintenance program.

Dik

 

[GeoPaveTraffic]

I have done several buildings with the lower level being a slab on grade. When we have done parking garages and there is a potential for heave we generally a) make sure that the lower level has several inches to several feet of additional head room to account for swell (we have some shales with very high swell potential) and b) use asphalt for the lower level. The asphalt is used because it is more flexible, easier to repair/patch isolated problem areas, and easier to remove and replace should the amount of swell exceed that allowed for.

There is at least one parking garage in Clayton, MO (a sub of St. Louis) where the lower level of the garage has swelled 18 to 24 inches. The asphalt dives down at each column, i.e. where the soil is restrained. However, the garage was constructed with enough excess head room that the swell has not caused a problem. The building is ~25 years old.

 

[TDAA]

All the time. As stated, slip joints and extra room.

 

[dik]

I've not had any problems with concrete, but the suggestion of using asphalt will be considered... I shy away from it due to damage caused by engine fluids... thanks.

Dik