Constructing a slab over shrinkable clay

[sybie99]

Hi Guys

If you had a deep underlying clay stratum, highly shrinkable, is there any other way of constructing the ground slab other than suspended, i.e. using beam and block, or piled foundation?

My concern is heave. We would like to offer the client more than one solution. One of the proposals is to support the heavy superstructure on piles, but the ground slab can be ground bearing. To deal with possible differential settlement between the ground slab and superstructure we will place isolation joints where the slab abuts against columnns or walls supported on piles.

If we were to take off say the top 500mm of soil and replace it with a well compacted fill material such as crushed stone, would this take care of any potential problems as a result of clay heave? If the clay below were to heave would it not still lift the ground slab? Is there a minimum thickness of fill that must be placed over the clay? I would also like to know what amount of heave (as a dimension) can occur from a clay stratum, are we talking 10mm or 200mm? I understand that a suspended slab with a void or compressible material below will not experience any loadings from heave, but what about a ground slab constructed on fill over highly shrinkable clay?

 

[countervail]

The voids for suspended slabs can be up to 150mm to avoid movement so you could be looking at heave movement of this order if you go for a ground bearing option. This obviously depends on the state of dessication in the soil (have you got trees etc), if you can take out any dessicated soil by digging out and replacing then I would think you would mainly be looking at shrinkage settlement as the soil dries slightly under the building footprint.

 

[hokie66]

Your geotechnical investigation report should give an estimate of shrink/swell potential.

For low rise buildings, generally residential structures or small commercial, it is common to use stiffened slabs, commonly called raft slabs. These are stiff enough to minimize differential movements, and when combined with articulation of the walls, have a good track record for small structures.

For multistorey structures, deep foundations are normally required, and the slabs at ground level should be supported on the piles and isolated from the soil by void formers.

Removing some of the clay and replacing with granular fill can make the situation worse, as the granular fill acts as a reservoir for water.

 

[a2mfk]

Where is the site and what does your geotech report say?

In the parts of Florida where we deal with plastic clays, their potential for unstable behavior may be neutralized by a constant and high water table, and by their depth. If they do not occur until 15-20ft below grade, often their effects are minimal.

Like Hokie said, work closely with your geotech on this one.

 

[sybie99]

Hokie, could you please explain on what you mean by the fill acting as a reservoir? Are you saying the fill becomes saturated as water doesnt flow into the clay as its less porous? What issues could this cause?

 

[JAE]

Here's what we used in Texas some years ago on expansive clays:

1. Smaller buildings (houses, etc.) Stiffened slab-on-grade per what hokie66 mentioned...basically a "waffle" slab with monolithic beams spaced at about 12 to 16 feet on center. Advantages: cheap and easy to build. Disadvantage: heaving will move the structure causing cracks...the stiffened slab is simply an attempt to minimize the damage.

2. Mid-sized structures - (retail stores, one story office buildings) - Support all perimeter walls and all perimeter and interior columns on drilled and belled concrete piers extended to better soils/shale below the expansive clays. Perimeter walls are supported by structurally spanning grade beams that span over the drilled piers. These grade beams were constructed with voids beneath them and fill retainers on either side to deter adjacent soils from sluffing in. Interior columns directly supported by drilled piers. Interior floor slab - concrete slab-on-grade.."floating slab" separated from pier stupported elements with expansion joints. Interior walls detailed to allow differential movement between exterior walls. Advantages - the overall superstructure is supported on non-heaving materials. Disadvantage - the interior slab will still heave a bit with variations in moisture in the soils - distress at exterior/interior wall interfaces.

3. Option 2 above but ADD removal and replacement of 2 to 4 (or more) feet of expansive clays with non-expansive soils. This is simply an attempt to minimize heaving in the interior slab-on-grade and surrounding sidewalks, etc.

4. Larger buildings - Completely suspended structures supported on drilled piers with a full crawlspace below the main level. Advantages - the structure doesn't move. Disadvantage - most expensive.

Other options:
a) There were companies that marketed chemical soil treatments that were injected into the soil to remove the expansive nature of the clays.

b) Some firms removed the top layers of soil, disced in lime in the remaining top feet of soil and then backfilled with non-expansive soils. The lime treatments were more common for parking areas.

For expansive clays - the key is to eliminiate moisture VARIATIONS in the soils. If the soil is constantly wet - or constantly dry - the volume doesn't change. That's why hokie mentioned avoiding granular materials because this allows the moisture to get in and get out more easily and thus affects the volumetric changes moreso.

 

[hokie66]

Yep, JAE has answered the question about the hole filled with granular material. Eliminating the variation in moisture is the key. In the same vein, keeping trees away from buildings is important, as they dry out the soil differentially.