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Part 7. PROBLEMS WITH SLABS-ON-GRADE . Cracked slab-on-grade built on expansive soils in south Orange County, CA. This problem was exacerbated by poor drainage about the perimeter of the slab.

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part 7
Part 7


Cracked slab-on-grade built on expansive soils in south Orange County, CA. This problem was exacerbated by poor drainage about the perimeter of the slab.
Edge curling (case 1) is usually associated with water infiltrating the perimeter of the foundation. It can also be caused by asymmetric framing loads, as shown here.
  • Interior doming (case 2) can be caused by soil moisture build-up beneath the interior of the slab, or by asymmetric framing loads.
All exterior concrete flatwork on expansive soilsshould be reinforced to reduce separations caused by differential heave. Rebar should never be more than 18” apart @ ACI Code, unlike the example shown here.

Perimeter trench subdrains are an effective means of limiting subsurface flow beneath a home founded on expansive soils

Under extreme circumstances, distressed homes may be detached from their original foundation so new, deeper, foundations can be constructed, which are more capable of withstanding expansive soils action.
Two kinds of slab-on-grade that can be employed on expansive soils are depicted here: 1) thickened post-tension slabs; and 2) double-T slabs with conventional reinforcement.
The heave potential of expansive soils is greatly diminished by conditioning near-surface layers to be well wet-of-optimum moisture content, as sketched here.
  • In this case soils dry of optimum were observed to swell as much as 51%, while those compacted wet-of-optimum swelled less than 20%.
In some areas, graded pads for post-tensioned slabs-on-grade are ponded with water for several weeks to elevate soil moisture levels, and thereby, reduce subsequent heave (at left).
  • Preparing PT slabs for pouring at the same site (right). This procedure usually includes placement of moisture membrane, sand cushion, excavation of perimeter footings, layout of embedded plumbing, and laying of the tension strands
Another method employs wax-coated boxes as disposable forms for crossing double-T footings, which create a deep waffle slab foundation.