Soil Heave – Protecting Slab During Construction

Protecting Slab During Construction

The diagram below shows what you should be looking for, during construction, to protect the ground under your slab from gaining moisture and moving upwards.

slab-detail-allowing-protection-against-soil-heave-drizoro-maxseal-superProtecting Slab During Construction, damp-clay-under-slab-detail-allowing-protection-against-soil-heave-drizoro-maxseal-super

“Drizoro Maxseal SUPER  to stop water penetration of your slab, Protecting Slab During Construction, especially where the clay remains in the water table”

The key issues in soil heave are making sure:

  1. The excavated surface falls away from the edge of the slab for at least 1m with a minimum drop of 75mm.
  2. Where the water will not continue to flow away from the slab an Aggi Drain in a granular backfilled trench should be provided. This drain should be a minimum of 100mm below the surface level of the clay and fall to a suitable discharge point*.
  3. Any trench in the area between the slab and the aggi drain should be topped with well-compacted clay to ensure there is no easy passage for water to penetrate under the slab.
  4. Temporary spoon drains
  5. Roof drainage should be connected to a suitable point of discharge as soon as possible after the roofing material is fixed. (See Temporary Downspout)

Although the requirement for an aggi drain is not as critical where the ground slopes away from the slab, it is nevertheless good practice to have one.

Note: Good practice also to use the best system that fully waterproofs your wall onto footings. Design the slab so that any water is directed away from the ground below your slab to prevent soil heave
*Aggi drains act like the sand filters eventually they will clog and can never be a substitute for the proper construction planning
After being actively involved doing waterproofing for many years I haven’t see the aggi drain placed this far from a footing let alone where it drained properly 

OnlineWaterproofingtShop Team


The most important step to prevent the soil from pushing or expanding up under your slab (soil Heave) is to direct the water away from the soil directly under your slab. Preventing water from draining to the soil surrounding the slab is critical.
Critical to install proper drainage to take all excess water off-site and well away from the soil beneath the slab.


Source: Soil Heave – Protecting Slab During Construction – A New House