In Australia the use of polystyrene waffle slabs on expansive soils is controversial.Fabio Parodi
Expansive soils are those that experience important volume change when the soil moisture is altered. Soil moisture can be altered by several factors which may act in combination with seasonal changes, the effects of trees, drains, roads etc. The swelling and shrinking of soils can adversely affect buildings. (1)
This problem is well known in several countries. In the United States alone, the cost to repair buildings damaged through expansive soils has been estimated to be twice the combined damages for natural disasters. (2)
The Australian Standards (AS2870) contains an entire section dedicated to construction on expansive soils.
In New Zealand and especially in the Auckland region, expansive soils can be detrimental for the durability of buildings constructed with the traditional methodologies.
A significant proportion of new residential construction in New Zealand has been designed with the NZS3604 concrete slab-on-ground floors.
However, the NZS3604 specifically excludes foundations on expansive soils from its scope and refers the designer to Section 17 of the Standard for additional information on expansive soils. (1)
Although the Australian Standards are an appropriate starting point for designing on expansive clays, New Zealand Chartered Professional Engineers are required to provide specific designs (which is not often obvious). (3)
NZS3604 slabs on ground and waffle slabs using polystyrene void formers have a similar interaction with heaving soils since the polystyrene void formers show a high-compression strength under short terms loads and they transfer most swelling pressure onto the slab.
Both systems have a large contact area and they are not the most effective foundation option for expansive soils.
The use of polystyrene waffle slabs has been inherited from Australia where they are included as an acceptable solution within the local standards.
In Australia the use of polystyrene waffle slabs on expansive soils is controversial. (4)
Foundations on piles under the edge beams and the internal bearing members may fail on expansive soils because the piles impede the foundation from floating and additional safeguard measures are recommended to protect the un-piled portions of the floor from receiving uplift forces and cracking.
Vice-versa shallow 2-way slabs such as the RibRaft® Xpod™ and the Armadillo Foundation System™ using plastic void formers have less contact area and they reduce the uplift pressure on the slab when soils heave.
When these systems for foundations are used the structure can float on top of ground and the voids act as relieve valves that absorb heaving soils reducing upward forces on the bottom of the slab and eliminating structure cracking.
When designed with an appropriate calculation methodology the RibRaft® Xpod™ and the Armadillo Foundation System™ can be up to 25% more cost-effective on expansive soils than other systems because instead of trying to resist against the swelling pressure they accommodate the ground movement thus limiting structural distortions.
Cresco engineers have developed a calculation methodology for stiffened rafts on expansive clay based on the equations and on the engineering principles for beams-on-mound underpinning AS 2870:2011.
This powerful tool can be also used to optimize the performance of foundation systems such as stiffened polystyrene waffle slabs, RibRaft® X-Pod™ and the Armadillo™ Foundation System in accordance to the New Zealand Building Code and the New Zealand ground conditions.
CPEng (NZ-AU) MIPENZ Dott.Ing(ITA) M.Eng(Hons)
CEO and Founding Director of Cresco
(1) ADDENDUM STUDY REPORT No. 120A (2008) SOIL EXPANSIVITY IN THE AUCKLAND REGION – https://www.branz.co.nz/cms_show_download.php?id=f52e73feb70417065fbbdaeb4bffc91ce1ef8461
(2) Effect of Wetting and Drying Cycles on the Behavior of Compacted Expansive Soils
Daniel Rosenbalm, Ph.D., P.E.; and Claudia E. Zapata, Ph.D., M.ASCE
(3) DBH Determination 2010/132 – https://www.building.govt.nz/assets/Uploads/resolving-problems/determinations/2010/2010-132.pdf