Great infrastructure starts with a solid foundation, and the soil behaviour underneath the foundation of our buildings has a huge influence on how they will perform over time. A team from Victoria University is investigating the impacts of seasonal changes of soil characteristics on building performance, particularly in the expansive soils that cover our major cities.
Approximately 20 per cent of Australia’s land, spanning six out of eight major cities, is cloaked in expansive soil. Expansive soils swell with moisture in wet months and shrink in dry spells, applying immense pressure on lightweight structures such as houses. The consequence? Cracks, instability, and a barrage of housing insurance claims. And it’s not just Australia, this is a global concern.
In an Australia-first study on this specific issue, a group of researchers led by Dr Wasantha Liyanage at Victoria University (VU) constructed a full-scale test foundation on an expansive soil site in Sunshine North, Victoria, with the aim of comprehensively evaluating soil behaviour underneath the foundation and the resulting foundation performance that lead to house cracks.
“The first-hand data from a full-scale foundation can help to realistically understand the foundation’s performance over time and develop a lasting solution. According to our knowledge, this is the first time in Australia that this topic is researched using a test foundation of this scale,” Dr Wasantha said.
The standard design practice
The Australian standard for residential slabs and footings (AS 2870-2011) is the key point of reference for engineers, architects, and builders involved in residential construction across Australia. This standard is a comprehensive guide and an important resource for all stakeholders.
“The growing number of house cracking issues, despite following the standard design and construction practices, is a concern and can be a product of a multitude of factors,” Dr Wasantha added.
In addition to insufficiently detailed site investigations, non- compliant construction practices and poor post-construction site maintenance, inadequacies in the foundation design procedure also contribute to the substandard performances of foundations.
According to Dr Bertrand Teodosio, a member of the research team at Victoria University for this project, “The design procedure of the current standard relies on several assumptions and empirical methods whose validity is worth verifying.”
As a part of this research, the design procedure of residential foundations was scrutinised, and the reliability of the underlying assumptions was evaluated against hands-on data from the full-scale foundation.
Waffle raft foundations
Waffle raft foundations, also known as ‘waffle slabs,’ are a type of foundation commonly used in house construction. They
are reinforced concrete slabs with ribbed patterns resembling waffles, and the void spaces within the grid of beams can house insulating materials, improving energy efficiency. Expanded polystyrene pods are typically used as void formers. Their cost-effectiveness, thermal insulation properties, and construction efficiency make waffle slabs popular in Australia.
“Waffle raft foundation structure helps to distribute the superstructure load uniformly on the ground. They are versatile and adaptable to various ground conditions, however, careful engineering and adherence to building codes is essential to ensure structural integrity and compliance with site-specific requirements,” Dr Wasantha said.
According to published data, the waffle raft and the stiffened raft account for around 65 per cent of Australia’s new and existing single-detached dwellings.
The popularity of this foundation technique prompted the VU researchers to select a waffle raft foundation for their study in addition to other engineering reasons.
“The seasonal variations of the soil condition below the waffle raft used in this research can be extrapolated to other types of shallow foundations through various techniques such as numerical simulations,” Dr Bertrand said.
The large-scale test foundation
In 2022, an ~8m x ~10m large-scale waffle raft foundation was constructed for the research on the premises of Victoria University’s Sunshine Campus in Sunshine North, Victoria, following the guidelines of the current Australian standard, AS2870.
The initial exploration of the ground profile confirmed the site is underlain by expansive soils typically found in the West of Melbourne. The ground below the foundation was fully instrumented prior to pouring the slab to measure soil moisture content, temperature and ground movement at strategically varied locations.
“Our aim was to develop a clear picture of geomechanical changes occurring below the foundation with seasonal weather changes and how these affect the foundation performance. A number of sensors were installed at different locations/depths under the foundation as well as outside of it for comparison,” Dr Wasantha added.
The constructed test foundation in 2022 was monitored for more than a year, spanning through all four seasons, yielding some significant initial findings.
According to Dr Wasantha, “The covered ground below the foundation should be largely immune to surface weather changes, and the AS2870 standard also assumes mound shapes for designs defining an empirically estimated distance from the edge where the foundation could deflect due to ground movements. However, the initial first-hand data of our study disputes those empirical estimates and suggests that soil moisture content changes can extend further towards the centre of the foundation than the empirically estimated edge distance in AS2870.”
These results have direct implications for the current standard design and construction practices of residential foundations in Australia. It indicates a possible disparity between the assumptions of the standard design practice and what is actually happening on the ground.
“Our data also suggests the overall seasonal weather conditions are more influential on the soil behaviour under and in the vicinity of the foundation than the short-term weather changes such as sporadic rainfall events,” Dr Bertrand added.
Through further analyses, VU researchers have observed that surface weather changes affect the ground deeper than what is recommended in AS2870. This depth is one of the key parameters used in designing foundations. It is climate-dependent such that different values are provided in the standard for different regions and cities in Australia. There can be a multitude of factors behind the observed discrepancy.
“The soil depth undergoing shrinking and swelling due to seasonal surface weather changes has important controls on the pressure applied on the foundation. Our data suggests that this depth can be higher than that is recommended to be used in designs in AS2870 for the Melbourne region,” Dr Wasantha added.
The way forward
The current AS2870 standard is the version revised in 2011, more than a decade ago, and many environmental conditions that affect the foundation performances can be different now, caused primarily by the impacts of climate change. Based on the initial outcomes of this research, the VU researchers believe that the time has come to review the standard and materialise necessary amendments.
The monitoring of the test foundation will continue for a few more years, and the VU researchers expect to draw further insights into the short- and long-term foundation performances. They are also currently working on developing advanced software simulations, which will further shed light on comprehensively understanding the engineering performances of foundations constructed on expansive soils.
“Currently, there is a lack of datasets to develop and validate numerical models of foundations built on expansive soils. This research will produce a robust dataset for future research initiatives in this field. The simulations we are currently working on will be validated based on the observed characteristics of the test foundation and extended further for investigations in the parameter space,” Dr Wasantha said.
The research team has also embarked on using the research outputs for artificial intelligence modelling, where the foundation’s performance can be potentially predicted for the future.
As Dr Bertrand said, “The dataset of this research is also a useful resource for developing artificial intelligence models to envisage how the foundations will react to seasonal weather changes in the future. This will set a strong base for making amendments to the existing standard.”
Overall, the VU researchers are confident that the outcomes of this project will contribute to future houses being more resilient against ground movements, reducing one of the key stress points home ownership can provide.
