Reactive soil: a slab of confusion
The type of earth you lay foundations on can have repercussions for your client’s homes for decades after it’s been built, and for the course of its whole life. Building Connection investigates the issue of reactive soils and what you need to know.
The term ‘reactive soil’ may conjure up images of radioactivity beneath the earth’s surface; however it shouldn’t. Reactive doesn’t indicate that the soil is contaminated in any way rather; it refers to the way it reacts to changing moisture content.
All clay-based soils have the potential to change volume and shift with changes in the amount of moisture in the soil – and are sometimes called ‘reactive soils’ because of this. The amount that soil is likely to shift defines how ‘reactive’ it is considered to be.
Some soils have a greater potential to change volume than others, and this amount of potential needs to be measured with a soil test to make sure footings are designed in a way that helps protect it from soil movement that might otherwise cause it to sink or buckle.
The issue of reactive soils is a national one though it really came to the fore in Victoria during the drought that ran from 1996-2009 with new homes that were experiencing settlement. Rain wasn’t falling, grounds were excessively dry and as a result, soils contracted and walls started to crack. And then the rains came, bringing with it, myriad other issues.
Swinburne University has been conducting research in the area of reactive soils to try and better predict the performance of houses and how we as a nation can improve house designs that better resist moving.
A consortium was formed including the Victorian Building Association (VBA), Office of Housing, the Association of Consulting Structural Engineers Victoria (ACSEV), the Foundation and Footings Society Victoria (FFSV), Housing Engineering Design & Research Association (HEDRA) and Swinburne University.
“Approximately 10 years ago, we were doing some work for the Office of Housing (Department of Foreign Services) who own and operate our public housing,” chair of department of civil and construction engineering at Swinburne University Emad Gad says.
“They maintain about 50,000 properties. They noticed that during that time they had high maintenance requirements for some of their houses that were experiencing settlement. That was during the drought. From mid 2000-2010 a lot of existing houses were beginning to show signs of distress in the form of cracks. It wasn’t across all of Victoria, but in a pocket where we have reactive soils. It was discovered that the edges of slabs and footings were dropping. This could have been the result of minimal rainfall or from trees absorbing moisture from the ground.
The other thing to remember is the fact that due to a growing population, we are building in areas we wouldn’t have 30 years ago, often in highly reactive areas. “We started the work in 2012. Around that time the drought had broken. Soon after that time slabs were beginning to heave – grounds that were dry had been filled with moisture and started to swell. This brought with it the opposite effect and now slabs were curling up.”
As a general rule, when the slab moves, the walls move and that’s when cracks begin to appear, doors start to jam, floors tilt and structural integrity diminishes. Then there’s the issue of home or building owners becoming disgruntled. “Our work changed focus from why shrinking occurred, to heaving,” Emad continues. “Most homes affected were those built toward the end of the drought around 2006-07. We began to look at environmental factors as well as soil classifications.”
To make sure you select the right type of subfloor, you need to ensure you read and understand test reports generated by the assigned geotechnical engineer on your project.
The engineer will take soil samples from the site, subject them to varying tests and then specify the correct site classification according to AS 2870/2011 – Residential slabs and footings. This rating indicates how reactive the soil is, and will help your structural engineer to determine what sort of footing system is suitable for your site in what are known as ‘normal wet’ and ‘normal dry’ conditions. Below is a list of these ratings indicating what each of them represents:
How soil classifications affect subfloors
For all ratings, it’s simply a matter of building a subfloor that’s suitable for the soil type, and for the type, shape and size of house you want to build – and an engineer will be able to figure out what’s most appropriate for your design and the site you’re building on.
Most Class A and Class S sites will only need a basic reinforced slab with concrete beam footings underneath the edges of the slab, while sites with other classifications will require the slab to be stiffened. For slabs, this will require a series of cross beams in the slab and sometimes piers too. This extra ‘stiffening’ helps to prevent the slab from bending and twisting as a result of moisture driven movements from underlying clays.
For ‘highly’ and ‘extremely’ reactive sites and problem sites (i.e. Class H1, H2, E or P), a structural engineer will need to do a special assessment and advise on how to safely build a house without the risk of damage due to soil movement. In many cases, this will involve installing concrete piers or screw piers into the ground underneath the footing beams on the slab for added stability and support, to a depth sometimes below the depth where soil moisture variations are minimal. Moisture variations tend to be lower at greater depth in clay soils.
Waffle slabs, also known as a waffle pod slabs, have caused much debate lately and many have pointed the blame toward them for the many Melbourne homes affected by reactive soils in recent times. These slabs are constructed entirely above the ground by pouring concrete over a grid of polystyrene blocks known as ‘void forms’.
Waffle slabs sit close to the surface hence they become more sensible to change in moisture. For this reason they are generally suitable for sites with less reactive soil, use about 30% less concrete and 20% less steel than a stiffened raft slab, and are generally cheaper and easier to install than other types.
These types of slab are only suitable for very fl at ground. On sloping ground or with more complex designs, a stiffened raft slab is normally the better option. Waffle raft slabs are suited to lightly and moderately reactive clay sites in particular, because they’re not embedded in the ground like stiffened raft slabs.
Trees and site maintenance
Trees and their roots are often the cause of problems for house footings too. Regardless of the soil classification where you’re building, you should take into account how the trees on or around a property might affect both the home being built and those nearby.
Ideally, you should consult an arborist, a geotech engineer or both to figure out what’s likely to be okay and what could cause problems – and what the right approach is likely to be. In some cases a deep plastic root barrier embedded in the ground can help to prevent roots from growing around a home, and protect the foundations.
You’ll also need to consider how the potential root intrusion area may cause moisture variations (and therefore soil movement) past your own residential fence line – because the owner of a tree can become responsible for problems it causes to neighbouring properties. Likewise – regardless of the site classification, making sure there’s adequate and appropriate site drainage (i.e. to stormwater drains) is a big part of protecting a home from reactive soil movement.
Working with the plumber and ensuring their design adheres to Australian Standards needs to be front of mind. For allotments containing reactive soils classified as H, E, H-D or E-D it is a mandatory requirement for compliance with AS/NZS2870, Section 5, Clause 5.5.4 (a), that pipes through external footings shall be lagged and (b), connection of stormwater drains and waste drains shall include flexible connections and in 6.6 (f), joints in plumbing pipes within 3m of the house under construction shall be articulated to accommodate ground movement without leakage.
The wording in this section of the Standard is rather vague; particularly when it comes to the word ‘shall’ and as a result, discussions and works are currently underway to create a more extensive set of instructions for plumbers to follow when faced with reactive soils.
Responsibilities of the builder
Builders need to comply with the Building Code of Australia, relevant Australian Standards, approved designs, specifications and contract documents. The Australian Standard for building footing construction permits minor wall and floor movements. If the foundation conditions are changed after construction, the floor and walls may move more than these standards allow for.
The designs for building footings in AS 2870 – Residential slabs and footings will perform adequately, provided the building site and surrounds have ‘normal’ foundation conditions that are maintained. If the building site and surrounds have ‘abnormal’ moisture conditions, special provisions must be followed by the design engineer, builder and owners (refer to AS 2870 for a description and examples of ‘abnormal’ moisture conditions).
Builders need to address the following important aspects (as advised by the VBA):
- Make sure there are well-drained foundation conditions which will create ‘normal’ soil moisture and maintain adequate bearing capacity of the footings as soon as work begins at the site.
- Where abnormal moisture conditions exist or are anticipated, the footings will need to be designed by a structural engineer to suit these conditions.
- Ensure that the floor level allows for proper drainage around the outside of the house, and that the property is protected from any adjoining water fl
- Slope the soil and paths away from the building by the minimum amount required by the Building Code of Australia to prevent water fl owing towards the house’s foundations.
- Special considerations may be needed if any excavations are to be dug near adjoining structures (i.e. when installing a swimming pool).
- Construct subsoil drains or moisture barriers on sloping sites to your engineer’s requirements, in order to prevent stormwater affecting the building’s foundations.
- In sites with highly or extremely reactive soil, AS 2870
- Residential slabs and footings requires mechanical flexible couplings for sub-surface drainage pipes and for aboveground connections from the downpipe to the stormwater and sewerage drains. These allow for the movement of the soil and minimise the risk of pipe joints breaking and leaking.
- Ensure owners understand that failure to maintain adequate drainage may result in damage to the structure.
Builders are also encouraged to provide owners with a plan showing the location of all ‘as installed’ sewer pipes, stormwater drains, water and gas lines. This will help with any future modifications or repairs.
As you can see, the issue of reactive soils is not straight forward and we would require many more pages to fully cover the topic. While Mother Nature is highly unpredictable, best building practices are not, so the next time you are faced with a highly reactive site, be sure to consult the Standards and Codes and work closely with the geotechnical engineers and plumbers to ensure you build the best structure possible.
Further research by the likes of Swinburne University into the effects of changing weather patterns and how future building designs can better cope with such patterns will hopefully lead to less impact on homes in the years to come.