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On the right footing

With building quality gaining media scrutiny, properly understanding site foundations, soil classification and drainage around sites cannot be understated. HOUSING takes a look at these issues, and what builders and contractors can do to avoid costly rectification issues down the track.

Author

Simon Croft

HIA Executive Director – Building Policy

Any builder who has been involved in rectification or repair work due to footing movements or settlement issues will tell you that trying to remediate issues of existing footing and slab construction is not a simple task. To find the root cause of the problem, along with subsequent rectification work, will generally involve costly and complex solutions.

Understanding site foundations and conditions, the bearing strength of soil and the past history of the performance of footing types for the area – as well as what was on the site prior to the construction of new home or extension – is essential before building work commences in order to avoid this. 

There has been increased political and media attention on building quality and compliance. Therefore, having a compliant design, understanding the detail prior to construction, knowing what to look out for, and what the requirements of the National Construction Code (NCC) and Australian Standards are for slabs and footings, has never been more important.

So, what can a builder or contractor do to give themselves confidence that the footing and slab they are using for a site will be suitable and perform as expected? Plus, how do you determine whether one footing or slab design is better or more suitable than another? 

Footings 101     

Without stating the obvious, a compliant and correctly designed and constructed footing or slab for any building type is one of most fundamental and critical parts of a building. Everything else constructed afterwards is largely reliant and inherently linked to the performance and capacity of the sub-structure of the building.  

Regardless of which construction system is used the purpose of footings is to support the foundation and prevent settling. Correct design, selection and construction of footings are especially important in areas with troublesome and reactive soils.

 

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Knowing what to look out for, and what the requirements of the National Construction Code (NCC) and Australian Standards are for slabs and footings, has never been more important

Footing and sub-structure design

In most states and territories, a builder or designer can design the base construction system of a low-rise building themselves once they know the soil classification for the site. They can use the NCC provisions or either the detailed design tables in AS2870 (residential slabs and footings) or AS1684 (timber framing – when using a sub-floor system).

However, the design and construction of footings is best left to a specialist designer, generally a structural engineer, who will assess the soil conditions, nominate the appropriate soil classification for the site, as well as embedment depth, bearing capacity for footings and/or edge and internal beams. 

A specialist can also assess the site conditions and decide on the most appropriate footing or slab type for the building design, as well prescribing the appropriate depth and width for the footings or slab beams. They can also determine the proper selection and type of steel reinforcement.

The design engineer will generally provide design details and recommendations on the cladding types and detailing based on expected movement. Included in their assessment will be details such as placement of articulation or expansion joints where using masonry elements to combat expected movement and the design may also include site drainage requirements to prevent ponding or directing water away from the building.

However, there are some restrictions on different base construction types, particularly slab types, set out in AS2870, which may limit, for example, an infill slab to only be used for certain sites due to limited expected movement of the site. Other restrictions may include only a minimal range of solutions for E and P class sites depending on the nature of the soil characteristics and expected movement of the site. 

Generally, most of the different base construction systems can be used for various site classes but the depth and width of beams or pads will get more substantive – as will the reinforcement types to be used, particularly for class M–P sites consisting of reactive clay soils. 

The dimensions of footings generally depends on the size and type of structure, and framing and cladding system to be used. Other factors influencing the design will include: the weight of the structure; the site classification and any unique site conditions or known site movement issues for that area; or characteristics unique to that area, such as salinity (salt level) of the soil which can influence reinforcement type or concrete selection. 

This last point in particular reinforces the need to not just assume all designs and sites are equal and why using someone with local experience can have a number of benefits. 

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We don’t often see outright failure but it’s not uncommon to see excessive settlement when soil bearing capacity is low

Settlement and movement  

Most of the time we take footings for granted, and sometimes we can: for typical soils, a common design applied project to project will perform as expected and is unlikely to have any long-term issues.

On the other hand, if you build on soft clay soil or if there’s a soft zone under part of your foundation, there can be trouble. A footing that performs well in good soil may not do so well in weak bearing conditions. We don’t often see outright failure, but it’s not uncommon to see excessive settlement when soil bearing capacity is low.

Further to this, correct selection, placement, installation and support for steel reinforcement may seem like a given but can be crucial to the footing/substructure’s long-term performance in resisting movement and any settlement issues. 

If the whole house settles slowly and evenly, some additional settlement will generally not cause any significant issues, but if settlement is uneven (differential settlement), there could be damage. 

It’s the unusual situations and sites that cause the most trouble. If you think there’s a problem ahead, you should stop and call the designer. 

In these tough cases, having a thorough understanding of footing and slab design and construction requirements and what to look out for becomes important. In very strong and stable soils, minor construction issues within acceptable tolerances are unlikely to cause major concern. In weak or marginal soils, however, it’s best to be cautious and pay extra attention since getting something wrong, even simple things such as incorrect concrete cover to reinforcement, or not properly tying laps in trench mesh or slab mesh prior to the pour, can be very costly and difficult to rectify afterwards. 

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