On the right footing

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.

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

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.