Many factors cause a house to move and cracks to occur. Although a solid structure, houses are set atop a footing or foundation ‘slab’ that can bend as the soil below it moves. This undermines the building’s structural integrity, causing cracks and safety issues.
So, why do soils move? Reactive soil is prone to present significant variation in volume, expanding and contracting based on fluctuations in moisture content. Also referred to as plastic foundation soils, mostly containing a clay-type, composite of microscopic plates. As clay absorbs water, it pushes the plates apart leading to swelling (and vice versa as it dries out).
Imagine reactive soils as a sponge brick. This swelling occurs during rainfall or garden watering (sinking the house) and the shrinkage happens in long hot spells or drought (pushing the structure upwards). Given Adelaide’s Mediterranean climate with hot summers and mild wet winters, as well as a large amount of highly reactive soils (classified as problematic), homeowners and builders need to be wary of this.
Figure 1: Soil movement – Swelling
Figure 2: Soil movement – Shrinkage
How reactive soils affect footing design
Soil types and movements (and the possible remedies) should be assessed before any construction work takes place. Soil tests and extensive engineering solutions are often required and must comply with Australian Standards (AS 2870 – 2011). Contract an Adelaide renovation management services team to guarantee the integrity of your building.
To obtain key details such as the soil layer profile, types and bearing capacity, an inspection with borehole logging and lab tests is required. The bore log determines the movement estimation of the soil. Calcareous materials testing, geo hazard evaluation and slope stability analyses are also necessary if the site is expected to be problematic. Only then can footing design begin, once all the required necessary information is collected.
When we talk about footing, we’re referring to the concrete steel base to support a building. All the loads are transferred to the ground through its footing. It’s designed to keep a building stable. This is a job best designed by structural engineers.
When we map out footing, we also take into active soil reactivity. For most sites with Class A and S, a basic reinforced slab with concrete beam footings is adequate. But for reactive soil (Class H1, H2, E and P), strip or raft footing are often utilised.
Strip, pad, raft and pile foundations
Wide strip foundations may be required where the soil is soft or has a low-bearing capacity, to spread the weight over a larger area. Where ground conditions are poor, settlement is likely or, if its impractical to create strip or pad foundations, the raft solution is leveraged – a thick concrete slab reinforced with steel, covering the contact area, much like a sturdy floor.
Raft foundations can be fast and inexpensive to construct, as they tend not to require deep excavations compared to strip or pad solutions. This footing uses less material as it combines the foundation with the ground slab. But it tends to be less effective where structural loads are focussed on a few concentrated areas, and they can be prone to erosion at their edges.
The concrete raft tends to include steel reinforcement to prevent cracking and may incorporate stiffening beams or thickened areas to provide additional support for specific loads. For example, below internal walls or columns (which may require punching shear reinforcement).
Beams can stand above or below the raft or may be ‘hidden’, strengthened by reinforced areas within the depth of the raft itself. These thickened areas are particularly useful where there are poor ground conditions, as the required thickness of the raft itself may be uneconomic.
If, however, extreme circumstances are encountered, pile footings are recommended to provide a stronger and more adequately stable support. When considering tree effect, loose fill, collapsing ground or soil with extremely low-bearing capacity, the normal shallow footings won’t suffice. Pile footings should be adopted to overcome extreme situations in reactive soils.
Soil assessments & construction services in Adelaide
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Our structural engineers are happy to offer advice, support and documentation to improve construction productivity. Start the process by booking a free 20-minute consultation with Naveesh, a leading structural engineer consultant in Adelaide.