The function of the footing is not only to distribute vertical loads from the building to the foundation, but to ensure that the building superstructure performs satisfactorily, particularly when it is on a foundation subject to movement due to moisture changes (reactive clay sites), settlement, etc. The most common movements arise as a result of moisture changes in reactive clay soils. As clay soils become wet, they swell or increase in volume, and as they dry out, they shrink. This is why cracks appear in clay soils during periods of prolonged dry weather. The more reactive the clay or the deeper the soil is affected, the larger will be the surface movement.

Constructing a concrete slab on the ground is like covering the ground with an impermeable membrane. Any moisture rising beneath the slab can no longer evaporate from the surface. The soil in the centre of the slab will therefore remain damp and the moisture content may even increase and, if reactive, will swell. However, around the edges of the slab, moisture will be able to evaporate during dry periods and permeate back under the slab during wet periods. Thus the edges of the slab will be subject to periods of heave and periods of loss of support when the slab has to cantilever out from the centre mound. It is important to determine how reactive a site is, and to what depth the soil will be affected. The amount of movement to be designed for can then be determined, and thus a footing provided that will adequately support the building superstructure and limit the movement that the superstructure has to accommodate.

It is important to understand that AS 2870 is not intended to provide damage- or crack-free performance of superstructures. Footing systems designed, detailed and constructed in accordance with AS 2870 are not intended to prevent cracking of the walls constructed on them due to possible foundation movement, but merely to limit cracks to a generally acceptable width and number, and at an acceptable cost.

Contact Melbourne Civil & Structural Engineers for your slab design requirements.

We approached MCS Engineers to investigate the cracked walls and brickwork at our home. A couple of builders and underpinners had already inspected the damages and provided me with their quotations to repair the damages and underpin the building. A Geotechnical Engineer had also recommended underpinning. Adam from MCS Engineers attended the property and carried out an extremely thorough investigation of the damages which no one else did, he also examined the building permits and engineering reports and found a series of issues and underlying causes of the damage. He got to the bottom of the issue and found the water leaks that were causing the damages. Had we not engaged MCS to investigate, we could have ended up spending thousands of dollars for the repairs and could have ended up with the same issue again as the underlying causes were not identified. Adam was very professional and he also took the time to explain everything in detail in a language that someone with no engineering/ building background could understand and recommended the most appropriate remedial action required. The fee for the investigation was money well spent as it saved us thousands in repairs that could have resulted in the same issue a few months down the track. It goes to show the importance of professionals and the expertise they carry and will definitely use MCS in the future and highly recommend them.

Slab Design

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