As NHBRC will tell you, foundations can make or break your house build.
Your biggest building risk when taking on a new construction tends to be what happens under ground. If your soil conditions are poor (for example dolomitic, clay, sandy, underground water) you could be in for major delays and cost.
Your appointed structural engineer will detail your foundation spec based on your site soil conditions. Many will now insist on getting two or three spot soil tests with a geotechnical engineer to make sure there are no hidden surprises when excavations start.)
The structural engineer then needs to provide foundation drawings, rebar schedule and concrete spec before any work can begin.
Your second, but smaller risk, is damaging existing underground pipes and cables when excavating. Hard to avoid knowing exactly where they are, so have numbers on standby to fix municiple water pipes, fibre connections or electrical supply.
Once the site is leveled and cleared, most foundation trenches in South Africa are dug by labourers. The key here is the plotting. Firstly, this should only be done by someone with experience and secondly, check and check again. If your house ends up being built in the wrong position by 300mm, you could be taken to court by neighbours or council.
Deep trenches for underground retaining walls on sloped sites can be very dangerous to dig in sandy conditions and you should take great care to dig it in stages with supports – both for the sake of the labourers and neighbouring houses.
Footings and plinth walls
Footing or foundation trenches for houses are between 500 – 1000mm wide (generally three times the width of the walls) and anything between 500mm -1500mm below the ground level (all soil dependent).
Once the rebar cages are in and concrete is poured, you can literally start building the plinth walls the next day, while the concrete cures. You do have to be aware that the NHBRC and structural engineer will need to inspect the site before any concrete can be cast (the same goes for slabs).
The foundation walls or plinths needs to clear the ground level by at least two/three brick courses to avoid damp in the future house (two brick courses will hold the concrete slab, which then needs to clear the top of the soil by at least 75mm).
Once bricked, the soil is placed back inside the trenches around the plinths and into the building platform where it needs to be compacted and prepared for the slab.
If you are using suspended, pre-cast floor slabs for the ground floor, you will not compact the soil in the building platform, but leave a substantial air gap.
Note: Any advise given here is very informal and superseded by professional, site specific specifications.