Footing Design

[ptyltd]

The engineer has specified 75x75x3.5 SHS embedded 600 into 450 dia. concrete pier 1200 deep. Concrete needs to be mechanically vibrated and F'C 20MPA. OK - So far so good, all makes sense . Then he says...."piers to be founded in natural material with a safe bearing capacity of 150KPA"......What The? :confused:

What exactly does this mean?

Am I supposed to make a deeper hole and fill it with some sort of fill up to 1200 and then poor the concrete so it sits on this or am I supposed to just poor the concrete into a 1200 hole (so the natural material will be whatever is at 1200 deep)?

I have checked AS2870 refered to in the geotest report but I am still :confused: .

Any hep clarifying appreciated.

[elphingirl]

Hi

The "founded in natural material" simply means that the ENTIRE 1200 depth of the concrete needs to be surrounded by original, untouched soil. This means not fill of any kind, no matter how long it has been sitting there. You should be able to determine this before excavating by augering a core sample (which should be dense and show gradual variations in colour and texture). Or after digging out your hole, you should be able to see the same in the walls of the hole.

The bearing capacity refers to the ability of the soil to not compress or shift when a load is applied to it. Presumably at some point in the house's history a soil sample was taken for engineering purposes. This should give you as a minimum a class for your soil, from which you (or your engineer) should be able to tell you the bearing capacity pretty easily.

This is all irrelevant if you have rock 'floaters' in your soil, and you'll defnitely need to go back to your engineer.

Good luck,
Justine

[bigdazza]

You need to bore the pier holes to a depth of at least 1200mm or until 150 kPa material is reached.

Nearly all naturally occuring (not fill) sand or clays would be capable of obtaining this capacity.

The engineer has basically put that note on the plans to cover his &&&& in the event that some unforeseen foundation condition occurs on the site ie there is a localised soft spot in the vicinity of the pier.

When the engineer inspects the pier holes let him make the call. Worst case is you will have to make it a bit deeper.

[ptyltd]

Thanks heaps for the prompt replies folks. Clears it up (except maybe the rock floaters :confused: ). I will get a level and then have them all bored 1200 from level and let the engineer or certifier make the call on each before the pour.

Cheers.

[Exador]

The engineer has specified 75x75x3.5 SHS embedded 600 into 450 dia. concrete pier 1200 deep. Concrete needs to be mechanically vibrated and F'C 20MPA. OK - So far so good, all makes sense . Then he says...."piers to be founded in natural material with a safe bearing capacity of 150KPA"......What The? :confused:

This is standard stuff. All it means is that the bottom of the hole must be below the fill you have on the site and that it should be tested _ probably with a DCP (dynamic cone penetrometer) and possibly also with a couple of soil property tests (PI, grading, CBR) f it looks dodgy. The reason is that the piers should be founded (based) on good soil that will bear the load ok without subsiding. Was the fill placed under an engineer's supervision and compacted in layers with testing as it was placed? If it wasn't, no engineer will allow a building to be founded on that fill and he may quite possibly increase things like slab beams and thickenings as well.