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Stiffen raft slab

Stiffen raft slab

Stiffen raft slab

I have to design footing for a residential 2 storey building with timber floor and roof. The external wall is 150 thick RC wall. The non load bearing internal wall is timber stud wall.
The ground floor is 1m above the ground.
The geotechnical report show that the uncontrolled fill (sandy clay) is approx. 3-3.5m from ground level with bearing capacity of 100KPa.

Based on the results of the laboratory test results and the empirical methods described in Section 2.3 of AS2870,
the calculated surface movement (ys) at the borehole location in response to normal seasonal soil suction
variation is between 0 mm and 20 mm. Additional long-term settlement in the fill is expected to be 80 mm under
a typical 10kPa house load, consistent with a class E classification. Based on the anticipated seasonal movements
and fill settlement high level footings and a stiffened slab could be adopted for a conventional house.

In the attachment, I sketched 2 option of the footing.
- Option 1: stiffend slab (preferred by geotehcnical engineer).
Stiffened slab with external deep edge beams.
My question are: with another fill (controlled fill) of 1m above ground + existing fill (uncontrolled) of 3m. Will they add more settlement of the footing ? The bored piers under the footing beams have to pass through the uncontrolled fill or not necessary ?. I think the soil layer under the fill is soft soil with the ame bearing capacity of 100KPa and similar to fill. Any comments about this option ?
- Option 2: Strip footng under the external wall and pad footing under the steel column.
The strip and pad footing can be connected together through additional strip footings. Bored piers are provided under the strip and pad footings.
My questions are: Will the footing experience a significant the long term settlement ? Footing will get a lot more settlement compared to option 1 ? Any comments about this option ?

RE: Stiffen raft slab

Your uncontrolled fill is expected to settle 80mm under a nominal 10kPa loading.

If you are now raising the ground with 1m of engineered fill, then yes, there will be serious additional settlement.

1m engineered fill = 20kPa = approximately 150mm additional settlement before you even get to the structure. Which is why it would be preferable to undercut the uncontrolled fill to -1m and replace it with an engineered fill.


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