Replacing shallow concrete foundations - worth it?

Hi,
Our N.Z. timber house is on a slope. The side of the house facing the valley has concrete foundations that are very, very shallow (probably no more than 8-10 inches deep). My guess is that when it is raining very very hard, and for a long time, the foundations can move a little bit. Another issue is when kids are playing with the garden hose... (ouch!)
We've had to replace one foundation that was kind of going down the valley actually. Four other concrete foundations could be replaced.
The inside of the house is quite new, I'm scared that the repiling process may damage the nice work that been done inside. Is the repiling process safe? Is the house likely to move when repiling?
The house is on clay.
Any opinions?
Thanks for any advice.
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Hi John, Usually on a slope grade a foundation is roughly the depth of..... level out 5 feet from the grade and measure the distance to the ground at 5 feet. The footing should be this depth plus 18 inches..... So if I leveled out from grade 5 feet and measured down to grade and had 2 feet I would build a footing that was 3 feet 6 inches deep. This is rule of thumb and not necessarily engineered for your soil type. A person could build a retaining wall out at a distance and backfill this to support the existing faulty condition also..... Good wishes. jloomis

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Hi, Thanks for the info but that wasn't my question. Do you think the repiling process (replacing the existing concrete foundation with a new one) is safe enough for the house? I fear the structure may move a bit and damage the indoor work done so far.
In other words how safe is the foundation replacement process for the house?

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John,
The process is called underpinning and is performed regularly without damage to the structure above. It's not a complicated job for a building the size of a home, but there are several methods and you really need site specific direction from a local engineer.
Tom

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far.
house?
If you really do need piles.... There are two ways to do it - drill them or hammer them. Drilling them causes a lot less noise and vibration than hammering.
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5
to
Interesting. I think that's quite a bit shallower then we would dig here in the UK on clay. Any trees around and you can be talking three times that depth - in which case piled foundations usually work out cheaper.
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