Digging under slab

I have a home that was once a ranch on a slab. A second floor was then added and when that was done, a utility pit was placed under the portion of the home that extended beyond the original footprint. I now need to expand the area of this pit. I want to jackhammer through the slab and dig out a 6-6 area. Could someone tell me the best method to go about this without displacing so mush soild under the rest of the slab that I have problems with settling (I am particularly concerned about the area of the slab that supports the chimney).
Any help is appreciated. If you need more details, let me know. Basically it boils down to how can I dig and brace the soild so I do not upset the area I am not planning to remove.
Thanks, Jon
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You might want to pose this question to an engineer you've hired who can evaluate your particular situation *on site* and advise you as to what you need to do.
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J Porter wrote:

school for this and has a college degree in engineering and insurance to cover any mistakes he makes with giving advice(for which he charges for).. or you can get free advice here and it will be worth every penny you pay for it.... just keep your fingers crossed so if the house falls down that you will be at work and will not get hurt... hope this helps.
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J Porter wrote:

Cut the floor with a masonry saw, don't jackhammer it. Then drive sheet piling down around the hole you want to dig at least a foot or so farther than the bottom of the pit. (how deep a hole are you talking about, by the way? Brace the piling in place, THEN excavate.
Depending on what kind of dirt you're on, and how deep you're going, you might be able to use 2x8s for the piles.
Stay the hell away from the chimney. Re-design the plumbing runs if you have to.
--Goedjn
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porter snipped-for-privacy@hotmail.com (J Porter) wrote in message

You will need an engineer to evaluate the kind of soil and the location of loads near the pit. Too much particular information is needed to answer the question at a distance. Tom Baker
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