Distance Between Helical Piers

Hi.
Our home is built on expansive clay. We have significant cracks in the foundation walls and floors. We have dry wall cracks and our windows and doors stick. Other houses in our neighbourhood have the same problems. We have had a soil boring test and a geotechnical engineered solution to fix the problem. The enigeer and subsequent 2nd opinions have confirmed our house is sinking due to the expansive clay. The recommendation is to retrofit the foundation with helical piers. We have a rectangular, poured concrete foundation approximately 30 x 40 in size. We have a full basement with 8" walls and 24" wide footers. The house is a 2 story of approximately 1650 sq ft. There is also a 2 car attached garage. It has a 4' foundation underneath - the basement does not extend underneath the garage.
The soil test shows that our soil is hard clay from the surface to about 8'. Then is it softer clay down to about 15'. From that point on, we have hard clay and then bedrock / glacial till.
The engineered solution calls for piers every 5 feet. There were a total of 32 piers recommended. We have installation estimates of about $2500 / pier. This adds up to $80K. I've read in a news group that a pier support weight 5 linear feet from the center of the pier.
If this is true, then wouldn't having piers 10 feet apart be enough? This would cut the cost approximately in half.
We want to do the repair right, but we also don't have a lot of money and don't want to spend more than is necessary.
Thanks in advance for your opinions, Rob
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Boils down to who you are going to trust-the engineer(s) who did all the soil testing and designed the entire system based on your structure, or some guys on a newsgroup?
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How much weight? Just because the span can be up to five feet doesn't mean that it can support all possible loads over that span. As well, the limit might not be the pier but the capacity of your house foundation. Even if the piers can hold the load at ten foot spacings, your house might require closer spacing unless you want to heavily reinforce the house.
Mike
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You might read this for information: http://www.concretenetwork.com/concrete/foundation_repair /
I think that the price quoted would suggest at least one more quote if possible.
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