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We have used them before on a project, but they were used in lieu of a block foundation with crawlspace. So it was trusses. So the foundation walls turned into columns w/ beams that where supported by the geopiers. The piers where under the foundation for each column.  And I believe this is what you are envisioning would need to happen. 

But the geopiers also work with a slab on grade. They just reach down into the good stable soil to help stabilize everything and avoid doing a lot of digging. Sometimes they have to go very deep. We use them a lot also in areas with very sandy soils where we need to bypass the sand to the bedrock but don't want to dig it all out.  The Geopiers.com website it has a lot of images and explanations as to how they work. They could also be used directly under a foundation under a foundation. 

Hello Andy

In addition to being a licensed Architect, I am also a licensed Land Surveyor. One of the services we offer is construction layout. We were involved with geopiers on a few projects all on the layout side - not the design side. The layout, installation and asbuilting was a little more involved than say structural piles. I cannot speak to how this system worked out with the footings and slabs, but I may be able to get some answers from the Structural Engineers involved. 

Best 

DJ McCutcheon, Jr. LS AIA

Thanks DJ.    My primary question/concern is can a building designed for typical spread footings and slab on grade be supported on geopiers with no change to the structural design?

Given my understanding of how geopiers work, I would think that wherever they were implemented would change the bearing condition to points loads thus requiring either a footing design that can span from pier to pier location - or a structural slab design if they were utilized under the slab.  

Andy

The following is a response from our Structural Engineer:

Without a clear understanding of the overall building structure I cannot answer the question.

For gravity loads on floor slab a load transfer mat / plate with crushed stone is required to arch over between the geopiers. No structural slabs are provided. At columns you need to have a footing / cap that will straddle the geopiers.

Lateral loads from bracing etc are difficult to dissipate. You have to develop resistance with passive pressure or friction between the concrete elements and the soil. The geopiers have no lateral capacity.

Similarly, for the uplift you have to add an uplift anchor rod that is bonded and grouted to handle the tension loads.

Hope this information helps. 

I am working on a 4 story condo project in Chattanooga, TN that is utilizing rammed aggregate piers.  We have column footings that were getting pretty sizable and were designed to 2000 psf soil bearing capacity.  The size the of column footings were able to be reduced by 40-50% by using piers as long as the soil bearing capacity could meet 4000 psf.  It did require our structural engineer to redesign the footings and coordinate with information from the pier designer.  We did not have an application that applied to continuous spread footings since the columns were carrying the majority of the building loads.    

Dawn Snyder, AIA