EvStephen,
Geopier is simply a discrete overexcavation and replacement method that densifies and increases lateral stress in the surrounding soil.  Geopiers are installed by drilling a 30-inch dia shaft and then ramming crushed stone into the shaft in 12 inch lifts.  So for a 15 foot shaft, you would have 16 to 18 lifts of rammed aggregate (2 to 3 lifts in the "bottom bulb" and 14 to 15 in the shaft).  The ramming process takes place with a modified 3,000 to 4,000 lb hydraulic hammer that has a 27 inch beveled tamper that delivers 300 to 400 blows per minute or 1 to 2 million ft-lbs per minute.  The ramming action pushes and embeds the aggregate laterally into the sidewalls of the drilled shaft.

High quality and high hardness aggregate is required for the ramming process or it is pulverized.  Pressuremeter tests have shown that the Geopier process increases the horizontal stress to near the passive pressure limit in the upper 3 to 4 diameters of the pier.  Typical spacing is 2 to 3 dia center to center under spread footings.  In CA, uplift steel can be added to the pier to resist uplift forces from seismic events and wind.

Actual performance is the telling tale.  I've seen a case history for 6-story parking garage in Sacramento, CA, where 12 to 15 foot Geopier elements replaced 75 foot drive piles.  Total settlements from the garage are less than 1 inch since the begining of construction in 1999.  Check out www.geopiers.com or www.farrellinc.com for more info.

Oh the good and the bad ... the good is they are cost effectve and provide 70 to 120 kips per pier for the support of shallow spread footings.  Installation rates average 40 to 50 per day.  Installers also provide performance warranties with the system.

The bad is they require a drilled shaft, so high ground water clean sands can be challenging.  Although, casing has been installed successfully ... albiet slower production than dry shafts.  As well, if you have a balanced site, off-haul of drilled spoil is required.

for GeotechGEinCA !!

I would agree with GeotechGeinCa, they have been widely used and are gaining acceptance.  The good: when used in cohesive soils that will stand open, they are cost efficient and relatively easy to design, install, and are cost competitive with stone columns.  The bad:  in cohesionless soils or below the water table, they are difficult to construct (must use casing) which makes them more expensive, and do not densify the surrounding soils as well as a vibrated stone column.  Be sure to gather all of the facts on both systems.  Both can be good options.

Our company has seen successful installation of Geopier rammed aggregate piers in cohesionless (sandy) soil in the Los Angeles areas as well as the Bay Area and Sacramento Valley without casing above and below the groundwater table.  Loose sands are good candidates for Geopier improvement, and cohisive clays are the best candidates.  geocontractor makes a good point, but doesn't consider the limitations of vibrated stone columns for foundation support in CA, eg. no uplift resistance and a lower stiffness than Geopier.

GeotechGAinCA,

I agree that stone column installers have not figured out how to install uplift resistance with their installations.  I continue to read about the relative stiffness of geopiers vs. stone columns.  How is this determined? And what testing has geopier done to cinfirm this information?  I have read that geopiers are on the order of 5 to 25x more stiff, which is quite significant.  How can this be?

Aren't these a bit like Franki-piles except that as you pull out the casing, you ram the concrete in above the intial bulb - like the initial bulb?  But of course, you are using aggregate - but same principal.

Yes, Geopier is alot like Franki-piles except that with geopier casings are only used at "caving-sand" sites.  The ramming and technical principals are the same as that of franki-piles.  Regarding the stiffness ratio between Geopier and stone columns, they have been tested side by side at Iowa State University by Prof. David White.

I have heard of the Iowa State test.  Is there a copy available online?  Also, regarding the stone columns that were installed, were they installed as vertical bearing elements or slide/shear resistant elements?  Maybe its in the report...

Try the following link.  

http://www.ctre.iastate.edu/research/detail.cfm?projectID=254

CTRE's website has been having some problems lately, but the final report is available as a link of this page, I believe.  I tried downloading it yesterday to make sure, but I got an error....trying again today.  

If you have further questions, I'm sure Aaron Gaul or Ken Hoevelkamp would be more than happy to answer your questions.  They were RA's for this project while they were at ISU, and are now Geopier reps.  Their link is below:

http://www.geopiers.com/

Densification is accomplished by virtue of the inclusion of the piers themselves.  The ability of the pier to engage the matrix soil shear resistance through it's bulbous shape is key to the soil-pier interaction process which results in a densified composite soil matrix.  Design is not smoke and mirrors at all, in fact, twice a year Geopier brings 30 or so geotechnical professionals to Phoenix to teach them how to design Geopier systems.  I recommend reading the link given by BigRedGeo.  I might even have a little familiarity with that study.  

Just as with any foundation system, Geopiers must be used with proper engineering judgement. We have seen instances where geopiers are being sold as a cure-all for all subsurface problems. On one project in Mississippi with very soft normally consolidated saturated clays to a depth of about 30',  15' deep geopiers were sold as a foundation solution for a multistory building and there were a lot of problems with installation and performance. In my opinion geopiers work best when the soil conditions generally consist of about medium consistency soils with some natural overconsolidation and geopiers can help enhance the composite modulus of the soil/geopier rammed rock matrix to allow use of higher bearing values and can also help reduce settlements where loads can theoratically dissipate within the improved (stressed) zone.  

Can any one send me " Methodology for Rammed Stone Column " as I need it for my project.

  Thanks,
  Capricon
 cgs3@redifmail.com