Effect of Stone Columns on Preloaded Silt
Effect of Stone Columns on Preloaded Silt
(OP)
Does anybody have any experience or could point me to a few papers that discuss the effects of stone columns installed into deep (20 m+ thickness) silt with a dense fill layer.
Should the stone columns be installed before the preload in in place or after the preload is removed?
Thanks.
Should the stone columns be installed before the preload in in place or after the preload is removed?
Thanks.





RE: Effect of Stone Columns on Preloaded Silt
RE: Effect of Stone Columns on Preloaded Silt
You're using the stone columns just for vertical support (minimizing settlement), right? (Not as reinforcement for lateral loads or for densification.)
We've found out the hard way that stone columns don't densify silt surrounding the columns the way they can do very nicely for sands with smaller amounts of silt, and the difference in silt content where they work and where they don't can be pretty small. For lateral reinforcement in silts (to prevent horizontal displacement), there is controversy about whether they work well. (The real answer is "It all depends.")
Big Harvey -
What would you use for the rigid inclusions? Pipe piles?
DRG
RE: Effect of Stone Columns on Preloaded Silt
Regarding stone columns : there is a big difference between vibroflottation which is mass compaction of granular soils ( ie improvement of strength betwwen treatment points ) and stone columns adapted for fine grained soils where no improvement happens between treatment points but construction of a soft inclusion.
Usually up to 5 t/m2 distributed loads down to 12 m max , stone columns are generally more economical. Otherwise it's often better to revert to rigid inclusion which basically are piles allowed to settle generally less than 25 mm. There is no norm regarding the design of rigid inclusions, therefore the designer must be experienced in this field ( generally finite elements models )
RE: Effect of Stone Columns on Preloaded Silt
The contractor will only install to a 20 m depth from the working platform, however bedrock is deeper than that in some areas. This is anticipated and taken into account in the design.
dgillette,
Your assumption is correct. The stone columns in the silts are just to reduce settlements.
I did anticipate that it would be hard to get through the crust, however our contractors are reliable.
The main concern is which will give us the least amount of settlement; preloading before or after stone columns? Is there even a difference? Can you cite a reference?
Thanks.
RE: Effect of Stone Columns on Preloaded Silt
RE: Effect of Stone Columns on Preloaded Silt
If the layer is thick compared to the column diameter, the columns become slender elements in bending, and have to be analyzed as such. The material has no tensile strength, so the bending stress on the tension face of the "beam" can't exceed the effective overburden stress. It would be an exaggeration to say that they don't work, but their benefit decreases with slenderness, and the analysis gets a lot trickier.
The bending issue applies to jet-grout and soil-mix columns as well. Guney Olgun, Jim Mitchell, and others at Va. Tech had written a paper about a jet-grouted site in Turkey, concluding that the JG columns had stiffened the foundation and prevented the problems that occurred at an adjacent area in the Kocaeli EQ. Subsequently, Guney and Jim Martin(?) reanalyzed it with FEM and concluded that they did not actually help so much. He presented that at a conference I went to sometime in the last few years, maybe the SFO EQ 100th anniversary conference in 2006. I've got a paper somewhere, but I think it's in a pile of stuff that I lent out. If you're interested, I'll see if I can track it down.
DRG
RE: Effect of Stone Columns on Preloaded Silt
RE: Effect of Stone Columns on Preloaded Silt
Are your comments specifically talking about stone columns in silt? It appears to me that moe333's question and comments are speaking more generally.
ht
RE: Effect of Stone Columns on Preloaded Silt
I would carry out the stone columns first and then preload while monitoring the settlements to check the performance of my design.
An average silt layer woild have a pressuremeter modulus of 4 MPa ( Young's modulus 8 MPa ). If you have a 20 m layer and a load of 5 T/m2 ( 2.5 m of fill ), this would lead to about 12 cm settlement without stone columns and stone columns would reduce this value to 3 or 4 cm. Is this, and the fifferential settlement going with it, acceptable for your structure, that's the question !
RE: Effect of Stone Columns on Preloaded Silt
moe333 - If the material is clean sand or sand with a small amount of silt, installing the columns with vibration (as is typical) can densify it quite nicely. We've used that at a number of sites (successfully, if the SPT, CPT, and BPT can be believed). The problem has occurred where there are layers of silt that don't get densified so well. {{Am I talking about the right issue?}}
We don't get enough big earthquakes to test out whether all of our designs and analyses really work.