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Stone columns....

Stone columns....

Stone columns....

Stone columns are being used for the stabilization of a natural slope for the construction of a bridge embankment. The stone column has 2 primary functions 1) increase shear strength along the slip surface and 2) provide pressure relief due to the location of an artesian aquifer below the construction site. The columns are being installed through a heavily over-consolidated predominantly silt or silt clay (CL-ML) with a potential shear zone of SILT and CLAY (CI-CH). The base of the stone column will be completed in a highly over-consolidated low permeability sandy silt till separating the artesian aquifer from the base of the column by 5-7 m. The stone gradation is very coarse and was selected primarily for its shear strength characteristics. My concern is with piping of fines into the voids of the stone column. Fines are >85% in the base soil, and there will be a high pressure gradient near the base of the column due to the artesian aquifer. My thinking is that actual seepage velocities should be low given the low permeability of the in situ soil, so I don't know if piping will be an issue. If piping is an issue could I be concerned with a piping failure? Would a geosynthetic provide adequate filtration if installed in the column prior to the stone? I would like to maintain the stone gradation for strength. Any good references on geotextile selection (proper AOS, permeability, filtration, etc.). Thanks in advance for any suggestions.

RE: Stone columns....

What do you mean by "piping failure" here?  I normally associate that term with dams and levees.  Do you envision a blowout of the artesian pressure from below through the 5-7 m of clayey material through the columns to the surface?  That seems rather unlikely unless the artesian head is huge.  

Experience has shown that if you install the columns with vibration to densify them, the surrounding material will intrude into the voids in the gravel.  This would reduce the permeability somewhat, but it sounds like you are trying to drain stuff with very low permeability so it probably doesn't matter much.

I don't know how to install geotextile in a stone column.  Most of the stone columns I have seen were installed from the bottom up (after jetting and vibrating the tool to the bottom), with vibration and compaction applied as the tool is removed and the gravel is fed through a little door.  I suppose you could but in a "sock," provided a drilled hole would stay open, then go back and fill it.  However, I would expect the sock to get shredded when you start compacting the gravel inside it.

One caution on use of stone columns for lateral reinforcement: Unless the weak layer is thin, less than 1/2 column diameter perhaps, the simple replacement-ratio approach (treating shearing through the column like it is a direct-shear test) does not work reliably.  The column is slender, and it can bend (imagine an extra-tall simple shear test), so you may not ever be able to mobilize the shear strength that you would get if it was like a direct-shear test.  I have much more faith in stone columns for vertical loads, or for densifying granular soils around them.

Am I understanding the questions properly?

RE: Stone columns....

Is your goal to use the stone columns for some shear strength increase or to drain water pressures that are weakening the slope?

If you are looking for drainage, I'd use a well graded sand or a sand and gravel mix like a concrete mix would require.  Then you need not worry about soil working into the voids.  When compacted these soil types are pretty strong in shear strength.

  For the shear strength option, are you prepared to accept that you do not get sufficient shear strength increase?  If so what is the next step?  Have you experience with this method and found it works elsewhere?

RE: Stone columns....

You are correct, blowout would be a concern; but, with a maintained upward pressure gradient our client is concerned that continued piping of material into the stone column voids could eventually lead to a blowout.  My feelings are the same as yours, ending the column in a lower permeability material will not have much for seepage, and the voids will likely plug off if/when material moves into the stone column.  Currently nearly all the head loss from the aquifer occurs at the base of the material that the shear zone exists. The stone columns will provide pressure relief by forcing the head loss to occur across the 4-5m of till below as long as they remain relatively more permeable than the in situ material.  I was uncertain on the ability to use a geotextile sock as well, so your input helps with that regard.  

As for the stone columns for slope stabilization, it is based on success elsewhere, also, the material we are installing in is highly over consolidated and very stiff.  The associated shear zones are very thin, (test pit indicated an the slip surface was just a few cm thick), so I feel like the shear strength of the columns will work to reinforce the slope in this instance as it will act similar to a direct shear apparatus.  I agree, if it was a thick layer of soft clay the stone columns would likely bend and move to a point beyond serviceability criteria without strength mobilization.    

The goal of the columns is to provide both an increase in shear strength and pressure dissipation to ensure a FS > 1.3 for all potential slope failures.  Due to the amount of fines in the base soil it is difficult to get a gradataion that provides good shear strength and filtration.  Any suggestions in this regard would be helpful.

RE: Stone columns....

There's this great site about 20 miles south of Syracuse New York where artesian pressures pushed up through an (improperly?) sealed borehole at a small bridge abutment.  The water came up and the sand layer also, not only wiped out the bridge abutment, also cause an acre or more of sand boils due to the instability spreading (along subsidence fractures, as a guess).  Past geology field trip.
Relief wells should be much more reliable for drainage than stone columns.  You can maintain them, there is not much questions about sealing or blocking with fines (you can clean out the well if you have to), your filter outside the well screen can be much more compatible with the likely soil type to keep in place. I would keep the wells a little away from the bridge.  Then the stone columns can be installed by a number of methods, for which maintaining them permeable is not a major concern.  I would think there is a possibility the double-function of the stone columns could mean they are not particularly useful for either purpose.

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