Effect of Stone Columns on Preloaded Silt

[RyGeotech]

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.

 

[BigHarvey]

Stone columns should be installed before any fill apart from working platform. Otherwise you could experience problems to go through a well compacted fill with the vibrator. This will also reduce the syone column length which seems to be quite respectable ( 20 m + ? ). I would personnaly rely on two contractors only in the world to make a good quality job to that depth. You could also look at rigid inclusions. They might prove more economical than stone columns ( definitely if you have more than 5 meters of fill ). Do not forget than stone columns will not reduce the total settlement by a factor greater than 4. With rigid inclusions you will be able to reduce the settlement to 2 to 3 centimeters ( all depends of the type of project you are dealing with : railway or highway embankment, structure , etc )

 

[dgillette]

Rygeotech -

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

 

[BigHarvey]

Any type of pile would be possible : rotary bored, CFA, displacement rotary, driven steel pile ( pipe or H ). It is more a question of design concept, soil resistance and local problems. I have used all these methods, each of them in a particular case. The last one was for 31 m diameter , specs called for steel H piles but no possibility to get them on time so we changed to CFA piles because we could ship only a 30 t rig . Water test under way for this project where total settlement at center of tank should be around 10 cm !
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 )

 

[RyGeotech]

BigHarvey,

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.

 

[moe333]

dgillette, regarding the use of stone columns for lateral load resistance; are you saying that they do not work to mitgate lateral spreading from an earthquake, or to mitigate a liquefaction induced flow slide in an embankment?

 

[dgillette]

Yes, that's the issue. If the silt layer is thin (maybe up to 1 column diameter thick), they probably work great as shear reinforcement, and can be analyzed by a simple area-replacement ratio (allowing for some edge effects, possibly including intrusion of excess PWP or liquefied 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

 

[moe333]

I would be very interested in the papers if you can dig them up. I have designed stone columns for liquefiable sites that were pretty flat so they were used more to decrease liquefaction induced settlements, but have seen many reports where they are used to increase factors of safety in a liquefied zone using limit-equilibrium slope stability methods. Sounds like we should be looking closer at this type of application, in terms of column spacing.

 

[RyGeotech]

dgillette,

Are your comments specifically talking about stone columns in silt? It appears to me that moe333's question and comments are speaking more generally.

http://www.accessmylibrary.com/coms2/summary_0286-32144316_ITM

 

[BigHarvey]

There is a difference because with stone columns you distribute the load between the columns and the surrounding soil ( about 10 times more on the columns than on the soil in terms of stresses ) and if you load the soil, unload and then put the real load, the Young's modulus of the silt will be multiplied by a factor of 3. But since the load is small on the soil, the influence over the total settlement should be small ( otherwise it means than stone columns might not be appropriate in this case because you have very large settlements !).
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 !

 

[dgillette]

RyGeotech - Thanks for sending the link. It shows that the ground (level) was stiffened a bit, so the cyclic strains and excess PWP were reduced. One big difference between that and an embankment foundation is that the embankment foundation is likely to yield under the slopes in a strong earthquake. (The loose silt would need undrained strength, so the yield acc. isn't very high.) At that point, it isn't stiffening that you need (to reduce strain and excess PWP), but shear reinforcement to prevent post-EQ instability.

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.