Vibro Stone columns

[Omah]

Dear colleague,
I am looking for any information about the following issues:

1- The quality control measures and the quality assurance requirements for
vibro stone columns that are to be constructed in hydraulic fill that consist of
soft to very soft clay (with consistency index of 0-0.1 and cu<20kPa), silt,
loose fine sand with various fines content. I heared that Becker Penetration
Test (BPT) has been used for that purpose in North America. But I don't know
what was the minimum No. of blows, and how it was used to check the
density, verticality & continuity of the columns.

2- For a project in a liquifiable formation, where the vibro stone columns are
proposed to be used to mitigate the liquefaction risk, the columns spacings
depend on some parameters including the soil permeability in the horizontal
direction (kh) in the design method of Seed & Booker (1977).
a- Is there an upper bound for the fines content of the soil in that formation,
beyond which the stone columns are not considered effective in mitigating the
liquifaction risk?
b- Is there any guidelines/research results about how much the ground
improvement should extend outside the project boundaries to prevent the
failure or large ground deformations due to liquefaction outside these
boundaries from extending to the improved ground with stone columns within
the project boundaries?

3- What is the lower bound of Consistency Index or cu of the soft clay, below
which we would consider the stone columns not practical to improve the clay
due to the lack of confinement by that clay to the columns? Some reports say
the technique is not effective for cu<20kPa. Prof. Das mentioned that it is
work more effectively where they are used to stabilize large areas with cu=10-
50kPa for the subsoil. However, I am looking for more information or case
studies, as I have cu=6-24kPa, with water content at, below , or slightly
above the liquid limit in many locations (Consistency Index, CI=0%) but CI can
be up to 0.24.

If you know any references (e.g. published case studies, or code of
practice,...etc.) that could help me with finding answers for these queries,
would you kindly send me information about these references.

Best regards
Ohama

 

[moe333]

Priebe (Germany I think) has a couple papers that are basically the standard for stone columns. Some software has his methods coded (Geologmiski sp?).

 

[BigHarvey]

with such low values of Cu ( less than 20 KPa ) it will be very difficult to control the quantity of stone that will be incorporated and you will not have very good filter parameters between the stone and the surrounding soils . Clay particles will migrate in the stone and will clog what is intended to be a draining column. I would suggest sand drains installed with a closed end steel tube or round prefab vertical drains depending on the lifespan of your project.

 

[dgillette]

1. BPT has been used, but not with great success for assessing the columns themselves. It is just fine for assessing the density achieved in surrounding clean GRANULAR material, where the stone columns can be used for densification, rather than reinforcement. There is a theoretical possibility of using BPT as a measure of density in columns in soft material, but you cannot use the same correlations that are used in normal soils. This is because you would be driving the BPT into a small zone of denser material surrounded by soft material and the stress boundary conditions are entirely different. Both the stone columns and the BPT tend to wander away from exact verticality, so the BPT may not stay within the column for its full depth.

2. If you have clay and silt in the formation, stone columns will not be successful in densifying the surrounding material. Neither will they be effective in dissipating excess pore pressure during the earthquake; no matter how pervious they are, the surrounding soil still needs to be quite pervious. Experiments have been done with slotted pipe drains for the latter purpose, but they have shown potential ONLY for very clean medium or coarse sand. The best stone columns can do for you is to proved vertical resistance to reduce settlement. They should not be used to reinforce fine-grained soil against lateral loads, such as stabilizing a critical slope, such as a dam.

Big Harvey is quite correct; in soft material, the stone will go where it pleases.

Yes, I have some strong opinions about this.

 

[sunny7uc]

You can utilize reinforcement effect of stone columns (proven by Finite Element Analysis), you can come up with a area replacement ratio (11-20%) to reduce liquefaction risk. You may also look at soil mix colums in lieu of stone columns. I will suggest you to call Hayward Baker.

 

[dgillette]

Sunny7uc - You should look at the more recent literature relating to reinforcement with slender elements. (Guney Olgun at Virginia Tech, for example. He's had second thoughts about the jet-grout columns at the Carrefour site in the Kocaeli earthquake, particularly since he did 3-d FEM analysis and found that the stiffer columns do not move in unison with the surrounding soil.) Reinforcement by columns should work fine if the layer thickness is a small fraction of the column diameter, but the simple area-replacement ratio doesn't account for bending when the layer thickness is much larger than the column diameter. The lateral stiffness of the columns is controlled by bending, and is therefore much lower than if the same amount of material was constructed as a shear wall.

Soil mix columns apparently worked well at the Oriental Hotel in Kobe. However, they had been constructed overlapping each other to create a lattice of shear walls, rather than as discrete, slender columns. Discrete soil-mix columns have practically no bending resistance, and they are brittle.

 

[sunny7uc]

I agree with you "dglittle" on the soil mix columns, that they need to be constructed as a "cell" element.

I made no commenst about jet groutng, but sure can be used in cells smililar to soil mix columns.

Stone columns shall work with simple area replacement ratio because they are not as stiff as soil mix/jet grout columns and will have similar strain during an earthquake.

 

[dgillette]

If the stiffness of stone columns is comparable to the surrounding material, they can do little to reduce the cyclic strain and liquefaction potential in the surrounding material. Remember also that the shear stress on the vertical outside surface of the column can be no greater than the shear strength of the surrounding liquefied soil. Therefore, the outer part of the column also has low shear stress on horizontal surfaces (like a beam loaded in shear) and cannot provide much shearing resistance for lateral loads such as from a slope. (This is why a direct simple shear specimen needs a low aspect ratio, like a hockey puck, so that the "outer part" with its lower shear strength is a smaller part of the total.) Finally, bear in mind that that with liquefaction, the columns are surrounded by material with very high excess pore pressure, which can intrude into the relatively pervious columns very quickly, lowering the effective stress and shearing resistance within.

The benefit from stone columns comes from either densification of the surrounding soil or vertical support under a structure (or both). I definitely do not believe they can be relied upon for reinforcement against lateral loading!

Neither can stone columns installed with vibration be counted on to relieve excess pore pressure because the installation process usually causes the surrounding material to intrude into the voids and reduce the permeability of the gravel. We have exhumed some columns (at least two sites) and found that happens.

Mechanically, jet grout columns are directly analogous to soil-mix columns, so they are relevant to this discussion.

 

[escrowe]

1. How did you manage a hydraulic fill consisting of clay? I suppose this was not a typical pumped dredge fill, which would be 95 to 100 percent sand.

WRT quality control of stone columns, was field inspection completed during installation? If not then the pooch is indeed screwed.

2. Suggest review of FEMA 274, which addresses concerns regarding ground improvement for liquifaction (such as adjacent site landsliding) and provides design references.

3. Stone columns are generally not intended to improve clay. In fact, stone column design generally should presume little improvement of surrounding soils. They are generally selected when other ground improvement methods (i.e vibro flotation, DDC) are not feasible.

Have you considered driven or augercast piles? Reinforced piles are generally more efficient for shear, not to mention uplift. Although I suspect your foundation design includes a mat? If not, and shear is significant, consider a modification.

Anyway, good luck!