settlement estimations for stone columns

[littlefrog]

What is the best method to compare settlements of the soil with and without stone columns with CPT. is the elastic theory the best to apply? Where can I find the good correlations to use?
thank you

 

[Geopacer]

littlefrog,

I'm assuming that you are refering to soil improvement using the vibro-replacement stone column method. If so, there are a number of estimation methods that have been developed that can be applied to settlement of soil treated with stone columns these include:

-BALAAM & POULOS (1983)
-CHARLES & WATTS (1983)
-GOUGHNOUR (1983)
-VAN IMPE & DE BEER (1983)
-GREENWOOD (1970)
-PRIEBE (1976)
-BALAAM, POULOS - Bremershaven
-BALAAM, POULOS - Teesport
-SCHWEIGER et al

However, the lack of well documented full load tests for stone columns in differing soil characteristics leaves a large scatter in the above settlement estimation methods. To further complicate the matter is the various vibratory equipment and installation techniques used by the different contractors out there. Also a factor is the column layout, spacing and diameter along with the backfill stone shape, gradation and amounts used. One of the more significant yet most difficult variables to monitor or specify, then reliably apply to settlement calculations is the compaction effort that is applied during the installation process. The difference in the degree of soil compaction and lateral confining stresses on the column will effect the settlement behaviour tremendously and is not clearly defined in any of the calculation procedures listed above.
To conclude, do some research before relying on any one settlement estimation method and it is best to find a vibro specialist in the area with local expertise to either backup or confirm your estimates.

Geopacer

 

[BigHarvey]

Stone columns is a good technique to reduce settlement but it has limits. It is generally admitted that the settlement will not be less than 1/4 of the settlement that would be obtained without treatment. Which means that if you have too large settlements without treatment then you might have to select other ground improvement or reinforcement methods.
If you use stone columns, this means that you are dealing with silty or clayey soils and CPT is certainly not the best tools to estimate settlement in these types of soils. If you have very soft soils , an oedometric approach would be more appropriate, if the soils are not too bad a pressuremeter approach could be used if you want to use in-situ tests.
You will find settlement reduction factors as a function of the cohesion, stone column diameter and spacing in the litterature already given to you. If you want a direct calculation of the settlement of the treated soil, the normal approach is to consider than the stone columns heads and the surrounding ground settle from the same amount. This implie a distribution of the load between the stone columns and the surrounding ground proportional to their respective deformation moduli. Knowing the stress applied and the deformation modulus you can then compute the settlement either in the stone column or in the surrounding ground ( and get the same result ).
It is now quite common to use finite elements programs ( PLAXIS ) to design these tratments.

 

[littlefrog]

>Geopacer and BigHarvey : thanks
My problem is the following : they will carry out an excavation of 4 meters of fill and sandy clayey silt, and put a load of 80kPa (circular slab, 35m diameter), so they think that only small settlements will occur. Do you think thiis hypothesis reasonable? is it possible that the reduction of pressure involve a swelling (decompression) of the soil?
I only have CPT and correlations to get oedometric moduli.
I think I'll use PLAXIS to understand the behaviour of the structure
Thank you, your advices are very helpful.

 

[BigHarvey]

Be careful when you consider that there is load compensation between excavation and structural load. You mentioned that you are excavating fill. The soil underneath the structure must be clayey since we are talking about stone columns. You can consider that the weight of the excavated material will be balanced by you structural loads only if the clayey soils had undergone full consolidation under the weight of the fill. You might find the answer from your oedometer test or if you know the history of the fill.

 

[Focht3]

You haven't indicated the strength of the soil; for the purpose of this post I will assume the average undrained shear strength of the soil is in the range of about 50 to 75 kPa and the soil is normally consolidated. In general, I would expect only about 10 percent of the settlement to be elastic.

It sounds like someone has decided to do some limited removal and replacement to improve the ground and to provide a "cap" over the stone columns. This may improve the performance of the perimeter of circular slab for the first 4 to 8 meters in from the edge, and improve differential movements that could be caused by the fill. And it should help transmit the slab forces to the stone columns. But it doesn't sound like it will do much more than that. The depth of influence is too great for a footing of this size.

If the subsurface materials are a "sandy clayey silt" then it is possible that the site will experience the majority of its' consolidation settlement within the first year of service. It may also be possible to improve the site by installing wick drains and preloading to about 120 kPa or so instead of using stone columns.

If the compressible layers are interbedded with "good" sand layers, then there may or may not be a settlement problem. It depends on the soil profile, the properties of the soils, stone column size, stone column depth, stone column spacing, etc.

 

[MarkDK]

Dear all,

I am working on design of stone columns (vibro replacement) to mitigate the effects of ground settlement due to liquefaction. The sub soils are fine gravelly sands.

The design approach I am planning to use constitute of estimating the elastic settlement of the stone column and checking the stress path in the in situ soils due to the lateral pressure generated by the column bulging. My understanding is that PRIEBE’s method is empirical and does not address the column bulging mechanism.

Has anyone there worked on a similar case? Is there any established analytical procedure for the design of stone columns?

For the people who used Plaxis, how do you model the stone column and how to estimate the plastic deformation of the in situ soils due to the dynamic pressures?

Your thoughts would be appreciated.

 

[ashjun]

Hi MarkDk,
In his latter work, Priebe has probably addressed the issue:
Reference:
Priebe, H.J. (1995). The design of vibro replacement. Ground Engineering 28, No. 10, 31-37.

You could also look into the reference:
Van-Impe, W.F., De-Cock, F., Van-Der-Cruyssen, J.P. & Maertens, J. (1997). Soil improvement experiences in Belgium: part II vibro-compaction and stone columns. Ground Improvement 1, 157-168.

Plaxis: How are you going to model 3D without going into simplified assumptions?

Whatever method you use: please note that the method of column installation has a very important role in estimating the settlement of the composite ground.