Lime soil mixing 1

[kieran1]

Can anyone offer any experience on this method of ground improvement for a filled site. The contractor is proposing this method of ground improvement on which traditional foundations will be constructed. We had originally specified vibro-compaction.

I have read up a bit about the method and i'm happy with the therory and methodology.

I wonder if anyone could point out any potential problems or things i should be looking for.

Kieran

 

[iandig]

Sulfates, sulfates and sulfates!!!
Lime treatemnt to modify soils is equivalent to drying a wet soil to an acceptable range and the long-term performance will be consistent with this. With the addition of quick lime, this will hydrate (the lime uses some of the water in the soil) and with the heat of hydration this will remove water from the soil(signifncnalty more than by hydration alone).
With increasing lime content, and where the pH is >12.4, the lime will start to break down the clay minerals and then with any additional lime it will begin to form calcium silicate hydrates and calcium alluminate hydrates (cements). Therefore as more lime is used the more likley you are to cement the soil (solidify if thinking about remediation of contaminants or stabilisation if just geotechnical).
Where lime alone cannot provide sufficient strength, then cement can be added. Where sulfates are present the use of GGBS or PFA can help.
If the Total Potential Sulfate is greater than 1.0%, DO NOT DO IT, you have been warned.
Modifying and stabilising soils will reduce consolidaiton and creep settlement of the fill.
Typical lime addition is up to 5%, and when mixed with cement you are probably looking at a ratio of 2:1 lime to cement.
Organic soils can be problematic, and normally you would not treat a soil with an organic content of more than 2%, but I have done human sewage with 60% organics using a combination of lime, cement, PFA and GGBS and engineered a general fill suitable for car-parking and areas of hard-standing.
All soils will need to be treated, so can involve large earthworks, although deep soil mixing is also possible, but you don't get the compaction of the fill in the same way as excavating and replacing.

 

[kieran1]

Thanks iandig.

This site has elevated PH levels up to 11.62. As a former brewery, some sulfates are present although the soluble sulphate content was not quantified.

The area of treatment is mostly for the stability of fill of between 3.0m & 6.0m over a firm to stiff clay.

I think some further testing will need carried out.

Kieran

 

[iandig]

Make sure when you do the testing you include the following:
Water soluble sulfate;
Acid (Total) sulfate;
Total sulfur;
organic content;
Initial consumption of lime (add lime in 0.5% increments and measure the resultant pH, where the pH stops increasing this is the ICL, add 0.5% to this and use this for intial mix design);
make sure you carry out assessment of swll of samples using the proposed mix design, HA 74/07 and BS 1924 recommend using CBR swell tests for this, but in lime saturated water the holes in the base of the CBR mould tend to clog up with limescale. As such I would also recommend that you consider the accelerated swell test to the BS EN (will look up the reference for you on this).
Make sure the Specilaist Contractor is the designer of the mix, and you as the Consultant check what they are proposing, don't try and design the mix yourself if you have not done it before.

 

[kieran1]

Thanks again

Kieran

 

[fattdad]

Well deserved star to iandig. This is absolutly correct, the presence of sulfur in any mobile form and the presence of lime has the potential to lead to the formation of ettringite, which is notorious for high swell pressure and damaging heave.

Just how will the contractor get the lime properly mixed into the soils that are 9 to 18 ft below the ground?

This sounds unusual - not that I'm some expert in lime stabilization. I've just seen it used for site improvement in the near surface.

f-d

¡papá gordo ain’t no madre flaca!

 

[Ron]

Commonly used in clayey soils as a surface-mixed treatment. Two notable problems...mixing and compaction.

Mixing is critical. It can only be done to a limited depth, usually 8 to 12 inches at a time, followed by compaction using a sheepsfoot roller.

For compaction control, take the samples after the mixing for the laboratory compaction testing, and make sure to take several samples to check the consistency of the mixing.

 

[msucog]

i would consult a local geotechnical testing firm to assist with the site specific design and field construction observation/quantification/documentation. spending the money to have them on site is usually well worth it (assuming you have a qualified firm)

 

[JedClampett]

I don't diasgree with any of the discussion above.
But I would say that lime treatment is in no way a substitute for vibro compaction. One is a surface treatment and the other is a deeper ground improvement.

 

[Zambo]

Some posts here refer to lime stabilisation at shallow depths. This can be to improve an existing soil so that it can be used as subbase or roadbase material. Application may typically be by removing the material putting it through a pugmill to mix with the lime, or by using a rotavator to mix insitu.

What the threadstarter seems to be describing is a system to mix at depth (about 6m). This suggests a technology more akin to jet grouting. This would produce columns of stbilised soil. Any internet links I can look at - sounds interesting.

 

[kieran1]

The system will involve mixing the lime with the existing clay being cut from areas of the site (also some brickwork & concrete rubble from demolitions) and compacting in layers up to a new level 6.0m above existing level. The site is on a slope so cut and fill operations are required. Originally we had intended to loose tip the fill from the cut areas and provide vibro stone columns down through. The other method would be to pile the houses however for cost, programme and insurance issues the contractor would prefer this method.

Kieran

 

[Zambo]

Ah got it now - big earthworks job! iandig seems to have a good grasp of the requirements.

If it was me I would aim to stockpile the excavated material, separating out the brick and concrete as much as possible. Then use a pugmill to mix the soil and lime before sending for grading and compaction.

I would have thought the drawback is the time required to compact in layers 3-6m of fill.

 

[kieran1]

Zambo,

Thats why we had suggested loose tip filling and vibro. However the works are now being phased. The soil stabilation occurs in phase 3. The contractor can bring in a specialist sub contractor now as an enabling works project and save time on the overall programme.

Kieran

 

[iandig]

This sounds very similar to a project I am aware of. Just a note of waring, check the underlying soils below the Made Ground and make sure of their engineering performance before embarking on widespread excavation and re-engineering.
If they are competant, should be OK but you should always check what the overall implications are from changing one key element, especially if you are not the orignal designer (if not, check with them on how they arrived at the solution in the first place to use VSC).
Hope that helps

 

[theCorkster]

Treating 6m of clay seems excessive. It would seem that the treatment depth would be determined based on the expansive characteristics of the clay?

This approach would be to figure out what thickness of clay will effectively negate expansion potential in the lower portion of fill, and consider treating the upper layer. Using swell tests and consolidation tests on the clay (at planned relative compaction) would provide the data you need to investigate this and create a design.

Also, deep lime slurry injection in clays has been performed in Texas to try to deal with treating the clays deep enough (moisture affected zone), although I have not experience with this approach myself.

Also, if you have to treat the clay being removed from the cut, what will you be exposing on the cut side of the pad? If its clay or some precursor (weathered claystone/siltstone/shale), the long-term effects of water migration into that material and its behavior should be considered.

And consider backfilling trenches in lime treated material with a controlled density fill; backfilling such excavations with excavated lime treated clay is easy to specify. However, the resulting product when compacted will behave differently than intact areas that were lime treated.