Soil stabilization with cement or lime 1

[yahoo123]

Can anyone tell me how the soil stabilization process works. Is it required for the ground to be graded first before applying the cement? Is the cement spread on the soil and mixed in place with the soil or does it require the soil to be excavated and mixed in the equipment before placing it back on the ground? If it is in-situ mixing, is water applied at the same time the cement is spread on the ground? Is there a delay time before compaction can begin?

Also does anybody if this is an expensive process?

How does it compares cost wise to excavating and backfilling with gravel?

Thanks!

 

[fattdad]

Soil stabilization is often used to modifiy otherwise-difficult-to-work native soils. For example, if you have soft and wet soils that would benefit from additional compaction, it may be very difficult to remove, dry and recompact the native soil to acheive the desired strength. By adding cement or lime, you essentially make the soil more workable at the original moisture content (I could go into more detail on this, but that's about all you need for this discussion). Thing is, it will only work if you can blend the lime or cement into the layer you are trying to improve.

BoMag makes a mixer that contractors can use. It looks like a big rotor tiller. For this method you'd distribute a desired layer of lime or cement and then set the equipment to mix the lime or cement into the desired layer thickness.

What you have to do is a bench test on the material. So, step one is to collect a sample and then make a few batches in the laboratory to see how a given percentage of lime/cement affects the moisture-density relations. Once you optimise the percentage, you can then figure out what it will take in the field.

Yes it's expensive. Then again, so is the wholesale removal and replacement of otherwise workable soil. Somewhere in this there is an engineering economic that must be considered.

Good luck. I'm sure others have different and valuable experience.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

There are cement as well as lime producers that very likely have nice brochures describing these procedures. Some of our members very likely will provide links to give more scoop about it.

Once you experience it even crudely, even where no lab work is done ahead of time, it is almost like magic the way things improve.

I will say one thing about cement stablization that soured me a little. A roadway subgrade stabilized with about 5 percent in a clay sure looked nice when built, but frost action later destroyed much of the strength.

 

[yahoo123]

So basically the cement or lime is sprayed ontop of the soil, then mixed insitu and water applied at the same time? How is it compacted?

 

[iandig]

The material is compacted using 'normal' compaction plant. In the UK, once the soils have been treated and rotavated, they will be compacted to a specification, often using a smooth-wheeled roller [even for cohesive soils]. Depending upon the material being treated, and whether or not you are looking at modification [short-term/improve handling/reduction in moisture] or stabilization [long-term/formation of cemetitious product] it will influence the the type of hydraulic binder used. The selection of the binder will also be dependant upon the material being treated. Clay soils high in silica and allumina are reactive with lime and this in isolation may be sufficient for your purposes depending on what you want the result to be. For higher strength, granular or soils with low silica and allumina then cement can be/is used. In addition to this other binders can be used such as PFA and GGBS. I am currently involved with a project where relic sewage sludge is being modified and stabilised using a combination of lime [2-3%] and PFA [12-18%] which is providing a material with a comparable stength in excess of 5% CBR. The material is being treated in order to leave it on site [thus preventing the need for off-site disposal - very VERY expensive in the UK] and is forming non-critical areas under hard cover on site for a larger commercial development.
Going back to the selection of the appropriate bindrs, if there are sulfates present in the ground, these can and will combine with water, calcium, allumina or silica and form expansive products. There are a number of accepted methods for dealing with this, however if you get it wrong, it really is a big problem. Some binders such as PFA and GGBS can help restrict the formation of expansive by-products, but with higher sulfate contents this can still occur. Total sulfates of < 0.2% should be OK subject to appropriate testing, sulfates between 0.2 and 1.0% MAY be ok but it needs extensive testing and sulfates over 1% are not reccomended for treatment with lime and cement.

 

[fattdad]

So basically the cement or lime is sprayed ontop of the soil, then mixed insitu and water applied at the same time?

Not too sure about adding water. More frequently, you are using the lime or cement to react against the water in the soil. To add water would defeat this purpose.

Going back to the selection of the appropriate bindrs, if there are sulfates present in the ground, these can and will combine with water, calcium, allumina or silica and form expansive products.

This is a very important warning!! Quite often marine clays (i.e., sensitive clays that really need stabilization) have disseminated (sp) grains of pyrite. Pyrite will readilyy oxidize and liberate sulfuric acid. Chemical reactions with liberated sulfides and lime or gypsum - expecially in a basic environment can lead to the formation of ettringite. The formation of ettringite can damage overlying structures.

f-d

¡papá gordo ain’t no madre flaca!

 

[Zambo]

mixing in-situ with a rotovator is one option. CSM can also be produced using pugmill continuous mixing plant.

 

[yahoo123]

What is more popular, mixing with rotovator or using pugmill continuous mixing plant?

 

[msucog]

i might disagree to some extent about not adding water. if the soil is not already wet of optimum by a fair amount, you've got to add water to help get hydration out of the cement. it's a fine line though...

 

[Zambo]

"yahoo123 (Bioengineer) 9 Nov 07 10:46
What is more popular, mixing with rotovator or using pugmill continuous mixing plant?"

It depends on the project and the type of plant available.

The first question is how thick is your pavement construction?

Other posters may correct me, but I believe the rotovator type insitu mixer will work to a maximum of a 150-200mm thick layer. So if you can grade off to finished sub-base level and then add cement (or lime) to the existing soil, compact and finish. Then go for the rotovator without question.

Next option you need 300mm thickness stabilised material. Then you have to remove the first layer. The lower layer can be treated by rotovator and the second layer either placed and again stabilised by spreading cement and rotovating, or it could be passed through a pugmill first for the cement to be added.

Another option is that the existing soils are not used at all. You remove say 300mm of existing soil. You find a nearby borrow pit where there is gravel which in it's natural state would not be suitable as a subbase material, but with the addition of cement is satisfactory. In this case again you either place the material in layers and spread cement followed by rotovator or you process through a pugmill.

If soils are to be removed and reused, or brought to site, I like pugmills for CSM as they are quick and accurate. If there is no need to remove the existing soil rotovators are the way.