Lime Stabilization of Silt

[dgillette]

Today I am involved in a risk analysis of an existing dam. Twenty-some years ago, lime was mixed with the silty (loess-derived) embankment fill to create erosion protection for the upstream slope. The material wasn't available for us to look at, and we are speculating on its properties.

None of us here have seen lime used to stabilize non-plastic or just barely plastic silt. Can one actually expect much strength from mixing lime and SILT, as one can with clay? I don't know anything about the chemistry of the process.

Thanks much,
DRG

 

[dgillette]

I should add that all of the guidance documents I could find with a quick internet search indicate that PI should be at least 10 or at least 15.

 

[moe333]

I don't think you would obtain any increase in silt strength from the lime. Fly ash may work with the silt and it is often incorporated with lime treatment when there are mixed cohesive/cohesionless soils to be treated.

 

[dgillette]

I was afraid somebody was going to say that.

Thanks, Moe333.

 

[SlideRuleEra]

In the early 1980's I was involved in the start-up of coal-fired electric generating station's flue gas desulfurization (FGD)waste disposal system. Fly ash and dewatered FGD sludge were stabilized by adding lime. FGD sludge as many of the same properties as silt. Here is a web page that gives a good description of the process and the product:

http://www.tfhrc.gov/hnr20/recycle/waste/fgd1.htm

Interesting that you said the material on your project had been used for erosion control. The stabilized waste (trade name Poz-O-Tec) was very susceptible to erosion even though it had some properties similar to weak concrete. In permanent landfill disposal we had to cover it with topsoil to protect the Poz-O-Tec from erosion from weather.

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[GeoPaveTraffic]

If a relatively large amount of lime was used, I would expect the silt / lime mixture to have a fairly high undrained strength and be fairly erosion resistant. However, if the amount of lime was small, I doubt it did much to improve the silt.

Without at least looking at the material and giving it the "boot" test, I don't think you have any way to know.

 

[fattdad]

WARNING: There is huge risk in mixing lime with any sulfur-rich materials (whether FGD residue or sulfur-rich soil). It can lead to the formation of ettringite and cause damaging swell. Refer to the current issue of Geo-Strata (ASCE Geo-Institute) for more information.

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

My understanding is the formation of ettringite was from lime treatment with high sulfate soils which I believe would be a bigger risk in clayey soils than silt as the sulfate may tend to be leached out of the silt, but I could be wrong.

 

[fattdad]

Moe, Whether clay or silt, if there's appreciable sulfate content and lime stabilization is being considered, there can be a problem. The earlier mention of FGD materials and use of flyash is another potential problematic mixture - mostly the FGD materials. There's enough published reference to anticipate these problems.

f-d

¡papá gordo ain’t no madre flaca!

 

[SlideRuleEra]

Looks like I had better clarify my earlier post. The use of stabilized FGD waste was NOT a recommendation that it be used as a specific purpose. The post was my attempt to answer the original question about mixing lime with silt. The analogy intended was that FGD sludge has similar properties to silt - that is all.

At the time mentioned (the early 1980's), the stabilized FGD waste was used for one, and only one, purpose - to "lock" the the pollutants in the FGD sludge, along with fly ash into a chemically stable form that could be placed permanently in a landfill dedicated solely to this one purpose. This solid waste was a big improvement over the ponds that had been the previous disposal method for both fly ash and FGD sludge.

Over time, this solid waste approach became more or less a dead end - perhaps because of the problems you are discussing. Today, fly ash is more commonly used directly in concrete and flowable fill. FGD sludge has become a raw ingredient for manufacture of gypsum wall board.

I presented the information based on my background with an electric utility, not from geotechnical prospective. In the future I will be more careful before offering analogies.

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[fattdad]

SlideRuleEra, Just to clarify, I have no problem with the mention of FGD materials and any potential reuse of industrial waste products. In today's LEED-credit driven design, these topics will continue to emerge. In some instances, blends with FGD can imitate the properties of "self-hardening flyash. For the case that such a blended product is being considered in civil engineering application, the goal is to identify the long-term swell potential and if it exists, reject the product. For the case that the broker is using proper inventory control (i.e. making the blend and putting it out to pasture to "cure"), then a potential reuse may be feasible.

¡papá gordo ain’t no madre flaca!

 

[moe333]

Fattdad, I agree with you, I was mainly stating that from my experience it has been clays that I have seen high sulfate contents from our testing, and I haven't seen high sulfate contentsit in silts, particularly non plastic silts.

 

[iandig]

Sorry, been on site for a couple of weeks and missed the original post, so you may have to scroll back up when you read this.
As for the chemistry element, when you add quick lime to a soil and increase the pH to over 12 [around 12.4 for quick lime in UK], any further lime you add will start to 'break' down the clay minerals [note, not clay size fraction]. This will allow the calcium [lime] to combine with the silica and/or allumina from the clay minerals, with water and form calcium allumina hydrates and silica allumina hydrates - i.e. weak cements. This accounts for the requirements for the PI as from experience, clay soils with PI in this range have sufficient allumina and silica within them that when they break down they will form the cementitious products. This also accounts for the 'Initial Consumption of Lime' [ICL] test, which is a measure of how much lime you need to add to increase the pH of the soil to over 12, after this any further lime is then free to combine with the silica and allumina.
If the soils do not contain sufficient allumina and silica, or where the lime addition is less than the ICL, then you are into 'modification' and not 'stabilization'. So if you need to assess the potential impact/imprvement of the material, you can begin by looking at the chemical composition of the silt [XRD etc..] and assess the clay mineral content.
On the plus side, when ettringite and/or thaumasite form from a detrimental sulfate reaction, they also use the silica and allumina, if they are not present in the parent soil, you may not get heave with sulfates. A bit left field but we have a site in the UK with very high sulfates but sulfate heave did not occur, no silica and no allumina to drive the reaction, unfortunately no stabilizaion either!!

 

[fattdad]

iandig: Interesting reply!

f-d

¡papá gordo ain’t no madre flaca!