Cement vs. Lime Treatment

[Geoduck]

I need some advice on subgrade stabilization. I have been asked to prepare some recommendations for lime treatment of some subgrade soil. I have generally been taught over the years that lime is typically most effective on clayey soils with a PI>20 and cement treatment is usually used on granular soils with less fines having a PI<20. I have used both methods several times to improve subgrade soils at numerous sites.

However, on this particular site, there are upto 5 thin soil layers in the upper 15 feet that include clayey gravels, clayey sands, sands, and even clays with less than 10% passing the #200 sieve and a PI around 22. Since there will be significant cuts to reach grade on this project, I would like to chose one method that would be effectrive on all these types of soil.

I am guessing that lime is not the best option here because of the sands and gravels. Would cement or fly ash be a better option for treatment? If so, would cement be effective in treating the clay soils with a PI=22?

 

[msucog]

are you trying to simply stabilize or create a quality subgrade for say a pavement?

also what part of the world is the job located?

 

[Geoduck]

Yes..it is for a pavement...California coast, so no freeze thaw concerns

 

[msucog]

i guess my question is: is someone requiring a DOT quality subgrade or are you simply trying to get a subgrade stable enough to construct the pavement? in other words, some county or city officials might require documentation that the process meets DOT requirements. for example, the last time i did a DOT quality soil cement, a design had to be done and the compressive strength of the samples had to obtain at least 450psi. so it might take 12% cement to achieve this for some silty soils around here. however, if the client is simply trying to get a "stable" subgrade, it might only take 3-8% cement for the same soil. it depends on the performance requirments. lime is often the choice if you're trying to get the stable condition since it's easier to work with...cheaper too but it takes more to have the same effect as cement.
contact a local firm and get their thouhts since they're familiar with the local soils. around here, the driving factor is the fines (and mica) in the soil. you gain a little if there is coarse material, so we've actually added a little GAB or pulverized ashpalt and incorporated it in to the material prior to adding cement so that the %cement could be lowered. this game works especially well for RAP.
try this link and go to the bookstore--search for soil cement or lime stabilization...there's several free publications to help.

http://www.cement.org/index.asp

 

[civilperson]

The mix of your various types of soil can serendipidously improve the individual components. Try a blend and test for CBR, etc. If a exterior component is required, flyash seems most cost effective.

 

[msucog]

and here's a thought that hasn't been expressed...the effeciency of the soil cement/cement stabilzation/lime stabilization depends greatly on the underlying conditions. in other words, don't try to build a 8" soil cement on top of 2' of horrible stuff...you'll spend lots of money to just have to go back in and do it all over again. in situations where you might have 2' of unstable material, remove 8" and them stabilize the underlying layer enough to make it hard (again, this would take much less cement than it would take to build a soil-cement). then put the other material back on top of the stable subgrade and build your pavement subgrade from there. i would suggest doing a test section before you go crazy doing this to a mile long road.

if you have a large area, try a reclamation contractor (here's one that i've worked with in my part of the country--

http://www.millergroup.ca/index.html)

that can pulverize large areas at a time. having jimmy joe bob grading to "tooth up" the area with his bobcat and throwing the cement by hand might work for potholes, but large areas require knowledgable contractors with the appropriate equipment to do the work efficiently.

 

[theCorkster]

Geoduck -

Years ago I did some work on predominantly granular soils (GM, GC, SC, SM) to find out if lime treatment would provide sufficient improvement from a structural perspective. Simply, when the fine content was low, and/or the fines were silty without much clay, we could only get a reaction by adding flyash - the flyash was the pozzzolan. As I recall, even with significant percentages of flyash, the strength gained was not very great.

Since you have lots of variability, select from the ends of the spectrum and work with the admixture (flyash and lime or cement and flyash) to get the strength gain you need. Just make sure it works for both ends of the soil spectrum.

 

[iandig]

Just going back to the original post "would cement be effective at treating the clay soils with a PI of 22". In answer to this, yes it would but it would be expensive.
When you start to add lime to a soil, it has two intial effects, firstly it uses some water to hydrate [and generate heat which reduces moisture - increases performance] and increase the pH. Once the pH reaches a maximum [typically around 12.4] then any additional lime begins to break down the clay minerals and allow the formation of calcium-silicate-hydrates and calcium-allumina-hydrates [i.e. cement]. By only adding cement, you end up using some of it to only increase the pH, which is why when cements are used, often lime will also be added. If you use PFA, this contains the allumina and silica which is present in clay soils [with a PI of over 20, or 15 in the UK].
Depending upon what the problems with the soils are [too wet/too weak etc...] and cost implications, a balance can be made between cost and end-perfromance.