Cement Treated Subgrade 5

[bjgottlieb]

Hi I need some help. I am looking at doing road reconstruction in my City and am wondering if I can use cement treated subgrade in lieu of base course. If this is not a good engineering option because of life cycle and maintenance concerns, I need to know before proceeding with these improvements. I am mostly looking to do this on collector roadways which receive a fair amount of traffic including school buses and tractor trailers. Any help that you could afford me would be greatly appreciated.

 

[LKS93]

Are you talking about cement stablizing the soil subbase or the stone subbase? I've heard subbase applied to both asphalt and stone.

B/c you say "base course" I'm inclined to think you mean asphalt.

I wouldn't recommend replacing your base course of asphalt with cement treated aggregate unless your traffic counts are in the tens of cars daily.

First, you may have issues just getting your DOT to approve it, assuming it's a DOT road. I know that Virginia DOT would have kittens if I recommended it.

Second, the structural numbers for CTA stone and asphalt are different and you'd have to redesign and regrade with whole roadway, very likely with a much thicker stone base.

That's my two cents.

 

[cvg]

around here, the "base course" is aggregate - not asphalt and so treating the subgrade with cement can be done to reduce the thickness of the ABC or to increase the strength of the subgrade. You would need a geotechnical investigation to determine how cost effective this would be as the cost and effectiveness would be partially determined on the strength of your subbase.

 

[PELS]

I agree with CVG. You need the recommendations of a geotech for both performance and cost analysis.

One similar alternative is lime-treatment or lime fly-ash, which also allows you to reduce the thickness of the pavement section and increase the base strength. Local conditions and geotech data should dictate your ultimate choice. Factors such as freeze/thaw weather, truck traffic, construction schedule, aggregate availability, available budgets, etc., come into play when considering this alternative.

 

[msucog]

there's several things to keep in mind (some of which were previously discussed). as far as the dot (and/or city), my understanding is that it differs state by state and area to area---check directly with the folks that will be making the decision as to whether it is acceptable or not and find out exactly what they want. and when they say soil cement, they mean a real soil cement (mix design, lab testing, field observation and field testing). it does not mean to spread a little cement and mix it around with jimmy joe bob's tractor. the existing subgrade condition and materials must be evaluated (along with the obvious things like traffic counts/loads). without knowing your soils, one thing that jumps out in my mind is that you typically cannot simply replace all the base course with soil cement due to cracking that propogates up through the binder. and this may happen any way if the cement is over applied (i prefer to leave some amount of base course in the design to help provide a sort of cushion between the soil cement and binder).
PELS hit a few good points...you can play with lime, lime/cement, cement, add flyash, etc...simply said, there's million different things you can do...i suggest checking with local folks and see what works best and is most economical for the use...so stay away from very complicated combinations if possible. i have had luck with reclaimed asphalt and base course in the mix since the strength seems to do better due to the larger aggregate. same goes for full depth reclamation. i have not dealt much with lime here other than drying back subgrade materials simply because cement performs better in my area. for cement (and i'm sure lime) PCA has very general correlations (and other manuals on the subject) between subgrade materials and the amount of cementitous materials needed so that cost estimates can be looked at on the front end. you local geotech may have more specific information (i have a pool of job test results relative to pill strength and it seems to do very well...it's sort of a homemade analysis and takes in to account several parameters).
my last point of advice is this: you should not plan to use soil cement to "bridge" over crap. i've seen people try this before and ended up spending a lot of money on cement and installation just to take it up again because they insisted that 8" would bridge over the 2' of crap below. if it's crap below, you likely will not be able to get the necessary compaction and the soil cement layer may "break" and fall out, in a sense of course.

these guys are about the best around:

http://www.millergroup.ca/

few links:

http://www.apwa.net/

(search FDR--full depth reclamation)

http://www.arra.org

good luck.

 

[Zambo]

so what are we talking about, to me:

subgrade is the natural soil at excavation level

subbase is a layer(s) of imported material on top of the subgrade

roadbase is a high specification of imported material on top of the subbase.

base course is the first layer of asphalt or bitumen macadam

wearing course is the final layer of asphalt or bitumen macadam

 

[BigH]

Zambo - I take base course as granular (usually crushed stone) layer placed on what you call subbase. For Asphaltic base course, I would say so (in India, they use DBM - dense bituminous macadam). Also, in many areas, they use a select subgrade which is a layer of material not up to subbase standards (sand and gravel) but which improves the subgrade so that the thickness of subbase and base course can be reduced. These are my accepted terms and definitions.

Personally, I would strengthen subgrade (meaning the natural subgrade) in order to reduce the thickness of subbase and base - but I would NOT do without subbase and base - unless, of course, the subgrade is a well graded sand and gravel - with a high CBR value !!!! Don't forget that a properly designed subbase will also act in drainage - keeping water out of the base and asphalt (by draining it away or by interception).

 

[cvg]

I believe all of these terms are somewhat specific to the region of the world that you are in. For instance, we do not use the term "road base" here (in Arizona), it is always assumed to be "aggregate base course" (ABC). In California it was shortened to just "Aggregate base" (AB). "Asphalt" is always referred to as "Asphalt Concrete" (AC) and "macadam" is a term that is never used here. The term "subbase" is rarely used here, it is generally just referred to as "fill" or "embankment" and once graded to the correct elevation and compacted it becomes the "subgrade".

 

[bjgottlieb]

First, I would like to thank you for all your help. Just for clarification and to help this good discussion, in Western Texas we do not typically use subbase. Our subgrade, which is the layer the base course is laid on, is typically good enough for road construction. If the R-Value is too low then we would do subexcavation and bring in fill material with a higher R-Value. Base course is exactly how BigH and cvg discribed. Hope this helps.