Slab on grade on expansive-shrinking clays

[Mccoy]

Hi all,
I may be involved in a project of a pretty large industrial building. An issue which I'm not familiar with is going to be design of base/subbase and improvement of subgrade, which is several meters of clayey soil which shrinks and expands with variations in moisture. Basic practical points: Replace or not replace with engineered subgrade, thickness of replacement, replace or stabilize chemically, tests to carry out on natural subgrade, how to avoid shrinkage and expansion issues. Any reference practical material and codes on such subjects? I'm asking this before spending hours in searching the whole eng-tips forum!

http://www.mccoy.it

 

[SlideRuleEra]

Mccoy - See U.S. Army Technical Manual (TM) 5-818-7.

Since there are several meters of problem soil and the project is for an industrial project, piling may be the "best" solution. Not necessarily the lowest cost, but a high level of confidence in success. Also, probably the quickest, least weather-dependent approach and a big help in maintaining the construction schedule. I would not be surprised is keeping on schedule is a high priority (high $$$ value) for an industrial project.

If replacement soil is considered, get an idea of availability of suitable material, the cost and transportation (including the haul distance).

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[GeoPaveTraffic]

My experience is with expansive soils is limited to the St. Louis, MO (USA) area; so take this with a big grain of salt.

My approach is to look at the Atterburg Limits, natural water content, and the amount of acceptable movement. If the liquid limit and the plasticity index are moderately active, say LL<65 and PI<35 and 0.25 inch or so of slab movement is acceptable, then I generally recommend a moderate remove and replace option. Something on the order of 2 feet of removal with a lean clay replacement soil. I don't like to use crushed stone or sand for the replacement. Reason being that those materials can trap water resulting in even more expansion.

If the soil is more active or the allowable slab movement is lower, then I increase the amount of removal and replacement. Maximum that I can recall is 5 feet.

I'm not a fan of chemical modification, but I know some have had great luck with it.

Lastly, I used a waffle slab on one project. Site had a history of a lot of expansion (shaley clay soil). Been in for over 10 years without any problems. I can post more info if you are interested, but a lot of work to design and install. You got to have really accurate load information on the slab.

Mike Lambert

 

[Mccoy]

Thanks guys for the info, but sorry, I've been imprecise, in that the issue regards the industrial pavement or non-structural slab on grade.

Most probably foundations will be deep ones, but any damage to the industrial pavement, in those conditions, may end up in high damage claims, above the US$ 130000 which is the max insurance threshold available for interruption of industrial activity.

Some of the suggestions contained in the US army tech manual are still valid, as well as the tips given by geopavetraffic, but probably that would be a real overkill. So, are there any pavement-specific procedures? I found a lot of material on road pavements, whereas pavements of industrial and commercial buildings are not very much discussed, except maybe the heavy-duty pavements.

It might be that it constitutes a non-issue, or a quasi-issue...

http://www.mccoy.it

 

[SlideRuleEra]

Mccoy - Our electric generating stations are typically on really poor soil (10 meters, or so thick), with high water table (less than 1 meter below the surface). However, we do not have expansive soil. Concrete paved areas for use by heavy equipment are 0.3 meter thick slabs with reinforcing each way, top and bottom. Also, have doweled joints. Below the slabs, remove and replace at least 0.6 meters of subgrade and replace with suitable material. In the worst areas, remove and replace 1.2 meters and use an appropriate geotextile fabric.

Around the power block (boiler, turbine, precipitator, scrubber, chimney, etc.) at or stations, anticipate a large truck crane, with outriggers deployed, may have to be used at some time in the future. Concrete paving needs to be substantial.

At the same time, have to keep in mind that their is buried piping which will require maintenance and modification in the future. Don't want make access to the piping too difficult.

For roads, we remove / replace and use locally available crushed limestone (ROC) which is similar to Fossilized Aggregate Base Course (FLBC). Keep adding material has the roads settle and compact from routine use. Selected roads are paved a year or two after being put into service. The most heavily loaded roads remain ROC, permanently.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[GeoPaveTraffic]

If the downside risk of the pavement moving is that large, then I would still go with a remove and replace option. Still would go with lean clay topped with a normal amount of crushed stone base.

How bad is this soil? What to the limits look like?

Mike Lambert

 

[Mccoy]

Presently there are two borings available, drilled to -21 m of depth (-63 ft) from ground surface plus various cone penetration tests.

Soil is silty clay with a few organic layers up to the above depth. Water table was about @ -6 m (-18 ft) in October (rainy but not so much), conspicuos fluctuations are expected.

A representative lab test on a sample taken a 1.5 to 2 m of depth (4.5 to 6 ft):

water content: 22%
Density: 1.92 grams/cubic cm (0.07 pci)
void index e= 0.74
Water Saturation: 81%
Liquid limit LL= 65%
Plasticity limit PL=23%
PI= 42%
CI= 1.02

Lab vane test: Su = 1.6 tsf
Clay 60%
silt 39%
gravel 1%

Goes thru 200 ASTM sieve 99% of soil

No limits profiles nor heave/swelling tests so far although they are probably going to be planned.

The crucial issue will be, in case of replacement, to provide thickness of natural soil to remove/replace and type of replacement. Chemical stabilization or use of geosynthetics may be an option but a last resort one since there is no local experience with those methods.
Design of pavement will be carried out by the structural engineers.
As far as the thickness goes, I saw that you guys pretty much agreed upon the worst case scenario (3.6 to 5 feet of replacement).

In the meanwhile I tried some google search, books on expansive soils tend to deal specifically with foundations (quite a few Indian authors), although there is one which seems to target pavements as well (hopefully not only road pavements). US$ 120 here, prior to purchase I'm going to request a written confirmation for my consultancy, LOL.

http://www.mccoy.it

 

[GeoPaveTraffic]

Soil looks to be pretty active. I would expect quite a bit of volume change with changes in moisture content.

If small movements are not acceptable, I would recommend something like 3 feet (1 meter) of remove and replace with lean clay. Not a fan of geosynthetics in this situation.

If the client doesn't want to spend the money, then point out how the soil could move with time ask if that is acceptable. As I'm sure you have seen, that can lead to some serious head scratching by the client.

good luck.

Mike Lambert

 

[SlideRuleEra]

Mccoy - Here is a link to used hard back copies of the book you located. Even when shipped to Italy, the prices begin in the $25 range:

"Expansive Soils: Problems and Practice in Foundation and Pavement Engineering" by Nelson and Miller

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Mccoy]

Thanks very much GPT and SRE (and yes, what the heck, I'm going to order that book used, a good title to have in the library!)

http://www.mccoy.it

 

[oldestguy]

Missing in the discussion so far is "What about maintaining constant moisture?". In my experience the main problems are from trees taking out the water. Otherwise movements usually can be lived with, but extra items such as reinforcing the structure to bridge or even out movements. Sealing surfaces also helps. Direct surface waters from roofs and rainfall away from the area of concern. Work with the inherent nature of the clay hold onto to water. Substituting shallow layers with granular material so capillary action is more reduced also might be considered.

On two jobs the situation was resolved by leaving trees in place, but dosing the area daily with water to overcome loss to trees that remained. Worked perfectly.

 

[Ron]

McCoy....the most prominent area of dealing with expansive clays and slabs on grade in the US is Texas. In many cases, the SOG is post-tensioned (for you Europeans...pre-stressed).

If you can provide a sand buffer of at least a meter, that will help.

 

[emmgjld]

McCoy; Nelsons book provides more of a review of a lot of papers and does not present any direct solutions. The biggest issue for pavements (& structures) is long-term moisture control. The review of some pavement field studies brings this point home. Nelson, being in Colorado, made use of several Colorado Highway Dept. (CDOT) studies. The Texas pavement experiences may provide more detailed advice for your use.

One of the listed CDOT pavement studies I was involved with in a minor degree, was the I-70 at Clifton experimental pavement sections. As I live in this semi-arid to near arid part of Colorado, I have kept tract of this & other local experimental pavements. Long-term Soil Moisture control is key.

My own experience with Overexcavation, up to 3-1/2 feet (about 1 meter), has been mixed, with some total heaves still reaching 5 inches and differential heaves within the project of about 3 inches in 100 feet being experienced. I believe the geology in my area is the largest issue as minor changes in clays, in-place density & subgrade permeability can play havoc with design assumptions.

 

[Mccoy]

@ OG: yes, thanks for reminding about constant moisture, trees, rainwater, all those issues must be addressed with specific suggestions in the report, even though they are probably going to influence the edges of the non-structural SOG mainly, since the pavement is going to be a wide one. Groundwater oscillations is one factor which may influence the SOG in its whole area.

@ Ron: good tip about Texas, that reminds me of Focht3, the very experienced Texan engineer who used to post in the main forum, going to try and retrieve some of his stuff if possible.

@emmjld: thanks for pointing out some important relevant aspects. Since there seems to be not very much specific material on common industrial pavements, I'm going to revise the material on roads, airport tracks, heavy duty pavements. What I would need, besides suggesting all possible measures against pavement damage, is a specific, published procedure (or more procedures) which would constitute a solid reference in case of future litigation.

http://www.mccoy.it

 

[geotechguy1]

In my area we have these clays all over the place. Generally slabs are constructed on a 150-300mm pad of gravel. We recommend that they carefully control drainage around the building, take measures to avoid things that would cause the clay to dry out (eg. having a heating duct running right next to it or something), avoid trees near to the building. In addition, we frequently require contractors to 'water the clay' during construction if it is going to be exposed to hot weather for extended periods. We also provide extensive CYA warning language that some degree of movement is expected, and that unless they are willing to dig out 10 feet of clay and replace it with gravel, they will have to accept that risk. Since these soils are everywhere in my area, essentially no one sub-excavates and replaces. Especially since gravel seems to be worth as much as gold around here.

Are swelling clays common in your area? That may impact your path forward. In my area the local practice lets us get away with doing things as I've described - if we didn't no one would be able to build anything ever.

 

[Mccoy]

Hi geotechguy,
thanks for chiming in, swelling clays in my area are not common and when we come across them basically we proceed empirically as you describe, usually overexcavating and replacing natural soil with crushed rock or gravel. Plus all the other recommendations. I reckon such material is less expensive here, with mountain quarries close by. So far no probs. Good idea about the ebxtensive CYA warning language.

The project I described is in another area which I do not know well and in this specific case the builder has some peculiar ways to proceed.

My colleaugues also called me a couple of days ago and told me that the job is canceled for some reasons, which they explained to me but I didn't understand completely.

Presently the construction arena is in a big slump in Italy and competition is fierce, with prices falling. Sometimes it is even advisable to turn a job down, if the risk of credit management is not acceptable.

http://www.mccoy.it