Plastic Soil? 3

[tractorguy]

I am getting ready to build my own house. I have been told that the probability is high that we will hit "plastic soil", the remedy for which is to overdig and backfill with 1" clean....I have also heard that the correct method is to use 2" white rock?

What is plastic soil? (The correct name) I assume it is some type of swelling clay? and Does the remedy sound right?

Thanks!

 

[Focht3]

Well, one inch of removal and replacement won't do you much good!

I'll bet you meant one foot of removal and replacement with crushed rock fill. Where is your home site located? What is the typical local practice? (Check around - don't rely entirely on your builder for this information.)

"Plastic soil" generally means the soil is predominantly clay - and the more plastic the soil, the more expansive it will likely be. The soil's plasticity is measured by two tests: the Plastic limit and the Liquid limit. The Liquid limit is always greater than the Plastic limit; the numeric difference between them is called the Plasticity Index - often referred to as the soil's PI. (There is no PI test.)

 

[nhyankee2]

Focht3,

The only PI test I know of is graduating from USMC Boot Camp at Paris Island!

Tractorguy,

It is usual in my area to use AASHTO #57 crushed stone as replacement for undercut footings. But, that is generally for more granular material up to and including decrepitating mudstone/siltstone. Clay presents a serious matter is water is allowed to come and go, which will allow it to shrink and swell. Therefore, the course gradation of the #57 stone may not be suitable. A local geotechnical engineer should be hired to investigate the site and possably inspect the footing subgrade prior to concrete placement. That would be my advise...

 

[SirAl]

A 'plastic soil' implies that the soil composition would include clay particles (as Focht3 suggests) that produces a cohesive, puddy-like consistency when moist.

A subcut alone is not necessarily the answer though. To illustrate by example, I was involved recently with a soils investigation to address a heaving slab-on-grade. The area of construction was geologically situated within a clay environment with moisture contents at or below the plastic limit value. The dry conditions persisted to depths of 8 to 10 ft. Part of the problem occurred during construction when the slab subgrade was subcut 1 to 2 ft. and backfilled with a well graded compacted crush gravel (well compacted). The intent was to provide an improved and more stable subgrade. Unfortunately, the deep granulars allowed free water to accumulate, triggering a swelling condition in the clays below. There were other factors that contributed to the severity of the problem but it illustrates a definite risk that could arise under certain circumstances.

nhyankee2 provides good advise. A local geotechnical engineer familiar with the local geology and local practices could provide valuable input.

 

[Focht3]

You clearly need a local consltant - on this we all agree.

SirAl has provided some valuable insight into the potential problems with the removal and replacement technique for dealing with expansive soils in relatively dry environments. This is definitely not "cookie cutter" engineering: proper design in this environment requires the consideration of many factors - some of which aren't particularly obvious.

But I want to be sure that readers of this thread do not get the impression that "remove and replace" is not a good approach - many times it is one of the three best options available: 1) a structurally suspended slab, 2) chemically treating the clay with a high pH solution containing KCl, and 3) removal and replacement. A fourth option, surcharging the site, is also possible. Surcharging alone is only about 50 percent as effective as removal and replacement; that is, you have to raise site grade about 4 feet in order to get the same reduction in swell potential achievable by removing 2 feet of expansive clay and replacing it with an inert material.

Structural slabs are expensive (and cost prohibitive for industrial warehouses); chemical treatment isn't available in much of the country (and there are only two firms I would recommend for this type of work.) That doesn't leave many options -

 

[BigH]

SirAl: Thanks for the case history!!

 

[coneboy]

Hello Tractorguy

In your intial inquiry you mentioned that you will probably encounter some plastic soil when you start digging for your house.

The fact that you hit plastic soil is interesting, but I don't think it is the most important item from a Geotechnical view point. As mentioned in previous responses you might want to retain a Geotech Consultant to solve your problem (if there is a problem?).

I know that it costs extra $ for a Geotech Eng report, but the construction savings will most likely more than make up for it.

A few items that you might think about;

1. It dosen't really matter if the soil is plastic if it has enough strength to support the footings for your house.
What is the soils strength?

2. Will the house settle in the next 2 years? 10 years?
If the soil strength is high enough you should'nt get any settlement. If you are putting in a basement then this process can help to reduce potential settlement.

These questions can all be answered by a Geotech Eng.

Hope all goes well
Coneboy

 

[Focht3]

The original post included the following:

I have been told that the probability is high that we will hit "plastic soil", the remedy for which is to overdig and backfill with 1" clean....I have also heard that the correct method is to use 2" white rock?

Given the problem as stated by [blue]tractorguy[/blue], I sincerely doubt that settlement is much of an issue at the site in question. Having said that, [blue]tractorguy[/blue] should retain a local geotechnical engineer to address all of these questions -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[sc]

Settlement isn't the only problem with "plastic soils" (read clays). The main problem that I have come across over time is shrinking (appears the same as minor settlement) or swelling. Clay in general reacts to moisture changes by changes in volume. The main reason that stiffened raft slabs are used in Australia is to restrict the effect of the variations in soil shrink/swell.

My experience tells me that it is near impossible to prevent soil moisture related movement, hence the need to limit the effect.

To me the excavation/replacement method would only be effective providing the soil moisture changes occur at a reasonably shallow depth (say <1.00m). I am wrong in this assumption?

regards
sc

 

[Focht3]

Well, our active zone can reach 6.5m (I've found 15mm wide vertical cracks that deep), but the problem zone seems to be the upper 1m to 2.5m. We have used removal & replacement as deep as 3m, but that's out of the ordinary.

You're right - the principal problem with highly plastic clays is shrink/swell, with swell dominant in the OC soils.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[sc]

Damm glad I work in Australia where 3-4m active depth is generally the worst case for residential buildings that needs to be considered.


regards
sc

 

[Focht3]

Perhaps now you can understand why Terzaghi & Peck cited San Antonio as an area with well-known expansive clay soils - and shrink/swell problems.



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[shreve17]

Now that you have all helped tractorguy, I'm hoping for some advice on the best approach to final grading my lot (just built a new house on it) which is completely covered with at least 12&quot; plastic soil and it has rained quite a bit making a tough. I have been waiting for a few consecutive days of sun to dry it out so the bobcat could work it but I haven't had any luck. I get one, maybe two days of sun but it's not enough to dry it out and make it workable (with a rubber tire machine, at least, which keeps getting stuck and sinking in areas where water has sat). I'm not looking for anything structural, just a way to work it to final greade and lay sod on it. Are there any additives that would help this process? Would a track machine do a better job?

I'm also thinking about mixing topsoil in with it to help the grass grow. Should I do that during final grading to mix it in with the clay or final grade and spread the topsoil on top?

Thanks