Soft Soils & Expansive Clay Causing Problems?

[SlideRuleEra]

I have taken on a potentially tricky project that three other engineers have withdrawn from. I would welcome discussion on the results to date.

The Client owns a 30 year old, two-story wood-framed, brick veneer residence. It is located within 75 yards of a tidal salt marsh creek. The house has experienced dramatic differential "movement" over its lifetime. Movement seems to be continuing today. For example:

1. The full-height brick chimney on the east side of the house has separated from the rest of the house and is leaning east.

2. A large covered screen porch on the south side has separated and is leaning south. A portion of the elevated wood-framed floor has buckled and heaved upward about 4 inches since I took on this project a couple of months ago. It has since "settled down", and is currently only about 1 inch high.

3. There are fresh cracks in the interior drywall,doors that no longer close, and floor irregularities throughout the house. There are numerous cracks in the brick work on all sides of the house.

To my surprise, none of the three previous engineers had any soil borings performed. So, I have had three slit-spoon samples taken at spaced locations around the house - per the Geotech's recommendations, one is 50 ft. deep, the other two are 20 feet deep. All three show essentially the same profile. Here is a rough description:

"N" values starting at 25 near the surface and decreasing to N = 4 at 8 feet below the surface. From 8 feet to about 28 feet "N" values were in the 1 to 4 range. The calcareous clay (locally called Cooper Marl) starts at 28 feet deep. This normally considered to be the significant bearing layer in the SC coastal regions.

The geotech report also says that the soils in the upper layers, from 2 feet deep to 8 feet deep are expansive clays.
The report does not identify an exact mechanism for the strange behavior of the house.

Could the combination of the upper expansive soils swelling and shrinking over time, combined with deeper very soft soils possibly allowing permanent settlement result in this continuing (and permanent) differential movement?

Other comments welcome, too.

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[BigH]

SRE: What is the tree situation like? Any large trees nearby? Or, perhaps older trees that have recently been removed? (see the recent thread on the effects of trees on structures). Do you have any soil tests done on the upper clays (Atterbergs, natural water contents)? India has a test they run called free swell - I'll try to find the specs for you. They have a lot of expansive clay there and use this test as a means of determining if the materials can be used for embankment fill. Is the clay at 10 ft and say 15 ft the "same" (other than N values) as at 8 ft and 6 ft? Seems like a desiccated crust to me. See if you can provide this info and I'll do some headscratching.

 

[SlideRuleEra]

BigH - There are trees, but this does not seem to be the cause of the problems.

The soils in the 2 ft. to 8 ft. depths are inorganic clay with sand (CH).

Then the soil changes to "very loose fine to medium sand with silt (SP-SM)". This goes from depths of 8 ft. to 15 ft., "N" for this region is 1 or 2.

From 15 ft. to 28 ft. there are various soft layers of inorganic clay or sand with silt. "N" in the 2 to 4 range.

So far, we have not perfomed any tests on the soil,everything has been visual. The lab has offered to run tests. At this time I am trying to come up with some logical reason for what is happening to determine if the expense of further investigation is justifed. There are a lot of other issues, such as long-term damage from water leakage - both rain water and sewage from pipe joints that separated because of the movement. Saving the house may not be cost effective. The Client is willing to accept this outcome if that is what the facts determine.

My wife, who works with me, has suggested to the Client to call an Exorcist.

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[SlideRuleEra]

The water table, at the time of the borings was reported at 7.1 feet below the surface. Steady-state is more likely to be a foot or two, or so below the surface.

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[emmgjld]

This sounds very familiar. My experience is probably more extreme due to arid environment, even though the majority of clay soil is low expansive Silty Clay (CL-ML).

I deal with dessicated surface soils with water tables from 4' to 10' deep. If the lower soils are sands, the 'capillary rise' is restricted and the upper crust behaves according to precipitation, daily/seasonal evapo-transpiration rates and landscape irrigation applications.

I have found that some very moist clays can undergo quite a bit of cyclic shrink/swell, even though the soil moisture changes are quite low. My interpretation is when the soils are at or right above the Shrinkage Limit (SL) some clays can experience quite a bit of volume change, and more expansive pressure than seems normal or logical.

I often use a Soil PVC Meter [Potential Volume Change] for soil swell evaluation. In this case I would use either a number of soil specimens, hopefully over a reasonable range of moisture contents or construct a number of remolded specimens, again over a range of moisture contents, trying to hold the dry density within a fairly tight range.

The original work with this apparatus is documented in "THE CHARACTER AND IDENTIFICATION OF EXPANSIVE SOILS" T.W. Lambe, 1960, Federal Housing Administration. I have the original document and ELE International (old SoilTest) sells the apparatus (SOIL VOLUME CHANGE METER, Model: 25-0390 (C-260)) and has a document titled 25-0390(C-260).pdf .

 

[VAD]

SlideRuleEra:

Seems like this one is for Sherlock and Dr Watson or as your wife said an Exorcist. Anyhow, it is imperative that you run the moisture contents and a few Atterberg limits. Generally the mineralogy of the soil constituents is important to know to decide whether the soil is potentially expansive. Local geology inf can help. Another thing I notices that the house is near asalt marsh. It is known that if the soil in the house area has been deposited in such material the structure may be such that it can collapse when infiltrated by rainfall water. You may want to check on this hunch as well.

Assuming that it is expansive soil versus collapsible soil I would have expected large pressures on foundation elements and perhaps much damage. Hence it is necessary to know the potentail expnsiveness of the soil. As well while it is true that dry and wet spells can result in cyclic movements of the soil, it is also known in some cases that once the soil has absorbed sufficient moisture and reached an equilibrium condition then the swell propensity lessens. Keeping such areas mist can in fact be a solution of some sorts. Anyhow these are rough thoughts and each site has its peculiarities that must be determined.

You face a difficult problem especially after such an extended period and will have to spend moretime that you will be compensated for on this issue, but it is fun for geotech forensic experts.

Sitting with the owner and getting the history would be important as well. I have in the past been involved with some of these and you do have to scratch your head.

My gut feeling is that your problem may not be expansive soil but its only a guess. You should compare soil moisture conditions away from the house as well to track any differences in stratigraphy and moisture at corresponding depths. AS you delve further, additional testing will be required and costs will go up. You need to discuss the ramifications of the investigation and possibly inconclusive results with the homeowner as soon as possible.

Regards and Best of Luck

 

[fattdad]

This does seem like a "problem" problem. For new construction we see these type of shrink-swell movements occurring in the first few years and they are often related to clearing, landscaping and the changes in moisture contents associated with residential development. As a shrink-swell concern, we (the engineering community in Central Virginia) feel that these dynamics take place in the first few feet (i.e., within 5 ft or so), where swings in moisture content occur during the year. Under this scenario, the soft soils at depth are not likely to be the culprit. To mitigate these concerns, we often recommend that the foundations bear at the depth of 4 ft or more (and the chimney foundation also is set at this depth.

Regarding the soft soil layer, I'm just wondering how this could be a contributing factor and all I can do is use my imagination. Everything I conjer up is pretty far fetched as I'd imagine that any settlement related to new loads conveyed to these depths would be small and would have occurred much earlier. With the ground water table at 6 or 7 ft, I wouldn't think that there'd be much concern on drying or wetting cycles on these lower soils. I also wouldn't think them to be "underconsolidated" unless the original construction included a thick fill. But even then, the settlements related to an areal fill would have attenuated over the last 30 years (or so it would seem).

I'd do Atterberg limits for the CH. I'd do Atterberg limits and natural moisture content for the low blow clays. I'd prepare the client for the need to do some followup work over time.

Not sure whether helical underpinning of the foundations would help, but they're used in Central Virginia for this purpose.

Just some random thoughts.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

Hi: Just for the heck of it compute the volume change in a given depth of the upper clay layer for one percent of moisture change.

I recall doing that on a job where the moisture changes were small and it really opened my eyes to what results.

In my experience more often than not the trees are the fault.

 

[SlideRuleEra]

Thanks to each of you for your comments to date. This weekend I will put together an analysis of the situation, as it now stands. Scheduled to meet with the Client on Monday to formally discuss our next step. Over the last few weeks I have pointed out to them that this next step could be anything from more testing to demolition of the house.

Based on your comments, will certainly cover the potential of tree root damage. Also will address the expansive soil.

The previous engineers on this project had considered helical underpinning, but I agree that it may be of limited value.

Will let you know what happens next. Thanks again.

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[BigH]

SRE - was thinking of a section in Malcolm Bolton's book on moisture change and effects - I'll find it and forward to you. If you think it is good one, you might post it on your site for the rest of the gang. Cheers

 

[oldestguy]

SlideRule Era:

I trust that was a slip of the finger saying "tree root damage". The damage I find from trees is moisture changes due to trees sucking out water, not root growth effects.

One job I was initally perplexed about was sideways movement of parts of the house, with lesser vertical differentials. Soil was weathered loess (silty clay) on top of weathered shallow dolomite bedrock. House was an old mansion where no reent activity took place changing things. The movement was towards two very large pine trees, pulling the house apart. I see that shrinkage can go sideways as well as down, when you see cracks open up. Before I got there the structural engineers visiting the site were completely without an answer. Upon seeing the big trees and the direction of movement, on the level, there wasn't any question about the cause.

I believe one of the Canadian fellows BigH recently quoted in another thread once wrote that a typical elm tree on a hot day can draw up 50 gallons of water a day, if available. It may have been "barrels" rather than "gallons", but memory is vague on that. I've seen even small trees do a lot of water removal.

One job was a high school gym. Taking elevations of the floor showed circularly shaped contours, centered at a corner, the location of an elm tree.

I've solved some of these by adding water and bringing the settled zones back up to usable levels, and removing the tree.

In summary, look first at the trees.

 

[SlideRuleEra]

oldestguy - Thanks for the clarification. I had not considered the trees drinking water as a drying mechanism. Will be certain to explain that to the Client.

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[BigH]

SRE - here are the references I had before. There was a good one from UK - seems to be a "hit topic" there about water and trees. Refs on other computer - will find.

http://irc.nrc-cnrc.gc.ca/pubs/cbd/cbd184_e.html

http://www.lga.sa.gov.au/webdata/re...ban_Environment_-_2005_Report_-_Section_1.pdf

http://www.golder.com/Archive/AssessingConstruction.pdf

 

[oldestguy]

BigH now you have me tryng to recall something I remember as to number of gallons a day a big elm tree will drink. Bozozuk was the fella who mentioned it probably in a Geotechinque article.

I find that Bozuzuk and Burn sure wrote a lot of articles on many subjects, but I am unable to find the issue of Geotechnique of March 1960 where I think I read it back about 1963.

I found this article that is based uopn the Geotechnique article I think, but it does not mention quantity of water per day sucked up by a tree. When I was first involved with shrinkage and settled buildings their work was real helpful.

http://irc.nrc-cnrc.gc.ca/pubs/rp/rp99/rp99.pdf

Any ideas?

By the way, I believe I was the first engineer (grad student) to use the neutron moisture meter for monitoring soil moisture changes at depth. It was first conceived by the physicists at Cornell about 1952. Imagine taking a lab slow neutron counter out on projects powered by a portable generator. I used the original device they "invented". It was crude, but it worked.

 

[oldestguy]

Some results found:

http://www.na.fs.fed.us/spfo/pubs/uf/lab_exercises/comparing_sun_leaves.htm

This article says a mature oak neeeds about 50 gallons per day.

Thus, I suspect the reference I recall was in the gallon category, not barrels, for elms.

In my current searching I found that one group of researchers determined that there is the possibility of getting some ground water elevation reduction using trees to take up the water, a biological pumping system I suppose.

 

[SlideRuleEra]

Met with the Client, for overall economic reasons recommended that they sell the property "as-is", with full disclosure of existing conditions - the location itself is very desirable and should have significant value. Will see what happens next.

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[VAD]

Interesting. I trust that full disclosure includes that a number of engineers had looked at the problem but could not determine a solution. In my opinion, unless a buyer wishes to use the land for other than a dwelling it would be a hard sell.

 

[SlideRuleEra]

VAD - The geotechnical issues are only a part of the problem. For example, when the cost of repairs to a house exceeds 50% of its value (as would be the case here) the house is required to be brought into compliance with the current building code. Since the house is located in a hurricane flood zone this entire brick veneer house (currently on a 3 ft. high crawl space) would have to the jacked up an additional 8 feet to allow for the storm surge specified in the current code. Needless to say, this would add even more expense, and result in a very odd looking house.

Driving piling for a properly designed new home on this property would be a reasonable solution to both the storm surge requirement and the expansive soils. The Client is just not interested in moving out for several months, doing the work, then moving back in - either for repairs or a new house. Potential buyers would not have this concern.

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[BigH]

oldestguy - I have the 1960 Bozozuk paper. Have an addy I can send to? Anyone else want, let me know.
Cheers

 

[SlideRuleEra]

BigH kindly sent me the Bozozuk paper and the publication on "Vertical Ground Movements Near Elm Trees". Both have been added to the "Foundations By Terzaghi Library" at this link (both are at the top of the page)

http://www.slideruleera.net/FBT-Library-index.html

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