Soil Dessication 3

[dik]

Does anyone have information regarding the change in moisture content over time with soils located within a building?

See thread: thread507-323989

thanks, Dik

 

[Ron]

dik...I read the other post as well. A few thoughts to consider....

Dessication will occur by one or both of two common methods...the first is lack of recharge because of the impervious nature of the construction above. The second is reduction of moisture by vegetation, particularly larger trees as they grow.

The impact of lack of recharge is related to the footprint size of the building. Obviously, the larger the building footprint, the more likely little or no recharge will occur near the center, thus more drying near the center. The lateral permeability of the soil will typically be less than the vertical permeability, so unless there is cross-flow drainage at the near surface level, dessication will occur.

Recharge piping under the building could be considered, assuming the construction occurred when the soil was in a swollen state. If not, recharge piping would likely cause swelling thus engaging the voidform to some degree. Recharge sources could be roof drains or surface drainage. A moisture content monitoring system can be employed to control the amount of recharge.

As for the vegetation, that can be controlled as long as you don't get into a fight with the landscape architect! A chainsaw usually cures that issue!

 

[dik]

The area within the building is well located and the building has a fairly large footprint. The area on the corner is a bit more of a concern, but not overly. I have to prepare a proposal/report for the client on the problem. It was a simple error in construction and I don't think there are any problems. I hadn't considered vegetation issues, although I've considered proper drainage away. We have problems with City sidewalks cracking down the middle... due to dessication on either side and too thin a sidewalk.

I've not been able to obtain information on the dessication within a building although I've seen it and know it happens. I also don't have information on the pressures caused by swelling clays and I suspect this varies with the clay.

Dik

 

[Ron]

dik...agree that the swelling pressures depend on the type and origin of the clay. This can be determined by lab testing. Many of the geotechnical texts discuss clay shrinkage but a quick look through my library did not reveal anything significantly related to your issue other than general statements.

Attached is a BRE paper on shrinkage of clays under low rise buildings. Hopefully something in there you can use.

 

[dik]

Excellent paper, Ron... you don't happen to have part 2 and 3 layin' around?

Dik

 

[Ron]

dik...I have Part 2, attached. Don't have Part 3.

 

[dik]

Thanks very much...

Dik

 

[shobroco]

I can't recall where I saw it but a google search might bring it up; a few years ago we had a very hot dry summer (southern Ontario) and I was dealing with numerous cracked basement walls from dessicated soils (footing subsidence). I found a quite detailed study of a regular seasonal problem in an area of Texas with particularly sensitive soils and there was also a large section dealing with dessication under an existing basement floor as a result of heat in a mechanical room; the centre of the floor was collapsing. It might help, even though I'm not very helpful in locating it!

 

[dik]

Thanks... recall about 8 or 10 years ago about dry conditions in Ontario... and did up a few reports myself...

I submitted my report on the project this AM and it appears that everyone is 'happy' with it... Found out, after the fact, that the reason the Contractor used the plastic void was because 2 prior cardboard attempts had 'soaked up, some liquid sunshine... I had noted in the report that the use of plastic was often to prevent this type of occurrance.

Dik

 

[Ron]

dik...way to go!

 

[fattdad]

horizontal permeability is typically 4 to 10 times greater than vertical permeability in water borne soils. In fills, I'd think they'd be similar.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

You might be interested in reading the opening of Malcolm Bolton's book, "A Guide to Soil Mechanics" where, on the first page, he starts discussing "The Case of the Collapsed Boilerhouse" - The first 17 pages should be required reading for all geotechnical engineers.

 

[dik]

Thanks FatPop... our clays are like a thin horizontal platlet, only a few molecules thick, with a significant electrical charge that helps bond the water molecules. They are not (mostly) random oriented but layed down in layers with seasonal influence over several thousand years (remnants of glacial Lake Agassiz).

I've included in the report that because we haven't experienced drought conditions, it's unlikely that the soil is in a 'pre-shrunken' state.

Is the greater horizontal movement of water to be anticipated for our varved clays (I realise you are not in our area).

Dik

 

[dik]

Thanks Shobroko... I missed that in my report... the added heat within the building... I thought I caught everything.

Dik

 

[fattdad]

This is my summertime desktop photo. It's from Panama and residuum from volcanic tuff. We used this stuff for earthwork (maximum dry density was less than 80 pcf).

Varved clays are more likley to have the 10 fold distinction between horizontal and vertical permeability.

Yeah, I'm in Virginia, but I've worked all over the country, except for Texas 'cause it's too hot!!!!!!

f-d

¡papá gordo ain’t no madre flaca!

 

[dik]

Thanks, FD...

Dik