Gypsum within clay soils below foundation

[eric1037]

Hi all!

I currently have two projects about 1 mile from each other in which we encountered isolated, gravel-size gypsum pieces with a clay matrix.

I know gypsum is relatively common in the rock in the area, but I have some concerns and I would like to hash them out on this forum to see if they are founded.

First, I have a concern that if the gypsum is exposed to water, it will dissolve and create voids. However, since this is a clay matrix with no visible fissures, etc. and the gypsum was encountered below about 10 feet. I'm not sure if I should be concerned.

Second, I seem to recall that gypsum, if dissolved, can actually break down the bonds within the clay structure. This may not be a problem if the gypsum doesn't dissolve.

Third, I seem to recall reading somewhere that the presence of gypsum may be an indicator of expansive clays.

Since I am in Michigan, and expansive clays are not generally a problem, I am not overly concerned about this.

To shed some light on the types of projects:

One of the projects is a lift station. We encountered the gypsum about 5 feet below the proposed bottom of the lift station.

The second project is a building. We encountered the gypsum about 4 feet below the proposed bottom of footing elevation.

I guess I just want a reality check. Currently, I am not overly concerned about it and I will probably recommend that they keep the soils free of water during and upon completion of construction. I just want some input from others that may have experience with similar soils.

Thanks in advance!

 

[emmgjld]

I deal with gypsum in soils on an everyday basis. Hopefully you don't have to deal with desiccated soils and gypsum cementation.
Please describe your soil profile (UCS, density, moisture %), amount of gypsum, is the gypsum also present as a 'caliche', water table (if known), and the real possibility of surface & near surface water migrating through the foundation soil.

 

[eric1037]

Soil profile consists of the following:

0-3' Very Stiff Brown Sandy Clay fill

3'-12.5' Very Stiff Brown Sandy Clay. Shear Strength 2.5 to 3.0 tsf. Moisture content 11.0 to 12.0%

12.5'-17.5' Very loose, gray, fine Sand. N-value of 3.

17.5'-22.0' Hard, gray, sandy Clay, trace gravel. Shear Strength 4.0 tsf

22.0'-32.0' Stiff gray, silty clay, occasional gypsum fragments. Shear Strength 2.0 tsf. Moisture content 22.2%

32.0'-35.0' Stiff gray, silty clay, with gypsum layers. Shear Strength 2.0 tsf.

Groundwater during drilling was recorded at 13.0 ft. I think it is perched in the fine sand layer.

I don't really think caliche-type materials are present. I wouldn't really expect them in this area anyway.

Since much of the soil is clay, I wouldn't expect there to be much migration after the lift station is constructed. However,during construction there is a possibility of water getting down to the gypsum.

The design bottom of the lift station is located at 30 feet.

Thanks for the assistance!

 

[fndn]

Good call to check your concerns. Below is alink I found in trying to learn more of gypseforous soils. By the way have you checked for heave calculations?

http://www.fao.org/documents/show_cdr.asp?url_file=/docrep/t0323e/t0323e03.htm

 

[emmgjld]

fndn
Excellent link!!! I am printing it out so I can study it tonight.
eric1037
I will return to your question later, as I have a few clients who think I should get onto their problems.

The Soil Scientists (I am very fortunate to have an 'old' NRCS man in my hire) can be a tremendous resource. While they are growing things and we are not, a tremendous amount of information applicable to Geotechnical Engineering can be had. All it takes is a little interpretation and asking a few questions. 30 to 50 years ago, the 2 fields talked to one-another. We need to regain the familiarity.

 

[soiledup]

Be aware that gypsum is a polymorph of selenite (and Vica versa) that has been partially attributed to sulphate attack on concrete in the UK. Read Building Reaserch Establishment (BRE Special Digest 1 ' Concrete in Aggressive Gound' 3rd edition 2005)