Effect of Moisture Content on Soils

[cbear1]

I often see the following excerpt in soils reports:

"Since water has a significant influence on soil, the natural moisture content provides an indicator of the soil's compressibility, strength, and potential expansion characteristics."

I never see any references to moisture correlations in the subsequent analysis, and am wondering where these characteristics come in to play. Does anyone have some resources where I can see how the moisture content affects the characteristics listed above, or is it just based on "rules of thumb"? Also, these correlations would never be accurate enough to substitute for testing, correct?

Thanks so much for your help.

 

[fattdad]

Natural moisture content is only relavent if you know other things about the soil. Natural moisture content with respect to the Liquid Limt or Plastic Limit; natural moisture content with respect to the "Optimum" moisture content; natural moisture content with respect to the ASTM soil description, etc. If I have a fat clay with a natural moisture content of 140 percent, I'd consider whether the soil is from a marine deposit and sensitive. I'd also consider to what extent there may be colloidal organic content. If I had a peat with a natural moisture content of 140 percent, I'd consider the affects of secondary compression.

Consider the following: If you have a fat clay with a natural moisture content of 10 percent, what information can you discern? If it began its life as a muddy ooze and dried out, it may have a moisture content of 10 percent. If it was an overconsolidated clay layer and dried out it may also have a natural moisture content of 10 percent. They will behave completly different however when re-hydrated.

It is incorrect to assume that rehydration of soils leads to loss of strength. It is correct to ask whether it is possible however.

f-d

¡papá gordo ain’t no madre flaca!

 

[msucog]

we have a nasty little band of soils here in the piedmont where it inherently has high void ratios and high moisture contents (40-50+%) relative to your typical piedmont residuum (~20%). expansion potential would low/zero. compressibility very high. strength depends partly on if the material is dried back and compacted (lowering the void ratio) but it's terrible for roads because it's often micaceous silt and just sucks as subgrade material (seen CBR=3 on this stuff).
this doesn't answers all your questions but thought i'd add a few comments to see if it might tie something together. the compressibility and strength apply to this stuff but expansion does not.

 

[BigH]

I'd be interested in who puts statements like the one quoted in a soils report. Never has been in any that I or any in my firm(s) ever did. Mmmm fattdad and msucog gave good comments. The one thing that is sometimes overlooked is that high moisture contents might not be indicative of the soil itself but of the sampling and bagging of the soil in the field by the driller (hope not) or the field engineer/technician (hope so). In wash boring especially, you many times will get a very saturated and 'disturbed' sample at the top of the recovered material in a split spoon or thin walled steel tube. This is not indicative of the soil itself but of "slop" from the drilling. If not removed, it can give false impressions after squeezing around inside the sample bag or jar. If the lab then does not take this into account in selecting the specimen, you can get some pretty wierd results. (sometimes you see "free water" in a jar of sand coming in from the field).

 

[cbear1]

Thanks for all the comments. Obviously I cannot divulge which firms are putting this statement in their reports, but there are several in my area. Usually this is accompanied by some borings, blow count data, penetrometer readings and just about no testing other than moisture content - that is why I inquired as to the significance of the moisture content, because they seem to be performing more qualitative than quantitative studies...which I guess brings me to another question: when is more testing needed?

It seems like the consultants I work with prefer to correlate blow counts with strength instead of performing shear tests and take Atterberg limits over expansivity testing - there was a post awhile back in the Soil Testing forum that asked whether professionals were performing fewer tests in general these days and the consensus seemed to be 'yes' - any thoughts on when more should be requested?

 

[BigH]

With respect to correlating blowcounts to strength - this is a time "honoured" tradition - especially for smaller jobs and in areas in which the engineer has expertise. Agree that for more complex projects where better soils data will lead to significantly reduced costs, then you would do more elaborate laboratory testing and also use in situ tests such as the piezocone or dilatometer. The geotech in your area, if experienced and well respected, should be the guide to lead you on the sophistication that one should bring to a particular project with respect to the geotechnical investigation. (P.S. wasn't implying that you name "names" - it would mean nothing to me anyway - it was more a rhetorical thought.)

 

[cbear1]

Thanks for your input, BigH. As a Civil Engineer who has not specialized in Geotechnical, it is difficult to know when to trust the soils engineer versus asking for more tests (what I learned in school..). Also, there are certain tests that are required by the Unified Building Code for foundation design (ie: Expansiveness must be determined by an expansion index test versus Atterberg limits - only one firm in the area has the equipment for this test and it is pretty expensive - kind of puts one in a tough spot..)

I guess time will help me get a feel for what I need in these reports. Thanks again for everyone's help - this is a great forum.