USCS Organic soil classes

[jumikas]

I have question about USCS, about organic soils (OH and OL). Does anybody know how big must be the organic material percentage do classify the soil as organic one, OH or OL?

 

[dgillette]

Good question. I looked a couple of places and only found loose definitions like "...sufficient organic content to influence the soil properties." Sort of like the Supreme Court justice's (Felix Frankfurter??) description of pornography: "I can't define it, but I know it when I see it."

Another definition "...its liquid limit value after oven drying is less than 75 percent of its liquid limit value before oven drying," to distinguish it from CL, CH, ML, or MH. That can be influenced by mineralogy as well, especially with tropical soils, which I know very little about.

 

[oldestguy]

An extensive study at the Wisconsin DOT for establishng the current method of classifying roadway soils for construction and pavement design by Robert Keyser in the 1960's found the following, quoted from the WDOT "Soils Manual":

"Organic silts and Clays"; "Organic content ranges from four to twelve percent". Organic content was determined by Loss on Ignition.

There also is a chart in the basic WDOT publcation showing relationship between Liquid Limit and Loss on Ignition percentage of oven dry weight. In general, the organic clays are shown with liquid limits from 15 to 80, with sedimentary peat LL from 80 to as high as 420. Loss on Ignition for the organic clays also is in the range of 2 to 13 on this plot with perhaps 25 plots between 4 and 13. The sedimentary peat loss on ignition plots are shown from 6 to 34, with the bulk of the plots from 10 to 25.

This would suggest the change from "orgainc soil" to peat is about 10 to 12 for loss on ignition. Keyser indicated that the better way to identify the difference between organic clay and sedimentary peat was with the LL test. So his 80 LL figure compares well with previous tip above at 75 by dqillette.

So, in general the statement above would seem correct for the USCS system because the Wisconsin DOT system of soils mapping is based upon the pedologic mapping methds.

It is known that Dr. Keyser had full cooperation of the US Dept of Agriculture Soil Conservation Service when he did his extensive study.

Be careful using Loss on Ignition at the low end for calling a soil organic, since heating that hot can drive off other components. Almost any soil will show some loss in this test.

 

[bmike]

ASTM D2487 is pretty clear on this. The principle question to be answered is, "Does the presence of organic matter influence the liquid limit?"

If the soil is inorganic, the liquid limit should be relatively insensitive to whether the liquid limit sample is prepared by air or oven drying.

Typically a liquid limit specimen should be dried at "low" temperatures" (see D4318).

D2487 tells us that if the soil has a dark colour and an organic odour when moist and warm, a second liquid limit test should be performed on an oven-dried (at "high" temperature specimen (D2217 Procedure B).

If the liquid limit of the oven dried specimen is <75% of the air dried one, it should be classified as an organic soil.

Hope this helps.