specific gravity test

Since the average range of specific gravity for soils is from 2.6 to 2.8, why is this test still requested? Especially, when you can determine visually (or by partical analysis) that the soil is mostly clay or mostly sand.

 

[emmgjld]

The first point is as you said, MOST SOILs, some very interesting surprises have been avoided by 'unusual answers'.

The other point is that a large number of calculations really do require an accurate determination of Specific Gravity.

I deal with Silty Clays (CL-ML)mixed with Silts and Lean Clays, Variable amounts of sand. It is not obvious what soil types I am dealing with. These soils also have a slight to very large amount of Soluable Sulfate Salts. The Specific Gravity is very important in my analysis.

 

[Ron]

I agree with EMMGJLD that the specific gravity makes a difference in many computations; however, the most difficult problem is an accurate determination of the specific gravity. I maintain that the specific gravity can be "guessed" about as accurately as the test can be run.

 

[emmgjld]

Ron brings up a very good point. Proper lab technique is crucial but, is often times lacking. I have found that the ASTM method works well for sands and gravels but becomes difficult to vague with clayey silts and near impossible with more plastic clays.

As I mentioned above, the addition of significant amounts of sulfate (gypsum) makes for some very interesting answers. Volcanics and some ores can throw things way off.

For most commen (non-specification) applications I use the Shrinkage Limit calculations for plastic soils. I have also used several methods of back calculating to 'guess' the Specific Gravity I do not believe that most computations require a Specific Gravity closer than 0.03 (resulting in a sequence of 2.60, 2.63, 2.66 ....) but enough check tests should be made to be reasonably sure that strange soils or materials are not being encountered and ignored.

 

[rasika]

SPECIFIC GRAVITY IS NOT ALWAYS BETWEEN 2.6 TO2.8 . IT MAY VARY ACCORDING TO THE PARTICLE SIZE. IT MAY BE 2.4, 2.45, 2.5 2.6, 2.65.....HOWEVER VERY ACCURATE VALUE OF SPECIFIC GRAVITY NEEDS FOR LOT OF GEOTECHNICAL CALCOLATIONS. SO SUCH RESULTS CAN BE VARY IF THE SECOND DECIMAL OF SPECIFIC GRAVITY IS WRONG.

 

[KAM_imported]

Try running a specific gravity test on peat--the fibers can float! Peat specific gravities can be as low as 1.2.