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Falling Head Test versus Constant Head TestHelpful Member!(2) 

Freddie81 (Civil/Environmental) (OP)
4 Mar 09 21:42
I have just performed a Constant Head Test and a Falling Head Test, now I am supposed to compare the result of the coefficients of permeability.  I was wondering if the result for the Constant Head is supposed to nearly double that of the Falling Head Test.
Helpful Member!  fattdad (Geotechnical)
5 Mar 09 9:51
Not sure if it is "supposed" to be nearly double, but if it is it's telling you a) just how imprecice either test method may be, b) that the data is within reason (i.e., at least it's not off by an order of magnitude), c) that there is an error, d) that the two samples are not equally prepared (i.e., degree of compaction or compaction moisture content), e) that you are using the wrong equations or boundary conditions, f) etc.

More detail would be helpful.

f-d

¡papá gordo ain't no madre flaca!

Helpful Member!  emmgjld (Geotechnical)
5 Mar 09 12:02
I wish to emphasize fattdad's comments and point out that sample preparation and sample similarity is very difficult to actually determine and to obtain.

Regarding your reported test results...I think they are actually very close, assuming the tests and computations were properly done.

I strongly recommend An Introduction to Measurement Theory and Experiment Design, D.C. Baird, 1962, Prentice-Hall, Inc.
  A very small book, loaded with insight. This will change how you look at testing and computations.
Freddie81 (Civil/Environmental) (OP)
5 Mar 09 20:25
I thank both of you for your insight in regard to this matter, but just for a follow up to fattdad's comment here is some of the data that I aqcuired.
I am not sure if this would have any effect on the test but there was a slight leak in the chamber was noticed when the vacuum was turned on, which I was told by my prof, that this would not make any difference with our outcome due to the speed of which a Falling head test is performed. Ottawa sand was used in both test which is air dryed.  The same procedure for compaction of the sample was used in both test.   The representaive k for the Constant head is 2.709x^10-4 and Falling Head is 1.831x10^-4.
Dry Densities:
Falling Head- 1.831
Constant Head- 1.722
iandig (Civil/Environmental)
6 Mar 09 8:25
The results look to be practically the same, the difference between 2.7 and 1.8 in the great scheme of things is probably very small, don't forget that these are x10-4 -> 0.00027 and 0.00018 or more simply 0.0003 and 0.0002. I would be more concerned over the variation in the dry densities and the fact that you seem to have similar permeabilities with such a range of dry densities. Were these performed before or after the tests and what was the moisture content.  
fattdad (Geotechnical)
6 Mar 09 11:01
Are you running these samples in a triaxial chamber?  If so, what head and tail pressures was applied prior to the beginning of the falling head test? Also, how much drop (i.e., cm) occurred during the test (i.e., 5cm or 25 cm)?  Were the gradients fairly similar between the two tests (i.e., deltaH/deltaL)?  I know that the gradient varies during the falling head test, but if there is significant pressure applied and limited "fall" in the pipe, you may be using formula for falling head, but be more close to the boundary conditions of a constant head test.

If, for instance, you are applying 10 psi head pressure, that's the equivalent to 23 ft of water.  If you measure 2 inches of "fall", that dims in comparison to the head applied by the pressure pump.  I say this as loosing perspective in the triaxial chamber is common when there is pressure involved.  Just something to consider.

Have fun with this.  The best learning takes place when you are stumped.  That said, I agree the two values are quite similar.

f-d

¡papá gordo ain't no madre flaca!

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