Drop Cone Penetrometer vs Casagrande test for LL in the lab 5

[RobPE]

Due to the difficulty of getting repeatable results and the time required to determine the liquid limit using the Casagrande LL device, our lab guys have explored alternatives to determining the LL in the lab and have come across what is known as the DROP Cone Penetrometer (for LL testing in the lab). With this device, the LL is determined by dropping a cone into a moist soil from a fixed height where the LL is defined as the water content at which the cone penetrates the soil 20 mm (I believe) when dropped from a standardized height. From what I gather, comparisons with the Casagrande method are favorable and the DCP is used extensively abroad. However, we haven't been able to find ASTM or AASHTO standards for this test. Do any of you have any experience with the drop cone penetrometer (for determining LL in the lab)? Comments with respect to repeatability of results, ease of testing, testing time in comparison with the Casagrande test, preferred DCP manufacturers, etc would be very helpful.

Below are a few links that present some info about the Drop Cone Penetrometer:

http://www.tecnotest.it/pdf/ING/cha...rop cone penetrometer tecnotest liquid limit"

http://sis.agr.gc.ca/cansis/publica...#search="drop cone penetrometer liquid limit"

http://au.answers.yahoo.com/question/index?qid=20060921082651AAVlPMZ

 

[BigH]

There is an Indian Standard on this. As soon as I get home (I'm in transit now in Hong Kong) I will look up the information. I once found some 7 pages of bibliography on using the "fall cone" - which is what we learned it was called at Cornell back in 1974. So, it has been around for quite a while. There is also a proposal out there - I ran across it the other day but details escape me, about using the fall cone, too, for plastic limit determination. This is done with the idea that the plastic limit and the liquid limit do, in fact, represent an undrained shear strength.
- and it is easier to use the cone.

 

[dgillette]

Joe Koester at the COE ERDC in Vicksburg (formerly known as Waterways Experiment Station) found the fall cone gives a LL that's 1-2 points higher than what the cup gives. For most situations, no big deal.

 

[dirtman85]

We have used the cone penetrometer for over 15 years and the method is explained in BS1377:1990 Part 2. The results have proven to be more accurate and it is certainly quicker.

 

[emmgjld]

A week ago, I received the August volume of Canadian Geotechnical Journal (vol 43, no. 8). K.Prakash & A. Sridharan have a Note 'Critical appraisal of the cone penetration method of determining plasticity'.

Their references include;
BSI, 1990, BSI 1377-2
Campbell, D.J. 1976, Journal of Soil Science, 27(3): 295-300
CAN/BNQ, 1986 CAN/BNQ 2501-092-M-86

Running through the article, I note they encourage comparative testing of local soils before drawing a conclusion of using the penetration method. I would personally avoid the penetration method due to the abundance of low plastic soils (CL-ML) I have been blessed to practice with. I make a lot use of Skempton's Activity and would prefer the best numbers possible.

Good discussion and I would encourage those concerned to read the whole article. If the authors will pardon my small quote, their conclusion:
'Soil Plasticity is due to to soil cohesion. Liquid & plastic limits of soils measured using the percussion and 3 mm thread rolling methods are contributed by the soil cohesion and hence, they represent the plasticity characteristics. Liquid limit and (or) plastic limit determined by the cone penetration method conceptionally do not represent the soil plasticity characteristics because they also represent undrained friction. For non plastic soils, the values obtained from the cone penetration method represent the contribution from the undrained friction only.

Use of the cone penetration method to determine the soil plasticity must be discouraged because it cannot determine the soil plasticity in its real sense for less plastic and nonplastic soils whose undrained strength is essentially due to undrained friction.'

 

[dgillette]

"...Liquid & plastic limits of soils measured using the percussion and 3 mm thread rolling methods are contributed by the soil cohesion and hence, they represent the plasticity characteristics. Liquid limit and (or) plastic limit determined by the cone penetration method conceptionally do not represent the soil plasticity characteristics because they also represent undrained friction."

This definitely makes sense w/ respect to PL and 3 mm worms (where there is an obvious conceptual difference), but I am less clear on the difference between undrained shear in the LL cup and undrained shear in the cone test. One is roughly analagous to Newmark seismic sliding analysis, where the number of thumps to move the slide mass is a function of undrained strength. The fall cone is a more direct measure of undrained strength, but the overall concept of the LL being governed by the undrained strength at a given water content is similar.

[BTW, I don't know Canadian copyright laws, but in the US, a quote like this is covered by "reasonable use." Besides, researchers all seem to love being quoted, in part because future research funding is influenced by how many practitioners use and cite their research.]

 

[RobPE]

dirtman85, could you please elaborate a bit on your experience? Where do you practice (I'm in AZ., US), and are you using the fall cone penetrometer deliberately in favor of the Casagrande method?

With respect to the theoretical notes on plasticity being due to cohesion rather than undrained shear strength which is sort of what the fall cone penetrometer gives, if the difference is in the 1-2% range as dgillette has referenced, then the difference may be negligible. This difference is in fact much less than ASTM's tolerances for Casagrande LL tests.

Emmgjld your idea of doing comparisons between the 2 methods with familiar soils is well recieved. Also, why would low plasticity soils preclude use of the fall cone penetrometer?

Bigh if you can post the Indian standards, I'm still interested. Thanks.

 

[dirtman85]

Hi RobPE. I've been a soil technician/supervisor for 15 years in England. BS1377 is the British Standard used by all soil laboratories in the U.K. The cone penetrometer has gradually replaced the Casagrande method in every soil lab I have been to and is now generally accepted to be the definitive method for determining LL. PL is rolled into 3 mm "threads" as per BS and AL's are one of the tests for which we hold UKAS (United Kingdom Accreditation Service) accreditation for this test.

UKAS are THE governing body in this country and the method of measurements/tracibility/calibration etc..is comparable with similar bodies within the EU and increasingly Internationally.

Hope this helps.

 

[BigH]

Check out:

http://www.ecs.umass.edu/cee/faculty/LLFallCone.pdf