Liquid Limit 1

[mickmac]

Can anyone explain why, when specifying , a granular sub-base material, a LL OF 20.5 is set for limestone crushed rock fines and LL of 21.5 is set for other rock types, even when the fines are non-plastic?

 

[BigH]

I have not seen this in specs before. I've posted a similar point in one of the forum that can a non-plastic soil have a liquid limit? Interesting answers - you should do a search for it. It might mean that they do not want the fines to be "sensitive" to water, e.g., erosion or movement when saturated - but this would be handled in a subbase if the subbase is filter graded within itself (see Terzaghi, Peck and Mesri, 1995). Sometimes, the subbase specs are so widely graded and non-uniform that the fines of the subbase can wash through. The specifyer might be trying to determine "how much water" it will take to move the fines. I think that in such a case, they are mis-using the test. I also find it interesting in that they have specified the LL to a 0.5 number when, according to test procedures, LL is to be rounded to nearest hole number. I have a suspicion on why, but . . . and, if someone can honestly say they thay can distinguish differences in hard geotechnical properties from the liquid limit of LL = 20 or LL = 21, I'd like to meet that person! Very interesting to say the least. Doubtful this spec was written by a geotechnical engineer or at least one with experience.

 

[mickmac]

Okay BigH I was misinformed on the 20.5 and the 21.5 LL. The figures are 20 & 21 respectively.

Here's the link below to the spec. Go to page 11 clause 804 type B. This is the specification for sub-base materials used in the Republic of Ireland.

I have a client with non-plastic granular material (LL 23-24 cone method) derived from a greywacke gritstone.

What would be the problem with using this material in relation to the spec?

http://www.nra.ie/PublicationsResou...ation/RoadDesignConstruction/file,1469,en.pdf

 

[BigH]

mickmac - you got me. I can't see for the life of me how the difference for NP subbase characteristics between LL of 20 and LL of 21 would be. For me, they are the same number. There is a similar LL<= 25 in Indian specs. Must be a British thing! jdonville???? panars????

 

[mickmac]

BigH,
It's not a British thing! The Brits just request non-plastic GSB.
The reason given for the use of LL as a control of sub-base materials in Ireland as given to me is as follows ;'The simple answer to your question is that when we used other methods of
>controlling fines such as you are proposing in crushed rock materials we >had
plenty of failures in unbound bases in Irish road pavements. I had up >to 20
years struggling with this problem and I found that technicians
>invariably had declared materials that caused the failures to be
>non-plastic when they were not. The plastic limit test is very subjective >and
the limits of 20 and 21 were found to be the most likely limits that >would give
some degree of safety to the Client and taxpayer. Incidentally >when I carried
out investigations on road pavement failures, about 95% of >the failed pavements
contained plastic fines and were the most likely
>contributors to the failure that had taken place'.

I'm lost as I have GSB materials with passing 0.063mm removed by washing which have marginal LL values to those specified.

 

[BigH]

What happens when you have a spec that is written for "other" reasons - such as poor quality testing.
Sad, but all too often.