Liquid Limit Test 1

[gemeda]

Can we determine liquid limit of a soil sample without going through so many trials as in Casagrande test?

 

[civilperson]

Yes, blow counts between 18 and 29 can be correlated to a 25 blow moisture.

 

[gemeda]

You mean,it need not necessarily 25 but between 18 and 29 blow counts can represent LL? or any chart you can correlate the LL for blow counts other than 25.

 

[civilperson]

One point calculation:
LL= W (N/25)<0.12 (between 15 and 30 blows)

 

[hokie66]

I am no soils engineer, but none of this makes sense to me. Direct correlation of boring blow count with Atterberg limits? Perhaps one of our geotech gurus can sort out what these guys are saying.

 

[BigH]

hokie66 - I know you understand it is the blow count of an Atterberg limit "cup" -- not from the SPT . . .

 

[hokie66]

Thanks, BigH. As I remember, my soils prof. called those taps rather than blows. Oh well.

 

[gemeda]

Civilperson-what are you talking about.LL is the water content of the sample(of Casagrande cup) at which a 13mm grove takes 25 blows to close up.Therefore,you need to try many times till you get that value.

 

[lovethecold]

gemeda,

What civilperson is saying is that you run the test until you get between 15 and 30 blows (or taps). Then use the formula he gave. The formula corrects to 25. If you get exactly 25 blows, then there is no need to correct.

Example:

You put your sample in the cup and obtain 27 blows, after which you put the sample into the oven and obtain a 35.7% m.c. Then, your results would be as follows:

LL = 35.7 x (27/25)^0.121 = 36.03 (or LL = 36).

civilperson uses a range from 15 to 35. We limit our range between 21 and 29. Some use 22 to 28 as a range in which they will use the above to correct to 25.

Hope this helps.

 

[fattdad]

Consider two or three trials. For a moisture content of 15 percent it takes 29 blows (taps) to close up the groove. For a moisture content of 25 percent it takes 21 blows. If you make a plot of blows v. moisture content you can straight line the data and see the liquid limit (i.e., moisture content for 25 blows would be 25 percent (i.e., LL=25). For our lab, we usually run three trials just to see whether there is actually a straight line. Rarely do all three line up, so we draw the best fit.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[civilperson]

There is two methods to determine the Liquid Limit:
1) Multiple trials with number of taps/blows at both below and above 25 with a curve fit or average.
2) AASHTO method of one trial with numerical adjustment to equal moisture at 25 taps/blows.

 

[gemeda]

lovethecold and civilperson-thank you for your explanaton.

 

[lovethecold]

We only do one trial. I am wondering how many folks are like fattad and do three trials? Usually for us we do the test just so there is a test to show that a soil is fat or lean. Rarely is there a soil that is borderline.

Would the three point method be primarily for soils that are borderline? Or for all types?

 

[fattdad]

We do multiple trials because you get better results and it's easy. Come on, it's mud pies for grownups! Once the mud is made, the time for extra taps and taring out weight cups isn't that hard.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

It also depends on the purpose of the test - to simply state "fat" or "lean" clay - it is not that important I agree from a practical aspect - for the most part a percent or two or even more will not change the 'fat' or 'lean' notation. But if you are wanting to develop, say, local correlations with triaxial tests, it is important to be as accurate as you can.