liquid limit

[alejo12]

need help understanding why a LL of 100 means that the weight of the water equals the weight of the soil. Same witha LL of 50.

Will appreciate any help. Thanks.

 

[Phalanx]

In their simplest terms, LL and PL are water contents in the soil. I won't get into what each one means, but here is what the CERM is talking about.

w = water content expressed in decimal form

mw = mass of water

ms = mass of soil

mt = total mass

w = mw/ms

mt = mw + ms

If LL = 100%, then: w = 1.0 = mw/ms => mw = ms

If LL = 50%, then: w = 0.5 = mw/ms => mw = .5*ms => mt = ms + .5*ms = 1.5*ms

Just for grins, let's say LL=50 (w=0.5) and the total mass = 1.2g. Using the above equations, this leads to ms = 0.8g and mw = 0.4g. With LL=50, the CERM states the total mass is 2/3 soil and 1/3 water. 0.8 / 1.2 = 2/3 soil 0.4 / 1.2 = 1/3 water

 

[alejo12]

Thanks a lot. This helps tremendously.

 

[sac_engineer]

I think it's important to know what the LL really means. It's the percentage of water that would transform the physical state of soil from plastic to liquid. This mainly applies to fine-grained soils like clays and silts. The PL (plastic limit) is the minimum water percentage for a soil to be considered plastic. LL is always greater than PL, and the difference (LL-PL) is known as the plasticity index (PI) which represents the range of moisture content over which the soil behaves as a plastic material. This is a very common value associated earthen structures such as dams and levees to determine how the structure will perform in saturated conditions. Soils used for engineering and construction are selected based on their PI values and solving for the LL and PL values are essential in ensuring that the soil will perform as per geotechnical design.

I'm not sure if a soil's LL value can ever reach 100% while retaining some plastic behavior, but I'm not a geotech engineer.

 

[alejo12]

now I will never forget that PI=LL-PL

now I have a better understanding. Its great to have another explanation rather than CERM.