Liquid Limit 1

[matest]

i am a lab tech and have being working in site lab for last 18 months. i have being carrying out liquid limits on clause 804 since then however no-one i ask can tell me the relevence of this test for this material. i understand the meaning of 'liquid limit' but why is knowing its value usefull and in my case why is the upper limit 20%

 

[BigH]

pick up a good soils book - Terzaghi, Peck and Mesri, for example. Wander through it. you will see that many many geotechnical parameters have been correlated with the liquid limit. The behavoiur of fine grained soils or fine grained fractions within a granular soil are dependent on the behaviour of the fine grained fraction - and the Atterberg limit (liquid and plastic) are the qualititative markers. If the natural water content is above the liquid limit, it indicates a very sensitive soil. (ps - almost all soils books will show correlations of engineering properties of fine grained soils with the liquid (and plastic) limit - but TP&M is a good one - not to "understand" - but by running your eye through it you will see how important the test has become in geotechnical engineering.

 

[fattdad]

+1 BigH!

In many respects, the liquid limit is a classification test. It allows the engineer to classify the soil being tested within the context of the engineer's experience and also to published reference. What's the relavence of hardness if you don't have a way of knowing there are items that are softer and harder, for example?

Because it is mostly a classification type test, folks have made correlations between liquid limit and other soil properties.

Not to dispute what BigH said - just to add, that is; if the natural moisture content is greater than the liquid limit, it can also mean that the soil is underconsolidated, just as if the natural moisture content is below the liquid limit it can mean that the soil is preconsolidated. These terms help an engineer anticipate just how settlement sensitive a soil layer may be to newly-imposed loads.

It think it's commendable that you want to understand the testing that you are performing!

f-d

¡papá gordo ain’t no madre flaca!

 

[iandig]

Argh... I see the problem. If by Clause 804 you mean from the Specifcation for Highway Works, Clause 804, then you are testing a Type 2 sub-base material. As a crushed rock (or similar), you are probably wondering why you are doing a soils classifcation test developed for cohesive soils. In summary the reason why you are doing this test is to find out if the fine material (passing the 425µm) is plastic in nature. If it is, the sub-base is likley to be frost suceptible and as such would fail the compliance test requirements from the SHW, much cheaper to do than a frost test so is usually done before spending the money for the rest of the required suite. In additon there is a general requirement for sub-base materials used in pavement constructon relating to the plastic limit and plasticity index. The requirements for a Type 1 sub-base are that the material should be non-plastic, however for a Type 2 the plasticity index needs to be less than 6. For a type 1 (clause 803) you would only do the plastic limit test, however for a Type 2 (clause 804) you would do the liquid limit as well (although only really needed if the fines are plastic). If you have done a plastic limit and the fines are non-plastic, then I do not see the justification for the liquid limit as well, however that will be down to the lab manager.