LIQUID LIMIT OF SOIL – WHAT, WHY & HOW?
What is Liquid Limit of Soil?
When water is added to dry soil, it changes its state of consistency from hard to soft. If we add water to a fine grained soil, then water will change its consistency from hard to semi hard. If we continue to add more water then again the soil will change its state of consistency from semi hard to plastic and finally reach a liquid consistency stage. When the soil reaches liquid consistency state, it has remain no cohesive strength to retain its shape under its own weight. It will start to deform its shape. So the amount of water which is responsible for this state of consistency of soil is calledliquid limit of soil. In other words we can define liquid limit as
“It is the minimum water content at which the soil is still in the liquid state, but has a small shearing strength against flow.”
From test point of view we can define liquid limit as follows.
“Liquid limit is defined as the minimum water content at which a pat of soil cut by a groove of standard dimension will flow together for a distance of 12 mm (1/2 inch) under an impact of 25 blows in the device.”
Why to Know Liquid Limit of Soil?
- Liquid limit of soil is a very important property of fine grained soil (or cohesive soil)
- Value of liquid limit is used to classify fine grained soil.
- It gives us information regarding the state of consistency of soil on site.
- Liquid limit of soil can be used to predict the consolidation properties of soil while calculating allowable bearing capacity & settlement of foundation.
- 液限土还用于它的价值te activity of clays and toughness index of soil.
How to Determine Liquid Limit of Soil?
There are two different methods are available for determining liquid limit of soil. These two methods are as given below.
- Liquid limit test of soil using casagrande apparatus
- Liquid limit test of soil using cone penetrometer apparatus
1. Liquid Limit Test of Soil Using Casagrande Apparatus
This test is done using casagrande apparatus. In this test a soil paste is formed using soil sample passing 425 micron sieve and water. This paste is then placed in the cup of casagrande device. The soil pat is then divided into two halves by making a groove using the grooving tool. Then the cup is dropped by turning the crank until two halves of the soil cake come in contact with each other. The test is repeated by adding different amount of water with the soil. Finally by plotting a graph we can determine the liquid limit of soil.
2. Liquid Limit Test of Soil Using Cone Penetrometer Apparatus
Prepare a soil paste by mixing soil sample passing 425 micron sieve & water. Transfer this paste to the cup of cone penetrometer apparatus. Set the penetrometer in such a way that the cone just touches the surface of the soil paste. Then release the cone to penetrate into the soil paste. Repeat this test by adding different amount of water with the soil. Then plot a graph and determine the liquid limit of soil.
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Suryakanta
Geotechnical-Material Engineer. You can follow me onInstagram. You can connect me onGoogle +. To know more about me just visitAboutMe
we know liquid limit analysis is for fine grained soil (size less than 75micron), but we are taking soil for testing is of 425micron ie caorse grained soil. why it is like that?
Sir I want to how to calculation of temperature correction of soil classification by hydrometer analysis.
Why liquid limit is found only for 25 blows??
It suposes that the shearing strength against flow is about 2,5 kPa. It is just 0,1 kPa/blow.
What is the aim of finding plastic limit and liquid limit of the soil ?
One of the uses is to compare the Natural Moisture Content (NMC) with the PL and LL. If the NMC is close to LL, there is large potential for settlements i.e. NC Clays, whereas if the NMC is close to PL, it suggests OC Clays with relatively less potential for settlements.
What is the aim of finding plastic limit and liquid limit of the soil?
to know the type of soil.
It is to determine at what moisture % that type of soil turns from a solid into a liquid, and then to see at what moisture % it turns from a plastic back into a solid. It helps classify the soil to use for designing roads and housing.
why in dam construction, soils with high liquid limits are avoided??
Hi Suryakanta,
I’m constructing an earth wall dam and have a non-cracking medium clay subsoil. i’ve been reading up on soil tests for determination of the suitability of in-situ soils and it seems that soil dispersion and shrinkage tests are most relevant for assessment of suitability, and the plastic limit is relevant for construction, identifying the optimum moisture content for compaction. I seriously don’t get the value of the liquid limit or the plasticity index – I already know it’s a non-dispersive medium clay, how does the liquid limit test help me to classify the bearing capacity or inform soil suitability or construction? I really don’t get the point of it?
Any advice or explanation is greatly appreciated.
Thanks, Gus