Liquid Limit Test as per IS- 2720 part-5 | Geotechnical engineering

 Liquid Limit Test in Soil Mechanics:  

Though soil mechanics is an important stream of civil engineering, as far as the design and stability of structures are concerned, little consideration has been given to its consistency, which can easily be gauged through different tests. With this view, one amongst them, which forms an intrinsic part of classification of cohesive soils, as well as understanding of the physical attributes of such soils, lies in the Liquid Limit Test. The Liquid Limit Test, as described in IS: 2720 (Part 5) – 1985, reaffirmed in 2006. 

What is the Liquid Limit? 

The liquid limit, or WL, is that moisture content at which a cohesive soil transitions from the plastic state to the liquid state. The point is determined by measuring the moisture content when a standard groove, formed by a grooving tool in a soil sample within a standard cup, closes for a distance of 12 mm after being subjected to 25 blows. Basically, it reflects the plastic limit of the soil and is a determinant of its engineering properties. 

Importance and Applications 

Soil Classification 

Liquid limit is basically a backbone of soil classification. With the in-situ moisture content known relative to liquid limit, engineers can observe the soil's behavior as follows:. 

Moisture content below WL: Indicates stiffer and brittle soil which can support the structures well, but tends to crack. 

Settlement Analysis 

Liquid Limit - Apart from classification, liquid limit also useful in determining the compression index and thus useful for settlement analysis in the foundation design. Generally, higher the liquid limits lesser the bearing capacity and higher the settlement value. 

 

Apparatus Required 

Liquid limit apparatus 

To conduct the liquid limit test, following apparatus are required: 

Sensitive Balance: Accurate to 0.01g 

Casagrande's Liquid Limit Device: Standard apparatus for carrying out the test 

Grooving Tool: For preparing the standard groove on the soil 

Mixing Dishes and Spatula: To prepare the soil paste 

Electrical Oven: Used for drying soil specimens 

Squeeze Bottle: Used to add distilled water 

Test Procedure 

The following steps explain how to perform the liquid limit test: 

Sample preparation A 250g oven-dried soil is mixed with distilled water; the wetted soil paste is uniform in consistency. It should be such that about 30 to 35 drops of the cup will close the groove. 

Calibration The drop height of the cup is changed to exactly 1 cm with an adjustment plate. 

Filling of the Cup Fill a very smooth layer of soil paste to a depth of 1 cm and trim to a nice shape. 

Grooving Cut a groove along the centerline of the soil pat to a depth of 1 cm to get clean, standard groove. 

Testing: Shake the container upside and downside at a rate of 2 cycles per second until the two halves of the soil meet at 12 mm. Record the number of blows (N). 

Determination of Moisture Content: Collect a representative sample of soil in the container, and repeat the test conducted above at varying moisture contents (5+ tests) to achieve sufficient data. 

Observations and Calculations 

During the test, major observations include are: 

Natural Moisture Content: This is the moisture content of the sample before beginning the test 

Weights: Weights of the container, wet soil, and dry soil are to be noted so that results for the moisture content of the material in question can be obtained with accuracy. 

Determination no.	1	2	3	4	5	6
Details of the Sample
Natural Moisture Content
Room Temperature
Container Number
Weight of Container (w1)
Weight of Container + Wet Soil (w2)
Weight of Container + Dry Soil (w3)
Weight of Water (Ww = w2 - w3)
Weight of Dry Soil (Ws = w3 - w1)
Moisture Content (%) = (Ww / Ws) × 100
No. of Blows

 

Calculation of the Results 

After the tests, plot a semi-log curve with water content as the y-axis and number of blows on the x-axis to obtain the flow curve. The moisture at 25 blows corresponds to the liquid limit (WL). The flow index (If) can also be obtained that is the gradient of the flow curve: 

Liquid Limit (WL): Obtained from the semi-log curve. 

Flow Index (If): Formula for this index is 

 

Where W1 and W2 are moisture contents.   

Accordingly, the Liquid Limit Test is one of the most important tools in soil mechanics and therefore soil behavior that must be known for safe and effective engineering practices. The understanding of the transition between the plastic and the liquid states in cohesive soils will give the relevant information that will be helpful to predict material behavior under a variety of loading conditions. Good execution and analysis of the test also supports soil classification as well as decisions regarding techniques and materials for construction, all of which will lead to stronger and safer structures.