Liquid Limit of Soil Mechanical Method (Using Casagrande Appratus)

A liquid limit of soil is the moisture content expressed as a percentage of the weight of over-dried soil, at which soil changes from a plastic to a liquid state.

---

Why to test Liquid limit of Soil?

• The liquid and plastic limits of soils are both dependent on the amount and type of clay in a soil and form the basis for the soil classification system for cohesive soils based on the plasticity tests.
• Besides their use for identification, the plasticity tests give information concerning the cohesion properties of soil and the amount of capillary water which it can hold.
• They are also used directly in specifications for controlling soil for use in fill.
• These index properties of soil have also been related to various other properties of the soil.

Apparatus

The apparatus shall consist of the following:

1. Mechanical Liquid Limit Device - It shall conform to IS: 9259-1979.
2. Grooving Too/ - It shall conform to IS: 9259- 1979.
3. Porcelain Eltaporating Dish - 12 to 15 cm in diameter.
4. Flat Glass Plate - 10 mm thick and about 45 cm square or larger (alternative to porcelain evaporating dish for mixing soil with water).
5. Spatula - flexible, with the blade about 8 cm long and 2 cm wide (for mixing soil and water in the porcelain evaporating dish).
6. Palette Knives - two with the blade about 20 cm long and 3 cm wide (for mixing soil and water on the flat glass plate).
7. Balance - sensitive to 0.01 g.
8. Oven - thermostatically controlled with interior of non-corroding material to maintain the temperature between 105 and 110°C.
9. Wash Bottle or Beaker - containing distilled water.
10. Containers - air-tight and non-corrodible for determination of moisture content.

---

Preparation of the Soil Sample

A sample weighing about 120 g. shall be taken from the thoroughly mixed portion of material passing through 425-micron IS Sieve obtained in accordance with IS : 2720 (Part I)-1983

Note - When no stones are present in the soil and practically all of the soil passes 425-micron IS Sieve, there is sometimes a practice of testing samples without previously preparing them. When soils are tested in the natural condition, the results will usually differ from those obtained with air-dried samples. The test record shall state that soil in the natural condition was used.

If this is done and stones are present, only the material passing 425-micron IS Sieve shall be used for the test; this can be obtained by rubbing the wet soil through the sieve until a sufficient quantity of the size passing 425-micron IS Sieve is obtained.

---

---

Adjustment of the Mechanical Device

• The liquid limit device shall be inspected to determine that it is clean, dry and in good working order, that the cup falls freely and it does not have too much side play at its hinge. The grooving tool shall also be inspected to determine that it is clean and dry.
• Using the gauge on the handle of the grooving tool or a separate gauge and by means of the adjustment plate of the mechanical liquid limit device, the height through which the cup is lifted and dropped shall be adjusted so that the point on the cup which comes in contact with base falls through exactly one centimetre for one revolution of the handle. The adjustment plate shall then be secured by tightening the screw.

Note: - If evenness of the base is not maintained in course of time, the base of the liquid limit device should be changed.

---

Test Procedure

1. About 120 g. of the soil sample passing through 425-micron IS Sieve shall be mixed thoroughly with distilled water in the evaporating dish or on the flat glass to form a uniform paste.
2. The pastes hall have a consistency that will require 30 to 35 drops of the cup to cause the required closure of the standard groove.
3. In the case of clayey soils, the soil paste shall be left to stand for a sufficient time (24 hours) so as to ensure uniform distribution of moisture throughout the soil mass (see Note 1)
4. The soil should then be re-mixed thoroughly before the test. A portion of the paste shall be placed in the cup above the spot where the cup rests on the base, squeezed down and spread into position shown in Fig. (Diagram Illustrating Liquid Limit Test), with as few strokes of the spatula as possible and at the same time trimmed to a depth of one centimetre at the point of maximum thickness, returning the excess soil to the dish.
5. The soil in the cup shall be decided by firm strokes of the grooving tool along the diameter through the centre line of the cam follower so that a clean, sharp groove of the proper dimensions is formed (see Note 2).
6. In case where grooving tool, Type A does not give a clear groove as in sandy soils, grooving tool Type B or Type C (see Note 3) should be used.
7. The cup shall be fitted and dropped by turning the crank at the rate of two revolutions per second until the two halves of the soil cake come in contact with bottom of the groove along a distance of about 12 mm (see Note 4).
8. This length shall be measured with the end of the grooving tool or a ruler.
9. The number of drops required to cause the groove close for the length of 12 mm shall be recorded.
10. A little extra of the soil mixture shall be added to the cup and mixed with the soil in the cup.
11. The pat shall be made in the cup and the test repeated as in (steps 4 to 9).
12. In no case shall dried soil be added to the thoroughly mixed soil that is being tested.
13. The procedure given in (steps 4 to 9) and in, this clause shall be repeated until two consecutive runs give the same under of drops for closure of the groove (see Notes 4 and 5).
14. A representative slice of soil approximately the width of the spatula, extending from about edge to edge of the soil cake at right angle to the groove and including that portion of the groove in which the soil flowed together shall be taken in a suitable container and its moisture content expressed as a percentage of the oven-dry weight otherwise determined as described in IS : 2720 (Part 2)-1973.
15. The remaining soil in the cup shall be transferred to the evaporating dish and the cup and the grooving tool cleaned thoroughly.
16. The operations specified in (steps 4 to 9) to (steps 14 to 15) shall be repeated for at least three more additional trials (minimum of four in all), with the soil collected in the evaporating dish or flat glass plate, to which sufficient water has been added to bring the soil to a more fluid condition.
17. In each case, the number of blows shall be recorded and the moisture content determined as before.
18. The specimens shall be of such consistency that the number of drops required to close the groove shall be not less than 15 or more than 35 and the points on the flow curve are evenly distributed in this range.
19. The test should proceed from the drier (more drops) to the wetter (less drops) condition of the soil.
20. The test may also be conducted from the wetter to the drier condition provided drying is achieved by kneading the wet soil and not by adding dry soil.

Note - 1 - Light textured soils (of low clay contept) may be tested immediately after thorough mixing of water.

Note - 2 - To avoid tearing of the sides of the groove or slipping of the soil cake on the cup, up to six strokes, from front to back or from back to front counting as one stroke, shall be permitted. Each stroke penetrate a little deeper until the last stroke from back to front scrapes the bottom of the cup clean. The groove shall be made with as few strokes as possible.

Note - 3 - With soils having low plasticity indices, it is sometimes difficult to cut a smooth groove in the soil with grooving tool, Type A. Grooving tool, Type B or Type C may be used in such cases. Grooving tool, Type B inserts a wedge into the pat of soil causing the two halves of the pat to slide at the cup-soil interface. During the test, the tendency is then for the soil of slide back again on this same face instead of flowing as it should do. It should, therefore, be used with caution.

Note - 4 - Some soils tend to slide on the surface of the cup instead of the soil flowing. If this occurs, the results should be discarded and the test repeated until flowing does occur. If sliding still occurs, the test is not applicable and a note should be made that the liquid limit could not be obtained.

Note - 5 - Care shall be taken to see that the soil paste does not dry out too rapidly between repeat tests as the number of blows for closure will increase gradually as the sample dries out.

---

Determination of Liquid Limit

Liquid Limit (W_L) "A flow curve" shall be plotted on semi-logarithmic graph representing water content on the arithmetical scale and the number of drops on the logarithmic scale. The flow curve is a straight line drawn as nearly as possible through the four or more plotted points.

The moisture content corresponding to 25 drops as read from the curve shall be rounded off to the nearest whole number and reported as the liquid limit of the soil.

Determination of Flow Index

Flow Index (I_f) - The flow curve (straight line) plotted on semi-logarithmic graph as in Liquid Limit (W_L) shall be extended at either end so as to intersect the ordinates corresponding to 10 and 100 drops. The slope of this line expressed as the difference in water contents at 10 drops and at 100 drops shall be reported as the flow index.

The flow index may be calculated from the following equation also:

Flow Index = $I_f=\frac{W_1-W_2}{{\log }_{10}\left({\displaystyle \frac{N_2}{N_1}}\right)}$

Where,

• I_f is flow index
• W₁ is moisture content in percent corresponding to N₁ drops
• W₂ is moisture content in percent corresponding to N₂ drops

Note:

For determination of Flow Index, one can randomly select any reading of water contents (W1, W2, W3 & W4) of the liquid limit test as W1 and W2 with condition that W1 must be greater than W2