How to Determine the Liquid Limit of Soils?

The following article will guide you about how to determine the liquid limit of soils by applying either of the two methods explained in this article. The methods are: 1. Casagrande’s Mechanical Method 2. Cone Penetration Method.

1. Casagrande’s Mechanical Method:

The mechanical method of determining liquid limit using Casagrande’s liquid limit apparatus is also known as the percussion method.

Apparatus:

The Casagrande’s liquid limit apparatus consists of a brass cup that is lifted by 1 cm on rotat­ing the handle of a cam and drops freely on a hard rubber base. The rubber base is made of vulcanized rubber with standard hardness of 86-90 IRHD and a resistance of 30%-40%. The radius of the brass cup is 54 mm and the maximum depth is 27 mm. The thickness of the brass cup is 2 mm. The rubber base is 150 × 125 × 50 mm³ in size. One end of the brass cup is connected to the sliding carriage of the cam so that the brass cup is in the inclined position when resting on the rubber base.

Procedure:

Following is the procedure for the determination of liquid limit by Casagrande’s mechanical method as recom­mended by IS – 2720 (Part 5) – 1985:

i. The soil sample to be used for the test is air dried, pulverized, and passed through 425 µm IS sieve.

ii. About 120 g of this soil sample is mixed thoroughly with distilled water in an evaporating dish to form a thin uniform paste.

iii. In the case of clayey soils, the paste is left undisturbed for about 24 h to ensure uniform distribution of moisture throughout the soil paste. However, light-textured soils having low clay content may be tested immediately after thorough mixing.

iv. The brass cup and the rubber base are thoroughly cleaned. The height of fall of the brass cup is checked and adjusted to exactly 1 cm.

v. Before conducting the test, the soil paste is remixed thoroughly. A portion of the soil paste is placed in the cup above the spot where the cup rests on the base, squeezed down, and spread in position in such a way that the surface of the soil paste is horizontal parallel to the surface of the rubber base and that the maximum depth of the soil paste in the cup is 1 cm.

vi. A clean sharp groove of standard dimensions is made in the soil paste using a grooving tool. This is done by keeping the grooving tool in touch with and normal to the surface of the brass cup at the top and rotating it along the diameter of the cup through the centre line of the cam follower.

vii. Two types of grooving tools are available. The Casagrande’s grooving tool (Type A grooving tool) is commonly used and makes a groove of size 2-mm wide at the bottom, 11 mm at the top, and 8-mm deep. In sandy soils, the Casagrande’s grooving tool tends to tear the sides of the groove and in such cases, ASTM tool (Type B or Type C grooving tool) is used. The ASTM tool cuts a groove of size 2-mm wide at the bottom, 13.6-mm wide at the top, and 10-mm deep.

viii. The cup is then lifted and dropped by turning the crank at a rate of two revolutions per second to apply blows on the soil until the two halves of the soil on either side of the groove come in contact at the bottom of the groove for a distance of 12 mm. This distance is measured with a ruler. The number of blows required to close the groove for the length of 12 mm is recorded.

ix. The water content of the soil is determined by collecting a sample of the soil in a container from the closed portion of the groove.

x. The experiment is repeated at least three more times, following the above procedure at higher water content, each time recording the number of blows to close the groove for a distance of 12 mm and determining the corresponding water content.

xi. The number of blows obtained in each of the trials should be in the range of 15-35. If the blows are more than 35, the trial is discarded and the procedure is repeated by removing the soil from the cup and using more water for the next trial. If the blows are less than 15, the trial is discarded and the procedure is repeated by adding dry soil powder to the soil to decrease the water content.

xii. For better results, it is preferable to conduct the test with increasing water content, by adding some more water in successive trials. The brass cup, the rubber base, and the grooving tool are cleaned before each trial.

xiii. The experimental data are plotted on a semi-log graph with number of blows on the x-axis on a log scale and water content on the y-axis on an arithmetic scale as shown in Fig. 5.3. A best-fitting straight line is drawn through the experimental points, which is known as flow curve.

xiv. The water content corresponding to 25 blows is read from the graph (flow curve) and is taken as the liquid limit, as shown in Fig. 5.3. The slope of the flow curve is determined, which gives the flow index. The liquid limit is reported to the nearest whole number.

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

One-Point Method:

It is possible to determine the liquid limit by the above procedure using a single trial instead of four trials for saving time, provided some conditions are satisfied.

IS: 2720 (Part 5) – 1985 permits the determination of liquid limit using the procedure with a single trial, using the following procedure:

1. The initial water content used for making the soil paste for the single trial shall be as close as possible to the estimated liquid limit of the soil. This comes out of experience of working with a number of soils for liquid limit determination.

2. The number of blows obtained in the single trial should be as given in Table 5.1.

3. When the above two conditions are satisfied, the liquid limit may be determined with a single trial using the following equation:

where ω_L is the liquid limit, N is the number of blows, and ω_N is the water content corresponding to the blows “N.”

Flow Index:

It is the slope of the flow curve, which is the relationship between number of blows on the x-axis on a log scale and water content on the y-axis on an arithmetic scale in a liquid limit test.

It can be mathematically expressed as –

I_f = [(ω₁ – ω₂)/log₁₀] (N₂/N₁) …(5.2)

where I_f is the flow index; ω_1, N₁ are the water content and the number of blows at any point 1 on the flow curve, respectively; and ω₂, N₂ are the water content and number of blows at any point 2 on the flow curve, respectively.

Flow index indicates the rate at which a soil mass loses its shear strength with the increase in water content. The higher the flow index, the lower will be the shear strength possessed by the soil at any given water content. Soils with higher flow index also lose their shear strength at a faster pace for a given increase in the water content.

2. Cone Penetration Method:

Cone penetration test is an alternate method to Casagrande’s method to determine the liquid limit of a soil in the laboratory. In this test, the liquid limit is correlated to the penetration of a freely falling calibrated cone into the wet soil mass.

Principle:

The principle of the method is to determine the penetration of a cone of standard sliding weight falling freely into a soil paste filled in a brass cup. The liquid limit is determined as the water content corresponding to 20-mm pene­tration. It has also been observed that the penetration depth, when plotted on a log scale, is an approximately linear function of the water content.

Apparatus:

The apparatus used in the cone penetration method. It consists of a stainless steel cone of apex angle 30° and length 35 mm fixed to the base of a mild steel plunger rod. The cone with the plunger rod can slide vertically along with a top cap on pressing a push button. The cone and the plunger rod along with the top cap is fixed to a vertical stand with the help of a clamping screw. The bottom of the stand has a base on which a cylindrical brass cup of 50-mm internal diameter and 50-mm internal height is placed.

Procedure:

Following is the procedure for the determination of liquid limit using cone penetrometer as recommended by IS – 2720 (Part 5) – 1985:

1. The push button is pressed and the plunger rod is adjusted to ensure that the pointer on the graduated scale shows zero penetration reading.

2. The soil sample for the test is prepared in the same way as in Casagrande’s liquid limit test. The soil sample to be used for the test is air dried, pulverized, and passed through 425 µm IS sieve.

3. About 150 g of this soil sample is mixed thoroughly with distilled water in an evaporating dish to form a thin uniform paste.

4. In the case of clayey soils, the paste is left undisturbed for about 24 h to ensure uniform distribution of moisture throughout the soil paste. However, light-textured soils having low clay content may be tested immediately after thorough mixing.

5. The soil paste prepared is placed in increments in the brass cup ensuring that no air is entrapped in the cup. The cup is completely filled with the soil paste and the excess soil above the top of the cup is removed. The surface of the soil paste in the cup is leveled.

6. The brass cup filled with wet soil paste is cleaned from outside and is placed on the base of the cone penetration device.

7. The plunger rod and the cone along with the graduated scale are moved vertically by releasing the clamping screw and holding the entire assembly carefully to bring the bottom of the cone in contact with the surface of the soil in the brass cup and the clamp is then tightened.

8. The push button is released and the cone is allowed to penetrate into the soil in the brass cup. The penetration (P₁) of the cone in the soil paste is recorded 5 s after releasing the push button by noting the reading against the pointer on the graduated scale.

9. The water content of the soil in the brass cup is determined and is recorded as ω₁ .The remaining soil in the brass cup is removed, and the cup and the cone are cleaned. More water is added to the soil, and the soil paste is thoroughly mixed.

10. The procedure is repeated to have at least four trials, and corresponding values of penetration (P₂, P₃, P₄) and the water content (ω₂, ω₃, ω₄) are determined.

11. The water added to the soil in each trial should be such that the penetration obtained is in the range of 14-28 mm. In case the water content in any of the trials is less than 14 mm or more than 28 mm, the result shall be discarded and another trial is made with different water content. It is desirable to make the first trial with smaller water content and the succeeding trials are made with increasing water content.

12. A graph is plotted with cone preparation on the x-axis and the water content on the y-axis. A best-fitting straight line is drawn through the experimental points as shown in Fig. 5.5.

13. The water content corresponding to 20-mm penetration is read from the graph and is taken as the liquid limit. It is reported to the nearest first decimal place.

As per IS – 2720 (Part 5) – 1985, the total sliding weight of the cone and the plunger rod is 80 g, and the liquid limit is taken as the water content corresponding to 20-mm penetration. However, as per the earlier version of the code IS – 2720 (Part 5) – 1963, the total sliding weight was 148 g and liquid limit was taken as the water content correspond­ing to a penetration of 25 mm. The test trials are made such that the cone penetration is in the range of 20-30 mm.

One-Point Method:

The depth of cone penetration is an indirect indication of the shear strength of the soil. The relationship between cone penetration and water content is unique and independent of the soil type.

IS – 2720 (Part 5) – 1985 recommends the following procedure for determination of liquid limit with a single trial:

1. The test is conducted in a single trial finding the water content ω_D and the corresponding cone penetration D such that D is the range of 16-26 mm.

2. The liquid limit is determined from any of the following equations:

ω_L = ω_D/0.77 logD …(5.3)

ω_L = ω_D/0.65 + 0.0175D …(5.4)

where D is the cone penetration in millimeters and ω_D is the water content corresponding to the penetration D. As per the U.S. Army corps of engineers, we get –

ω_L = ω_N(N/25)^tanβ …(5.5a)

tan β = 0.121 …(5.5b)

where N is the number of blows to close 12.7-mm groove length and ω_N is the water content corresponding to N. Here tan β is not equal to 0.121 for all soils. Equations (5.5a) and (5.5b) yield good results if N is in the range of 20-30. ASTM D-4318 recommends this procedure as the water content involved for N = 20 – 30 is in a small range. The cone penetration method, also known as fall cone method, is popular in Europe for the determination of liquid limit and is recommended by BS 1377.

When cone of total sliding weight of 240 g is used, the fall cone method can be used to determine the plastic limit of soils. Here, the water content corresponding to a penetration of 20 mm gives the plastic limit.

Advantages of Cone Penetration Method:

Compared to the Casagrande’s method, cone penetration method has the following advantages:

1. The test is simple and easy to perform.

2. The method is applicable to a wide range of soils.

3. The result obtained is reliable and does not depend on the skills of the person performing the test.

In Casagrande’s method, the result is affected by the:

i. Way in which the soil is paced and leveled in the brass cup.

ii. Way in which the groove is cut.

iii. Rate of application of blows.