Liquid Limit Test

6.1 lntroductionWhen a cohesive soil is mixed with an excessive amount of water, it will be in a somewhatIiquid state and flow like a viscous liquid. However, when this viscous liquid is driedgradually, with the loss of moisture it will pass into a plastic state. With further reduction

of moisture, the soil will pass into a semisolid and then into a solid state. This is shown inFig. 6.1. The moisture content (in percent) at which the cohesive soil will pass from a liquidstate to a plastic state is called the /iquid limit of the soil. Similarly, the moisture contents (in

percent) at which the soil changes from a plastic to a semisolid state and from a semisolidto a solid state are rcfenedto as plastic limit and shrinknge limit, respectively. These limitsare referred to as the Atterberg limits (Atterberg, 1911). In this chapter the procedure todetermine the liquid limit of a cohesive soil will be discussed.

Moisturecontent(increasing)

SL = Shrinkage limitPL = Plastic limitLL = Liquid limit

Figure 6.1. Atterberg limits.

37

38 Soil Mechanics Laboratory Manual

6.2 Equipment1. Casagrande liquid limit device2. Grooving tool3. Moisture cans4. Porcelain evaporating dish5. Spatula6. Oven7. Balance sensi t ive to 0.01 g8. Plastic squeeze bottle9. Paper towels

The equipment (except for the oven) is shown in Fig. 6.2.The Casagrande liquid limit device essentially consists of a brass cup that can be raised

and dropped through a distance of 10 mm (0.394 in.) on a hard rubber base by a camoperated by a crank [see Fig. 6.3(a)]. Fig. 6.3(b) shows the schematic diagram ofa groovingtool.

\ F I

r\

Figure 6.2. Equipment for l iquid l imit test.

Liquid Limit Test 39

(b)

Figure 6.3. Schematic diagrams: (a) Liquid l imit device. (b) Grooving tool '

6.3 Procedurel. Determine the mass M1 of three moisture cans.2. Put about 250 g of air-dry soil, passed through a No. 40 sieve, into an evaporating dish.

Add water from the plastic squeeze bottle and mix the soil to the form of a uniform

paste.3. ilace a portion of the paste in the brass cup of the liquid limit device. Using the spatula,

smooth the surface of the soil in the cup such that the maximum depth of the soil is

about 8 mm.4. Using the grooving tool, cut a groove along the centerline of the soil pat in the cup

tFig.6.a(a).1.5. Tum the crank of the liquid limit device at the rate of about 2 revolutions per second.

By this, the liquid limit cup will rise and drop through a vertical distance of 10 mm once

for each revolution. The soil from the two sides of the cup will begin to flow toward

-N.% K-

" lL . - - ' . \ | \ iI

' . : . _ . ) z z "

t t mm ) r " ^

k- 50 mm -----+

40 Soil Mechanics Laboratory Manual

--+ 11 K-mm

(b)

Figure 6.4. Schematic diagram (plan) of soi l pat in cup of l iquid l imit device.(a) Beginning of test. (b) End of test.

the center. Count the number of blows N for the groove in the soil to close through adistance of 1/2in. (13 mm), as shown in Fig.6.4(b).

If N equals about 25 to 35, collect a moisture sample from the soil in the cup ina moisture can. Close the cover of the can and determine the mass of the can plus themoist soil, M2.

Remove the rest of the soil paste from the cup to the evaporating dish. Use papertowels to clean the cup thoroughly.

If the soil is too dry, N will be more than about 35. In that case transfer the soil tothe evaporating dish using the spatula. Clean the liquid limit cup thoroughly with papertowels. Mix the soil in the evaporating dish with more water, and try again.

If the soil is too wet, N will be less than about 25. In that case transfer the soil in thecup to the evaporating dish. Clean the liquid limit cup carefully with paper towels. Stirthe soil paste with the spatula for some time to dry it up. The evaporating dish may beplaced in the oven for a few minutes for drying also. Do not add dry soil to the wet-soilpaste to reduce the moisture content in order to bring it to the proper consistency. Nowtry again with the liquid limit device to get a groove closure of 1/2 in. (13 mm) between25 and 35 blows.Add more water to the soil paste in the evaporating dish and mix thoroughly. RepeatSteps 3, 4, and 5 to get a groove closure of 1/2 in. (13 mm) in the liquid limit device ata blow count N : 2O to 25. Take a moisture sample from the cup. Transfer the rest ofthe soil paste to the evaporating dish. Clean the cup with paper towels.Add more water to the soil paste in the evaporating dish and mix well. Repeat Steps 3,4, and 5 to get a blow count N of between 15 and 20 for a groove closure of 1/2 in.(13 mm) in the liquid limit device. Take a moisture sample from the cup.

--+ k-2 m m

(a)

6.

7 .

Liquid Limit Test 41

8. Put the three moisture cans in the oven to dry to constant masses, M3. (The caps of themoisture cans should be removed from the top and placed at the bottom of the respectivecans in the oven.)

6.4 CalculationsDetermine the moisture content for each of the three trials (Steps 5,6, andT),

w(vo\ : M'- M'

, looM t - M t

(6.1)

6.5 GraphsPlot a semilog graph for moisture content (arithmetic scale) versus number of blows N(log scale). This will approximate a straight line, which is called, the flow curve. Frcm thestraight line, determine the moisture content lr (7o) conespondingto 25 blows. This is theliquid limit of rhe soil.

The magnitude of the slope of the flow line is called.Ihe flow ind.ex F1,

F r :w1(Vo) - w2(Vo) (6.2)log Nz - log Nr

Typical examples of liquid limit calculations and the corresponding graphs are shown inTable 6.1 and Fis. 6.5.

6.6 General CommentsBased on the liquid limit tests on several soils, the U.S. Army Corps of Engineers (1949)observed that tlre liquid limit ZZ of a soil can be approximately given by

LL - w11 (Vo) (6.3)

where w17 is the moisture content, in percent, for 1/2-in. (13-mm) groove closure in theliquid limit device at N number of blows.

ASTM also recommends this equation for determining the liquid limit of soils (ASTMTest Designation D-4318). Howevet the value of w,ns should correspond to an N value ofbetween 20 and 30. Table 6.2 lists the values of (N t25\o tzt for various values of N.

The presence of clay contributes to the plasticity of soil. The liquid limit of a soil willchange depending on the amount and type of clay minerals present in it. The approximateranges for the liquid limits of some clay minerals are given in Table 6.3.

( * ) " " '

Table 6.1' Liquid Limit Test

Description s1 ssil GroY siltt/ cloy Sample no' ----4--

Location

Tested by

Liquid l imit l l

Flow index F737 - 33.7

- 1 6 . / 1

;s

o

E.9.

40

38

36

34

32

301 0 1 5

Figure 6.5. Plot of moisture content vs. number of blows for liquid limit test results

reported in Table 6.1.

20 25 30 35 40

Mass of can, M1 (g)

Mass of can + moist soil, M2 (g)

Mass of can * dry soil, M3 Q)

Moisture content,

w(vo) -m " rco

- Int

Liquid Limit Test

Table 6.3. Liquid Limits of some ClayMateria ls

Casagrande (1932) concluded that each blow in a standard liquid limit device corres-ponds to a soil shear strength of about 0.1 kN/m2. Hence the liquid limit of a finegrainedsoil gives the moisture content at which the shear strength of the soil is approximately2.5 kN/m2 (N52lbtfP).

Table 6.2. Variation of (y'y'l25)0'121 Values

Liquid LimitTestSample no. -

DescriDtion of soil

Location

DateTested by

Mass of can, M1 (g)

Mass of can * moist soil, Mz G)

Mass of can * dry soil, Mg (g)

Moisture content,

w(vo) -ffi " no

Number of blows, N

Liquid limit lL

Flow index Fr

181