llMrl

Plastic Limit Test

7.1 IntroductionThe fundamental conceptof plnstic limit was introduced in section 6.1 (see Fig. 6.1). Plastic

limit is defined as the moisture content, in percent, at which a cohesive soil will change

from a plastic state to a semisolid state. ln the laboratory the plastic limir is defined as the

moisture content, in percent, at which a thread of soil will just crumble when rolled to a

diameter of 1i8 in. (3.2 mm). This test might be seen as somewhat arbitrary and, to some

extent, the result may depend on the person performing the test. with practice, however,

fairly consistent results may be obtained'

7.2 Equipment1. Porcelain evaporating dish2. Spatula3. Plastic squeeze bottle with water4. Moisture can5. Ground glass plate6. Balance sensitive to 0.01 g7. Oven

The equipment (except for the oven) is shown in Fig. 7'1.

7.3 Procedurel. Put approximately 20 g of a representative air-dry soil sample, passed through a No' 40

sieve, into a porcelain evaporating dish'2. Add water from the plastic squeeze bottle to the soil and mix thoroughly'

3. Determine the mass M1 of a moisture can in grams and record it on the data sheet.

45

46 Soil Mechanics Laboratory Manual

Figure 7.1. Equipment for plast ic l imit test.

4. From the moist soil prepared in Step 2, prepare several ellipsoidally shaped soil massesby squeezing the soil with your fingers.

5. Take one of the ellipsoidally shaped soil masses (Step 4) and roll it on a ground glassplate using the palm of your hand (Fig. 7.2). The rolling should be done at the rateof about 80 strokes per minute. Note that one complete backward and one completeforward motion of the palm constitute a stroke.

6. When the thread being rolled in Step 5 reaches 1/8 in. (3.2 mm) in diameter, break itup into several small pieces and squeeze one piece with your fingers to form again anellipsoidal mass.

7. Repeat Steps 5 and 6 until the thread crumbles into several pieces when it reaches adiameter of 1/8 in. (3.2 mm). rt is possible that a thread may crumble at a diameter largerthan 1/8 in. (3.2 mm) during a given roling process, whereas it did not crumble at ihesame diameter during the immediately previous rol\ing.

8. Collect ttre sma\\ crumbled pieces in the ".oist\',e

can and, put ttre correr on the can.9. Take rt\e other e\'rpsoid.a\ so\\ masses tormed in Step 4 and repeat Steps 5 through g.10' Determine the mass of the rnoisture can prus the wet so , M2, in grams. Remo've the

cap from the top of the can and place the can in the oven (with the ;ap at the bottom ofthe can).

Calculate the olastic limit PZ.

7.4 Calculations

Plastic Limit Test 47

Figure 7.2. El l ipsoidal soi l mass being rol led in a thread on a glass plate.

11. After about 24 hours. remove the can from the oven and determine the mass of the canplus the dry soil, M3, in grams.

mass of moisture Ml - MzDr - - - ----:-----------: \. lrYl

mass of dry soil Mt - M t(7.1)

The results may be presented in tabular form, as shown in Table 7.1. If the liquid limit ofthe soil is known, calct:Iate the plasticity index PI

PI _ LL- PL (7.2)

7.5 GeneralCommentsTable 7.2 lists the approximate ranges for the plastic limits of some clay minerals. Theliquid limit and the plasticity index of cohesive soils are important parameters for classific-tion purposes. The engineering soil classification systems are described in Chapter 9. Theplasticity index is also used to determine the activity A of a clayey soil, which is defined as

A : P I

.Vo clay-size fraction (by weight)

The range of activity A of some clay minerals is given in Table 7.3.

Soil Mechanics Laboratory Manual

Description

Table 7.1. Plastic Limit Test

ot soil Groy cloyey silt Sample no. .3

Date

Location

Tested by

Can No. t 03

Mass of can, Mr (g) 13.33

Mass of can + moist soil, M2 (g) 23.86

Mass of can * dry soil, M: (g) 22.27

M o - M qPL: M;fr X 100t7.78

Plast ic i tv index Pt: LL- PL: 34- 17.78 : I878

Table7.2. Range of Plastic Limits

Clay Mineral PL

Kaolinite 2040

Illite 35-60

Montmorillonite 50-100

Chlorite 35-40

Hydrated halloysite 40-60

Table 7.3. Range of ,4 Values

Clay Mineral A

Kaolinite 0.3-0.5

Illite 0.5-t.2

Montmorillonite 1.5-7.0

Hydrated halloysite 0.1-0.2

Plastic Limit Test

Liquidity index tI is a term that defines the relative consistency of a clayey soil in itsnatural state. It can be expressed as

w - P LL I -

u - P L

where w is the natural moisture content.For sensitive clays Zl may be greater than 1. Heavily overconsolidated clays may have

LI <O.

Plastic LimitTestDescriDtion of soil Sample no. -

Location

DateTested by

Plasticity index Pl: LL - PL -

Mass of can, M1 (g)

Mass of can * moist soil, Mz G)

Mass of can + dry soil, M: (g)

, r : f f i , r c0

183