Highway Engineering

The penetration of the needle into the sample is measured to the nearest tenth

of a millimetre. The moisture content of the sample is then determined.

The process is repeated four more times, on each occasion with a sample of

differing moisture content. A relationship between cone penetration and

moisture content can then be established, allowing the moisture content

corresponding to a cone penetration of 20 mm to be determined. This moisture

content is termed the liquid limit of the soil under examination. See BS 1377

for further details of the cone penetrometer test.

Plastic limit

The plastic limit is defined as the moisture content at which the soil in question

becomes too dry to be in a plastic condition. The plastic limit test, as defined

by BS 1377, involves taking a 15 g soil sample, mixing it with water, and rolling

it into a 3 mm diameter thread. (The rolling process will reduce the moisture

content of the sample.) This process is done repeatedly for different samples

until the point is reached when the sample just crumples when rolled into a

3 mm diameter thread. The moisture content of the sample in question can be

taken as the plastic limit of that soil.

Plasticity index

The plasticity index of a soil is defined as the liquid limit of a soil minus its

plastic limit:

Plasticity index (PI) =Liquid limit (LL) -Plastic limit (PL) (7.1)

It denotes the moisture content range over which the soil is in a plastic state.

Using plasticity index to derive CBR

If it is not possible to derive the CBR of a soil using the standard test referred

to in section 7.2.2, its plasticity index can be used as a means of assessing it

(Black & Lister, 1979). This method determines the long-term CBR of various

subgrades, as shown in Table 7.3.

Notes to Table 7.3:

(1) A high water table is one situated less than 300 mm below formation level

(2) A low water table is one situated more than 1 m below formation level

(3) Poor conditions denote the situation where the lowest layer of the pave-

ment is laid on weak soil in heavy rain

(4) Average conditions denote the situation where the formation is protected

during adverse weather

(5) Good conditions denote the situation where the soil is drier than its likely

service conditions during construction

198 Highway Engineering