Atterberg limits are excellent indicators that describe the relationship between consistency and moisture content of the fine-grained soil. Figure 1 illustrates the changes in consistency of the cohesive soil with changing its moisture content.

Figure 1: Atterberg Limits of Fine-Grained Soils

▪ Liquid Limit: is a moisture content at which a soil changes from a plastic state to a liquid state.

▪ Plastic Limit: is a moisture content at which a soil changes from a semisolid state to a plastic state.

▪ Shrinkage Limit: is a moisture content at which a soil changes from a solid state to a semisolid state.

A soil property which relates between the liquid limit and the plastic limit is defined as a

plasticity index (PI or Ip). The plasticity index is the amount of water that must be added to

change soil’s moisture content from plastic limit to liquid limit:

PI = LL − PL

Another parameter which can be used to describe the relative consistency of fine-grained soil

is called a liquidity index. Table 1 shows the relationship between liquidity index and in-situ

soil behavior of the soil. The liquidity index is given as follows:

▪ If a soil has LI > 1, field moisture content is greater than the liquid limit of the soil.

▪ If a soil has LI < 0, field moisture content is less than the plastic limit of the soil.

▪ If a soil has LI = 1, field moisture content is equal to the liquid limit of the soil.

▪ If a soil has LI = 0, field moisture content is equal to the plastic limit of the soil.

Table 1: In-situ Behavior of Cohesive Soils based on LI

The swelling potential of fine-grained soils can be identified using a correlation model which relates the plasticity index of the soil with the percentage of clay particles finer than (i.e., 0.002 mm). Table 1 shows typical values of the activity for different clay minerals. The

correlation equation is given by:

Table 2: Swelling Potential of Cohesive Soil

Table 3: Typical Activities of Clay Minerals

EX-1:

A soil has the following Atterberg limits and natural water content. Determine the PI and LI and comment its general activity.

Solution:

PI = LL − PL = 13 − 8 = 5

The soil is very sensitive and highly active.