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Plasticity and Moisture Content of Cohesive Soils
المؤلف: Prentice Hall
المصدر: Foundation Design: Principles and Practices
الجزء والصفحة:
29-1-2023
786
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.