As a dry, clayey soil takes proctor compaction test procedure pdf increasing amounts of water, it undergoes distinct changes in behavior and consistency. In each state, the consistency and behavior of a soil are different and consequently so are its engineering properties.
Thus, the boundary between each state can be defined based on a change in the soil’s behavior. Distinctions in soil are used in assessing the soils that are to have structures built on them. Soils when wet retain water, and some expand in volume. These tests are mainly used on clayey or silty soils since these are the soils that expand and shrink due to moisture content. Clays and silts react with the water and thus change sizes and have varying shear strengths. If it is an expansive soil, this soil will also begin to swell in volume as this moisture content is exceeded. Increasing the water content beyond the soil’s plastic limit will transform it into a malleable, plastic mass, which causes additional swelling.
The soil will remain in this plastic state until its liquid limit is exceeded, which causes it to transform into a viscous liquid that flows when jarred. The shrinkage limit is much less commonly used than the liquid and plastic limits. The procedure is defined in ASTM Standard D 4318. If the soil is at a moisture content where its behavior is plastic, this thread will retain its shape down to a very narrow diameter.
The sample can then be remolded and the test repeated. As the moisture content falls due to evaporation, the thread will begin to break apart at larger diameters. The plastic limit is defined as the moisture content where the thread breaks apart at a diameter of 3. A soil is considered non-plastic if a thread cannot be rolled out down to 3. 2 mm at any moisture possible. However, the transition from plastic to liquid behavior is gradual over a range of water contents, and the shear strength of the soil is not actually zero at the liquid limit.
The precise definition of the liquid limit is based on standard test procedures described below. A groove was cut through the pat of clay with a spatula, and the bowl was then struck many times against the palm of one hand. Casagrande subsequently standardized the apparatus and the procedures to make the measurement more repeatable. The cup is repeatedly dropped 10 mm onto a hard rubber base at a rate of 120 blows per minute, during which the groove closes up gradually as a result of the impact. The number of blows for the groove to close is recorded. The moisture content at which it takes 25 drops of the cup to cause the groove to close over a distance of 12.
The test is normally run at several moisture contents, and the moisture content which requires 25 blows to close the groove is interpolated from the test results. The liquid limit test is defined by ASTM standard test method D 4318. It is based on the measurement of penetration into the soil of a standardized cone of specific mass. Europe due to being less dependent on the operator in determining the Liquid Limit. The values of these limits are used in a number of ways. There is also a close relationship between the limits and properties of a soil such as compressibility, permeability, and strength.
This is thought to be very useful because as limit determination is relatively simple, it is more difficult to determine these other properties. Thus the Atterberg limits are not only used to identify the soil’s classification, but it allows for the use of empirical correlations for some other engineering properties. The plasticity index is the size of the range of water contents where the soil exhibits plastic properties. W is the natural water content. W is the existing water content.