6.3.5 Consistency of soil

 

Soil consistency is the resistance of a soil at various moisture contents to mechanical stresses or manipulations. It is commonly measured by feeling and manipulating the soil by hand or by pulling a tillage instrument through it. The consistency of soils is generally described at three soil moisture levels: wet, moist and dry. Consistency of a soil sample changes with the amount of water present. Such change in soil consistency may be accurately measured in the laboratory following standard procedures which determine the Atterberg Limits. This limit corresponds to the moisture content at which a soil sample changes from one consistency to another. Two of the Atterberg Limits are important: liquid limit and plastic limit.

 

Atterberg Limits are basic measure of the nature of a fine-grained soil. Depending on the water content in the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state, consistency and behavior of the soil is different and so are its engineering properties. Thus, the boundary between each state can be defined based on a change in the soil's behavior.

 

Shrinkage Limit: Shrinkage limit (SL) is the water content where further loss of moisture will not result in any more volume reduction. The testing standard to determine shrinkage limit is explained by ASTM International D4943. Shrinkage limit is much less commonly used than the liquid and plastic limits.

 

Liquid limit (LL): It is defined as the percentage moisture content at which a soil changes with decreasing wetness from liquid to plastic consistency or with increasing wetness from plastic to liquid consistency. It is the water content at which a soil changes from plastic to liquid behavior. The importance of this liquid limit test is to classify soils.

 

Plastic limit (PL): it is the percentage moisture content at which a soil changes with decreasing wetness from the plastic to the semi- solid consistency or with increasing wetness from the semi-solid to the plastic consistency.  Plastic limit is the lower limit of the plastic state. A small increase in moisture above the plastic limit destroys cohesion of the soil. Plastic Limit test is defined by ASTM standard test method D 431.