Tag Archives: plasticity index

Defining Terms Weight–Volume Problems

AASHTO classification system — A classification system developed by the American Association of State Highway and Transportation Officials that rates soils relative to their suitability for road embankments, subgrades, subbases, and basis.
Atterberg limits — Water contents at which soil changes engineering behavior; the most important ones in classification are the liquid limit and plastic limit.
Boulders — Rock particles larger than 9 to 12 inches or 200 to 300 mm.
Clay — Fine-grained soil that exhibits plasticity.
Coarse grained — Soils that are retained on a No. 200 sieve.
Coarse fraction — In the Unified Soil Classification System, that portion of a soil sample retained on a No. 200 sieve.
Cobbles — Rock particles smaller than a boulder but larger than 3 inches (75 mm).
Coefficient of curvature — A mathematical parameter, D2
30/(D60D10), used as a measure of the smoothness of a gradation curve.
Coefficient of uniformity — A mathematical parameter, D60/D10, used as a measure of the slope of a gradation curve.
D10 size — The grain size, in mm, for which 10% by weight of a soil sample is finer.
Effective grain size — Another name for the D10 size.
Fat clay — Highly plastic clay; clay with a liquid limit greater than 50.
Fine fraction — In the unified soil classification system, that portion of a soil sample passing a No. 200 sieve.
Fine grained — Soil passing a No. 200 sieve.
Grain-size analysis — The determination of the relative proportions of soil particles of each size in a soil sample, performed by passing the sample over a nest of sieves.
Grain-size distribution curve — A plot of percent finer or coarser versus soil-grain size. Grain size is plotted on a logarithmic scale.
Granular material — In the AASHTO classification system, soil with less than 35% passing the No. 200 sieve.
Gravel — Soil or rock particles smaller than 3 inches but retained on a No. 4 sieve (Unified Soil Classification System) or on a No. 10 sieve (AASHTO system).

Lean clay — Clay with low plasticity; clay with a liquid limit less than 50.
Liquid limit — The water content above which soil behavior changes from a plastic solid to a viscous liquid.
Median grain size — The grain size for which one-half of a soil sample, by weight, is larger and half is smaller.
Nest of sieves — A stack of sieves of different sizes, having the largest opening on the top and progressing downward to successively smaller openings.
Peat — A highly organic soil, dark brown to black in color, with noticeable organic odor and visible vegetable matter.
Plastic limit — The water content above which the soil behavior changes from a brittle solid to a plastic solid.
Plasticity — The ability of a soil, when mixed with water, to deform at constant volume.
Plasticity index — The difference between the liquid and plastic limit.
Sand — Soil particles retained on the No. 200 sieve that pass the No. 4 sieve (Unified Soil Classification System) or the No. 10 sieve (AASHTO system).
Shrinkage limit — The water content at which further reduction in water content does not cause a further reduction in volume.
Sieve analysis — A grain-size analysis using a nest of sieves.
Silt — Fine-grained soil having a low plasticity index or not exhibiting plasticity.
Unified Soil Classification System — A descriptive classification system based on Casagrande’s airfield system and now standardized by ASTM D 2487-93.

Buoyant unit weight — The apparent unit weight of a submerged soil, obtained as the total unit weight
minus the weight of water.
Compaction mold — A metal mold, typically 1/30 ft3, used to determine the density of compacted soil.
Degree of saturation — The ratio of the volume of water to the volume of void space in a sample of soil.
Density — The mass per unit volume of a soil or one of its components.
Dry density — The ratio of the mass of solids to the total volume of a soil sample.
Dry unit weight — The ratio of the weight of solids to the total volume of a soil sample.
Effective unit weight — Another term for buoyant unit weight.
Porosity — The ratio of the volume of void spaces to the total volume of a soil sample.
Saturated — The condition in which all of the void spaces in a soil are filled with water and the volume
of air is zero.
Saturated unit weight — The unit weight obtained if a soil sample is saturated by adding water at  constant total volume.
Specific gravity — The ratio of the density of a material to the density of water; usually refers to the
specific gravity of soil solids.
Total unit weight — The total combined weight of solids and water in a unit volume of soil.
Unit weight — The ratio of the weight of a material to its volume.
Void ratio — The ratio of the volume of void space in a soil sample to the volume of solid particles.
Water content — The ratio of the weight of water to the weight of solids of a soil sample.

Atterberg Limits and Plasticity

Atterberg limits, named after the Swedish soil scientist A. Atterberg, are water content values at which notable changes in soil behavior occur. The liquid limit , denoted LL or w L , marks the transition between liquid and plastic behavior. At water contents above the liquid limit the soil behaves as a viscous liquid; below the liquid limit the soil behaves as a plastic solid. The liquid limit is determined in the laboratory by partly filling a standard brass cup with wet soil and cutting a groove of a standard dimension in the soil. The liquid limit is taken as the water content at which the groove closes a specified amount when the cup is lifted and dropped 1 cm exactly 25 times. The details of the test are given in AASHTO T 89 and ASTM D 4318-93. The plastic limit , denoted PL or w p , is the transition between plastic and brittle behavior. It is determined in the laboratory as the water content at which a 1/8-inch diameter thread of soil begins to crumble when rolled under the palm of the hand. Details of the liquid limit and plastic limit tests are provided by AASHTO T 90 and ASTM D 4318-93. The shrinkage limit , denoted SL or w S , is the water content below which the soil no longer reduces in volume when the water content is reduced. Although Atterberg limits are water contents and are properly decimals or percentages, they are usually expressed as an integer percentage without a percent sign. Thus, a liquid limit of 40% is usually reported as LL = 40.

The plasticity index , denoted PI or I P , is the difference of the liquid limit and the plastic limit: