CHARACTERIZATION TESTS USEFUL IN DETERMINING ENGINEERING PROPERTIES

Technical Note DOER-C2

May 1999

Limits, either individually or with other soil properties, can be correlated to other properties such

as compactability, compressibility, shear strength, or permeability. The water content above which

a dredged material is in a semiliquid state is its LL. The water content that is the lower limit of the

plastic state and the upper limit of the semisolid state is the PL. If the water content of the dredged

material is below its PL, it becomes brittle and breaks into fragments when remolding is attempted.

The plasticity index (PI), liquidity index (LI), and activity index (AI) are derived from the PL and

LL. The PI is the difference between the LL and PL. Materials with a large PI have more plasticity

than those with a smaller PI. The PI is directly proportional to the clay content. The LI is some

dimensionless number that indicates the ratio of the water content w of a cohesive soil minus the

ratio of its PL to the PI (LI = w - PL/PI), and it normalizes the water content relative to the plasticity

index. The following relationships are noted for remolded soil: (a) when the LI = 0, the soil is at

its PL water content and is stiff, (b) when LI = 1.00, the soil is at the LL water content and is soft,

and (c) when LI > 1.00, the soil is liquidlike (slurry). The AI is the ratio of the PI to the percentage

of clay and is useful in identifying the type of clay minerals present in the dredged material: AI =

0.3-0.5 for kaolinite, AI = 0.5-1.0 for illite, and AI = 1-7 for montmorillonite. Each clay mineral

has a unique behavior. Knowledge of the clay mineral type aids in determining the behavior and

water-holding capacity of the dredged material.

Organic Content/Organic Matter. The organic content in a soil can contribute to high

plasticity, high shrinkage, high compressibility, permeability, or low strength. Soils with significant

amounts of organic matter generally have lower shear strength and higher compressibility than those

composed mainly of inorganic minerals. An organic soil is one where the LL of the oven-dried soil

is <75 percent of the LL of the soil before it was dried. While a certain amount of organic material

can be desirable (e.g., enhanced buffering capacity, immobilizing contaminants), it can make

characterization of dredged material more difficult since there are many forms of organic materials,

and, depending on the origin, each has distinctive attributes.

CHARACTERIZATION TESTS USEFUL IN DETERMINING ENGINEERING PROPERTIES

Compaction Tests. One of the basic and least expensive construction procedures used for soil

stabilization is compaction. Compaction mechanically increases the amount of solids per unit

volume of soil. It improves the engineering properties of foundation material so that the required

shear strength, structure, or void ratio are obtained, while decreasing the shrinkage, permeability,

and compressibility. Compaction is often required when building subgrades or bases for airport

pavements, roads, embankments, earthfill dams, or similar structures. The Proctor and California

Bearing Ratio (CBR) are two commonly used compaction tests. Three basic Proctor (compaction)

tests are used depending on the amount of compaction anticipated: the standard, the modified, and

the 15-blow compaction tests. The standard compaction test is generally used in routine foundation

and embankment design to simulate field compaction; the modified compaction test is used when

a higher level of compaction is desired; and the 15-blow compaction test is used when lower levels

of compaction are required. These tests aid in determining the percent compaction and water

content necessary to obtain the desired engineering properties for construction. Before a dredged

material is used as a fill for road bases, foundation pads, or embankments, it is vital that the amount

of compaction needed to obtain the required shear strength, compressibility, and permeability is

determined.