CHARACTERIZATION TESTS USEFUL IN DETERMINING PHYSICAL PROPERTIES

Technical Note DOER-C2

May 1999

CHARACTERIZATION TESTS USEFUL IN DETERMINING PHYSICAL PROPERTIES

Grain Size, Particle Shape, and Texture. Grain size and particle shape are useful in

determining the stability, resistance to shear, permeability, compressibility, and compactability of

the dredged material. Grain size can be determined mechanically with sieves (direct) or indirectly

with the hydrometer or pipette methods. Sieving is not practical for silt- or clay-sized particles

since they tend to clog the screen. When conducting grain-size determinations on silt- or clay-sized

particles, sedimentation in water (hydrometer or pipette methods) is preferred. Grain-size distri-

bution and particle shape significantly impact on the weight-bearing capacity of soil or dredged

material. Angular particles tend to interlock, forming a stable dense mass capable of bearing more

weight than rounded particles, which tend to slide or roll past each other. Dense soils have greater

weight-bearing capacities than loose soils. The strain required to reach failure is approximately

twice as large for angular-shaped particles as that required to reach failure for spherical particles.

The texture of a soil is its appearance or "feel" and depends on the relative size and shape of the

particles, as well as the range or distribution of those sizes. Soil texture is affected by the mineral

content, organic matter, soil aggregates, and moisture present in the soil. Soil texture contributes

to the water-storage capacity, water-infiltration rates, aeration, fertility, and ease of tilling, as well

as compressibility. The texture of dredged material can limit its beneficial uses. For example,

predominantly sandy dredged material can be used as a fill material or in dike construction, but

might not be suitable for vegetation establishment because of its low nutrient content and water-

holding capacity.

Water Content and Permeability. Water content and permeability are interrelated and have a

significant influence on the suitability of a dredged material for use as a fill, subgrade, or foundation

material. Water content (w) is one of the most important factors affecting the properties and

behavior of dredged material. Water content is the ratio of the weight of water to the dry weight

of the solids in a mass of dredged material, expressed as a percentage. Soil must be compacted to

obtain the required strength and density while the water content is maintained at the optimum level

during construction projects (e.g., embankments, highway subgrades). The behavior of fine-

grained soils, like silt or clay, is influenced by the water content.

Permeability is one of the factors that determine shear strength and is a measure of water or air

movement through the dredged material. Permeability is determined by mineralogical composition,

particle size and distribution, void ratio, degree of saturation, and pore fluid characteristics. Very

fine-grained materials (clayey) generally have low permeability rates to water, and this is a desirable

feature when dredged material is used as fill or foundation material in landfills. However, if the

material is to be used for revegetation projects, coarse-grained material would need to be added to

clayey material to enhance aeration and root penetration.

Atterberg Limits (Plasticity Tests). Plasticity tests are conducted on dredged material that is

finer than 0.425 mm to determine the range of water content in which plasticity is exhibited. The

types and amounts of clay particles present and water content, as well as the physicochemical

interactions of clay particles, determine the plastic behavior of a dredged material. The Atterberg

Limits consist of the liquid limit (LL) and plastic limit (PL) and can be used to assess the amount

of dewatering needed before a dredged material can be handled and processed. The Atterberg