19 June 2001
Textbook DM 7.02, Foundations and Earth Structures for further information on
Description. A complete description of a soil includes classification,
density, shear strength, moisture content, and mineralogic content. For soils
used in waterfront structures, it is often sufficient to classify them according to
size (clay, silt, sand, and gravel.) The density, plasticity, and moisture content
are important for the finer-grained soils, while soundness and gradation are
applicable to the coarser-grained soils and rock fills.
The particle size, which marks the boundary between the fine-grained,
generally cohesive soils (silts and clays) and the coarse-grained, granular soils
(sands and gravels), is approximately the minimum size retained on the No. 200
standard sieve. Organic soils, such as elastic silts and peats, are never used in
the construction or repair of engineering structures.
Maintenance problems increase as the grain size of the soil gets
smaller. Finer-grained soils in the cohesionless range are extremely susceptible
to leaching and erosion, whereas fine-grained cohesive soils are difficult to
compact satisfactorily and may undergo undesirable shrinkage or swelling.
With granular soils, gradation is important. Uniformly graded soils with
a narrow range of particle sizes are difficult to compact, are extremely porous,
and have lower densities and strengths than soils with a broader distribution of
particle sizes. However, where compaction of sands and gravels is involved,
large, oversize cobbles can interfere with the compaction of the finer materials
present. Such large particles should be removed from the compacted fills and
used as riprap or slope protection.