Page 171 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
8 Soil Fabric and Structure

8.1 SOME IMPORTANT RELATIONSHIPS

8.1.1 Relevance in Engineering
Probably no characteristic of soils is more changeable and has a greater influence
on engineering properties than fabric, which is the spatial arrangement between
soil particles and soil voids. Larger aspects of fabric such as bedding, cross-
bedding, shrinkage cracks, and shear surfaces are defined as soil structure, or
more specifically, macrostructure. However, structure and fabric often are used
interchangeably. Mitchell (1993) defines structure as fabric that includes its
strength and stability.

While the engineering importance of fabric has been recognized for many decades,
the difficulty of measurement and of defining meaningful measures has put fabric
almost in a category of afterthought, and it is only recently that marked changes
in soil behavior have been observed that have brought fabric to a microscopic
center-stage.

For example, the most common rudimentary treatment of soil, compaction,
requires that the existing soil structure must be broken down in order to expel air
and push grains together. Compaction, if properly performed, makes soil stronger
and less compressible under load. However, if the soil is compacted too wet so
that voids are full of water, remolding can cause the soil to dramatically become
weaker. If a soil is compacted too dry it can retain an open structure that will
collapse upon wetting even though a specified minimum density has been
achieved. Both of these problems involve soil fabric.

Fabric is closely related to particle size and mineralogy, and therefore is
conveniently considered in relation to whether a soil is granular, cohesive, or has
attributes of both categories of soil.

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