Page 26 - Geotechnical Engineering Soil and Foundation Principles and Practice

Igneous Rocks, Ultimate Sources for Soils
Igneous Rocks, Ultimate Sources for Soils 21

2.1.8 Random Fill vs. Engineered Fill
Soils that have recently been moved from their place of origin are redeposited
as fill. The soil loses most of its identifying features and substitutes other
characteristics that are forensic clues that it is recent fill, such as containing
random bits of bricks or glass. Identification of random fill is critical because
it has not undergone geological processes of compression and cementation that
give a soil strength. Random fill therefore includes some of the weakest, most
compressible, trashiest, noxious, combustible, and least predictable materials that
can be encountered by engineers and builders. There are better places to build
than on top of old newspapers and bedsprings. More details on recognition of
random fill are presented in the next chapter and are among the more important
tools used by the geotechnical engineer.

Engineered fill is selected and compacted under controlled conditions to increase
its strength and decrease its compressibility. Fill that has been placed under these
controlled conditions in layers is a dense, competent construction material used
for foundations, highway embankments, and earth dams.

2.1.9 Relative Abundance of Sedimentary Rocks and
Sediments
Although sedimentary materials comprise only a thin skin around an igneous
earth, that skin covers most of the exposed areas of continents. As a general
picture, a deep igneous rock complex is overlain by sedimentary rocks that
occur in discrete layers. The top layer can be residual soil developed on either
of these types of rocks, or may consist of a blanket of geologically younger
clayey, silty, and sandy sediments that have been deposited by wind, water, or
glacial ice.

In the last century soil erosion has been rapidly accelerated by cultivation,
such that the upper layer of sediment on river floodplains often can be identified
as ‘‘post-cultural.’’ The soil that presently is being carried down streams and
rivers is rapidly filling reservoirs behind dams, or building deltas extending
outward into the sea. Post-cultural eroded sediment often contains agricultural
chemicals that are a threat to fisheries and can create ‘‘dead spots’’ in the sea close
to the delta.

2.2 WEATHERING AND SOIL MINERALS

2.2.1 Inherited Minerals
The mineralogical composition of sediments relates to the source rock and
weathering. For example, the composition of a pile of rock fragments that have

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