Page 100 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
Soils That Are Sediments 95

4.9.4 Collapsible Loess
A common problem associated with many loess deposits is collapsibility, meaning
that it collapses when wet with water. This is consistent with the hypothesized
eolian origin from wind such that it never has become saturated. Ground-
water tables in collapsible loess are perched near the bottom of the deposit, and
soil below a groundwater table is dense and not collapsible.

The problem for the engineer is to keep a collapsible loess from ever becoming
saturated. Saturation can occur at leaky joints in pavement gutters, near leaky
water or sewer pipes, or near downspouts. Close attention must be paid to surface
drainage to prevent ponding, and roof water should be carried several meters
away from foundations.

Damages from collapse also can be prevented by presaturating in order to induce
collapse prior to construction, or by founding structures on deep foundations
such as pile. Presaturation is the obvious choice for dams and canals, but after
collapse the wet soil remains soft and compressible and may require surcharging.

4.9.5 Changes with Distance from a Source

Collapsible loess occurs close to a source area. Farther away, loess deposits are
thinner, more clayey, more likely to have been saturated, and less likely to be
collapsible. As the loess was carried by wind, the larger silt grains settled more
rapidly, allowing clay to become more concentrated farther away, where its
sedimentation was aided by rain. A slower rate of deposition also provides more
opportunity for weathering during deposition.

Because the properties of a loess deposit gradually change with increasing
distance from a source, loessial soils exhibit a spectrum of behavior from
collapsible close to a source to relatively stable to expansive farther away. This is
referred to by geologists as a ‘‘facies change’’ (facies from Latin for face).
Expansiveness or collapse can be measured in a laboratory compression test or
can be indicated from the soil density and classification that are discussed in later
chapters.

4.10 MAN-MADE FILL

4.10.1 Overview

As the best building sites become used and re-used, random fill becomes more
commonplace and can contain everything from concrete rubble to tree limbs and
old refrigerators. The recognition of landfill can be one of the easiest but
nevertheless most critical duties of the geotechnical engineer. Random fill usually
is not compacted, and even if it is compacted it can contain open voids.

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