Page 185 - Geotechnical Engineering Soil and Foundation Principles and Practice

Source: GEOTECHNICAL ENGINEERING
9 Soil Density and Unit Weight

9.1 DENSITY AND MOISTURE CONTENT

9.1.1 Heavy Is the Soil
Soil density and unit weight are basic to many soil and foundation problems, as
the weight of soil involved can eclipse that of other materials used to build
structures. For example, the weight of soil removed for a basement usually
exceeds the weight of the house that will stand in its place. This does not ensure
that the house will be safe from settlement because its weight is not evenly
distributed, but is mainly carried by the foundations.

The most critical component of a foundation is not concrete, which obviously
is much stronger than the soil on which it rests. Nor does the weight of the
building simply derive support from the soil underneath the foundation, as
foundations often are extended to a width that enables the soil alongside to keep
the soil underneath from squeezing out. Fig. 9.1 illustrates what can happen if the
soil alongside is removed. Support for foundations therefore is not simply a
matter of soil strength but also involves a calculated balance that takes into
account both the weight and strength of the soil. In Fig. 9.1 a basement was
excavated next to the foundation for a municipal fire station with predictable
consequences.

Soil weight also contributes the ‘‘body forces’’ that are the driving forces for
landslides, and exert pressures on buried pipes and tunnels. Lateral bulging of soil
under its own weight is opposed by support from retaining walls.

Soil weight also can have a positive influence because it contributes to friction that
helps to restrain a landslide and support a building foundation. Soil weight
therefore can appear on both sides of the equation.

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