Chapter-3-Types-of-Fnds - Chapter 3 TYPES OF FOUNDATIONS...

Chapter 3
TYPES OF FOUNDATIONS, FORCES AND DESIGN CRITERIA
A thorough geotechnical investigation will usually suggest
the ideal type of foundation to be used at a site. This implies
functionality, safety and cost effectiveness. Usually, the final
report will suggest the best two alternatives.
Typically, a building is primarily designed for function.
After that criteria is satisfied, then the two subsequent
criterions considered by its designer are its aesthetics and its
structural requirements. Finally, after all these are satisfied,
foundations are accommodated around the above constraints.
Rarely are foundations considered at the outset. Exceptions are
in seismic severe areas, and in predominantly civil works, such
as earth dams, deep excavations, etc. This has not always been
so. The influence of foundations on architecture has existed
since prehistoric times, and become notable in chaldean and
egyptian public works. In contrast, most greek monuments, so
influential to modern architecture, ignored the fine foundation
experience developed by older builders, primarily because the
Greeks benefited from their peninsula's surficial rocks.
The structural solution of a building must consider the
behavior of the soil over time. For example, a reinforced
concrete building behaves poorly in consolidating clay, when
compared to a more flexible steel structure. Foundations must
therefore be visualized as a natural connection between the
structure and the soil, minimizing poor connectivity by adding
flexibility to the structure.
For example, the use of basements should be viewed both
from a geotechnical and a space requirement points of view. The
designer should answer these questions:
1. Is the rock shallow?; does it require blasting?
2. Is the bearing stratum within 4 meters from the
surface? These 4 meters are easily reachable if you had planned
for a basement, but expensive otherwise. Also, a basement may
wipe out a good surficial stratum, forcing the use of deep
foundations.
3. In very soft ground, use a floating or
compensated mat foundation. The economics of a compensated mat

suggests the use of one basement per each six stories of
building height. However, two or three levels below the water
table may require the use of expensive cut-off walls.
TYPE OF BUILDING
The average cost of a foundation for a multistory building
ranges from 3 to 8% of the construction cost. Special
structures, however, or difficult soils may raise the cost to
20% or higher. Light structures, such as 2 or 3 story buildings
can be supported on inexpensive shallow foundations. Taller
buildings might require more expensive solutions, such as mats
or deep shafts to minimize differential settlements between
columns.
Very tall buildings (skyscrapers) experience large
horizontal forces from wind and seismic activity, and require
greater constraints, such as:
a) Reduce soil deformations to minimal magnitudes, if
necessary, via extensive soil improvement (for example,
consolidation and grouting);
b) Take the building load to much deeper levels, in order