15 AUGUST 2005
SELECTION OF FOUNDATION TYPES
SELECTION OF FOUNDATION TYPE
Foundation Selection Considerations. Selection of an appropriate
foundation depends upon the structure's function, existing soil and groundwater
conditions, construction schedules, construction economy, the value of basement area,
and other factors. On the basis of preliminary information concerning the purpose of the
structure, foundation loads, and subsurface soil conditions, alternative types of
foundations for the bearing capacity and total and differential settlements should be
evaluated. Some foundation alternatives for different subsoil conditions are
summarized in table 3-1. When making foundation selections consider the following:
Some foundation alternatives may not be initially obvious. For example,
preliminary plans may not provide for a basement, but when cost studies show that a
basement permits a floating foundation that reduces consolidation settlements at little or
no increase in construction cost, or even at a cost reduction, the value of a basement
may be substantial. Benefits of basement areas include needed garage space, office or
storage space, and space for air conditioning and other equipment. The last item
otherwise may require valuable building space or disfigure a roofline.
While mat foundations are more expensive to design than individual
spread footings, they usually result in considerable cost reduction, provided the total
area of spread footings is a large percentage of the basement area. Mat foundations
may decrease the required excavation area, compared with spread footings.
The most promising foundation types should be designed, in a preliminary
manner, for detailed cost comparisons. Carry these designs far enough to determine
the approximate size of footings, length and number of piles required, etc. Estimate the
magnitude of differential and total foundation movements and the effect on structure.
The behavior of similar foundation types in the area should be ascertained.
Final foundation design should not be started until alternative types have
been evaluated. Also, the effect of subsurface conditions (bearing capacity and
settlement) on each alternative should be at least qualitatively evaluated.
A checklist of factors that could influence foundation selection for family
housing is shown in table 3-2.
Adverse Subsurface Conditions. If poor soil conditions are
encountered, procedures that may be used to ensure satisfactory foundation
performance include the following:
Bypass the poor soil by means of deep foundations extending to or into a
suitable bearing material (chap. 11).