Foundation - Selection Considerations.

Selection of an appropriate foundation depends upon the structure function,  soil and groundwater conditions, construction schedules, construction economy, 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, evaluate alternative types of foundations for the bearing capacity and total and differential settlements.  Some foundation alternatives for different subsoil conditions are summarized in table 9-1.

Foundation Possibilities for Different Subsoil Conditions
Table 9-1.  Foundation Possibilities for Different Subsoil Conditions

      a. 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.

      b. 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.

      c. 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.

      d. 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.

      e. A checklist of factors that could influence foundation selection for family housing is shown in table 9-2.

Checklist for Influence of Site Charateristics on Foundation Selection for Family Housing
Table 9-2.  Checklist for Influence of Site Charateristics on Foundation Selection for Family Housing

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