15 AUGUST 2005
Design the structure foundations to accommodate expected differential
settlements. Distinguish between settlements during construction that affect a structure
and those that occur during construction before a structure is affected by differential
Remove the poor material, and either treat and replace it, or substitute
good compacted fill material.
Treat the soil in place prior to construction to improve its properties. This
procedure generally requires considerable time. The latter two procedures are carried
out using various techniques of soil stabilization described in Chapter 16.
Cost Estimates and Final Selection
On the basis of tentative designs, the cost of each promising alternative
should be estimated. Estimate sheets should show orderly entries of items,
dimensions, quantities, unit material and labor cost, and cost extensions. Use local
labor and material costs.
The preliminary foundation designs that are compared must be sufficiently
completed to include all relevant aspects. For example, the increased cost of piling may
be partially offset by pile caps that are smaller and less costly than spread footings.
Similarly, mat or pile foundations may require less excavation. Foundation dewatering
during construction may be a large item that is significantly different for some foundation
The most appropriate type of foundation generally represents a
compromise between performance, construction cost, design cost, and time. Of these,
design cost is generally the least important and should not be permitted to be a
controlling factor. If a lower construction cost can be achieved by an alternative that is
more expensive to design, construction cost should generally govern.
Foundation soils pretreatment by precompression under temporary
surcharge fill, regardless of whether vertical drains are provided to accelerate
consolidation, requires a surcharge loading period of about 6 months to a year. The
time required may not be available unless early planning studies recognized the
possible foundation cost reduction that may be achieved. Precompression is frequently
advantageous for warehouses and one-story structures. Precompression design should
be covered as a separate design feature and not considered inherent in structure