TM 5-818-1 / AFM 88-3, Chap. 7
d. Treat the soil in place prior to construction
age space, and space for air conditioning and other
equipment.
The last item otherwise may require
to improve its properties. This procedure generally
valuable building space or disfigure a roofline.
requires considerable time. The latter two procedures
b. While mat foundations are more expensive
are carried out using various techniques of soil
to design than individual spread footings, they usually
stabilization described in chapter 16.
result in considerable cost reduction, provided the total
area of spread footings is a large percentage of the
9-3.
Cost estimates and final selection.
basement area. Mat foundations may decrease the
a. On the basis of tentative designs, the cost
required excavation area,
compared with spread
of each promising alternative should be estimated.
footings.
Estimate sheets should show orderly entries of items,
c. The most promising foundation types
dimensions, quantities, unit material and labor costs, and
should be designed, in a preliminary manner, for detailed
cost extensions. Use local labor and material costs.
cost comparisons. Carry these designs far enough to
b. The preliminary foundation designs that are
determine the approximate size of footings, length and
compared must be sufficiently completed to include all
number of piles required, etc. Estimate the magnitude of
relevant aspects. For example, the increased cost of
differential and total foundation movements and the
piling may be partially offset by pile caps that are smaller
effect on structure. The behavior of similar foundation
and less costly than spread footings. Similarly, mat or
types in the area should be ascertained.
pile foundations may require less excavation.
d. Final foundation design should not be
Foundation dewatering during construction may be a
started until alternative types have been evaluated. Also,
large item that is significantly different for some
the effect of subsurface conditions (bearing capacity and
foundation alternatives.
settlement) on each alternative should be at least
c. The most appropriate type of foundation
qualitatively evaluated.
generally
represents
a
compromise
between
e. A checklist of factors that could influence
performance, construction cost, design cost, and time.
foundation selection for family housing is shown in table
Of these, design cost is generally the least important and
9-2.
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
9-2.
Adverse subsurface conditions. If poor soil
should generally govern.
conditions are encountered, procedures that may be
d. Foundation
soils
pretreatment
by
used to ensure satisfactory foundation performance
precompression under temporary surcharge fill,
include the following:
regardless of whether vertical sand drains are provided
a. Bypass the poor soil by means of deep
to accelerate consolidation, requires a surcharge loading
foundations extending to or into a suitable bearing
period of about 6 months to a year. The time required
material (chap. 11).
may not be available unless early planning studies
b. Design the structure foundations to
recognized the possible foundation cost reduction that
accommodate
expected
differential
settlements.
may be achieved.
Precompression is frequently
Distinguish between settlements during construction that
advantageous for warehouses and one-story structures.
affect a structure and those that occur during
Precompression design should be covered as a separate
construction before a structure is affected by differential
design feature and not considered inherent in structure
settlements.
design.
c. Remove the poor material, and either treat
and replace it or substitute good compacted fill material.
9-2
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