TM 5-818-7
CHAPTER 8
CONSTRUCTION TECHNIQUES AND INSPECTION
inches or more of impervious nonswelling soil on the
8-1. Minimization of foundation prob-
lems from construction
site prior to construction of the foundation to promote
drainage and trafficability on the site. Dehydrated
Many problems and substandard performance of foun-
lime may also be sprinkled on the surface of expansive
dations observed in structures on expansive soils occur
soil to promote trafficability. Sumps and pumps
from poor quality control and faulty construction prac-
should be provided at the bottom of excavations if nec-
tice. Much of the construction equipment and proce-
essary to remove rainwater or subsurface drainage en-
dures that are used depends on the foundation soil
tering the excavation. Provision for after normal duty
characteristics and soil profiles. Careful inspection
operation of the pumps should also be made.
during construction is necessary to ensure that the
structure is built according to the specifications.
cent and preferably 5 percent, to promote drainage of
a. Important
elements of construction techniques.
water away from the structure, should be provided
Construction techniques should be used that promote
around the perimeter of the structure. Low areas
a constant moisture regime in the foundation soils dur-
should be filled with compacted backfill. Runoff from
ing and following construction. The following ele-
roofs should be directed away from the structure by
ments of construction are important in obtaining ade-
surface channels or drains. Subsurface drains should
quate foundation performance in expansive soils.
be constructed to collect seepage of water through per-
vious backfills placed adjacent to the foundation.
pleted as quickly as possible to the design depth and
b. Considerations of construction inspection. Table
protected from drying. An impervious moisture barri-
8-1 lists major considerations of construction inspec-
er should be applied on the newly exposed surfaces of
tion. Inspections related to concrete reinforced slab
the excavation to prevent drying of the foundation
and drilled shaft foundations, the two most commonly
soils immediately after excavating to the design depth.
used foundations in expansive soil areas, are discussed
Sides of the excavation should be constructed on a lV
below.
on lH slope or an appropriate angle that will not trans-
mit intolerable swelling pressures from the expansive
8-2. Stiffened slab foundations
soil to the foundation. The foundation should be con-
Items in table 8-2 should be checked to minimize de-
structed in the excavation as quickly as practical.
fective slab foundations.
(2) Selection of materials. Selected materials
should conform to design requirements.
a. The inspector should check for proper site prep
(a) Backfills should be nonswelling materials.
aration and placement of the moisture barrier, steel,
(b) Concrete should be of adequate strength and
and concrete. All drainage systems should be inspected
workability.
for proper grade and connections to an outlet.
(c) Reinforcing steel should be of adequate size
b. Posttensioned slabs require trained personnel
and strength.
and careful inspection to properly apply the postten-
(d) Moisture barriers should be durable and im-
sioning procedure. For example, anchors for the steel
pervious.
tendons should be placed at the specified depth (lower
(3) Placement of materials. All structural materi-
than the depth of the tensioning rods) to avoid pullout
als should be positioned in the proper location of the
during tensioning. Tendons should be stressed 3 to 18
foundation.
days following the concrete placement (to eliminate
much of the shrinkage cracking) such that the mini-
expansive soil should be compacted to minimize effects
mum compressive stress in the concrete exceeds 50
of volume changes in the fill on performance of the
pounds per square inch. Stressing should be completed
foundation. Backfills should not transmit intolerable
before structural loads are applied to the slabs.
swell pressures from the natural expansive foundation
soil to basement or retaining walls.
8-3. Drilled shaft foundations
Items in table 8-3 should be checked to minimize de-
be prepared to avoid ponding of water in low areas.
fective shaft foundations. The foundation engineer
Consideration should be given to compaction of 6 to 12
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