TM 5-818-7
should visit the construction site during boring of the
lems of this method. Stiff or very stiff cohesive soils
first shaft holes to verify the assumptions regarding
with no joints or slickensides are usually needed. Un-
the subsurface soil profile, e.g., the nature and location
derreams are vulnerable to caving and should be con-
of the subsoils. Periodically, he or she should also
structed as quickly as possible.
check the need for the designer to consider modifica-
(3) Problems with the casing method. Slurry
tions in the design.
should be used while drilling through caving soil prior
to placement of the casing and sealing of the casing in
a. Location of shaft base. The base of the shaft is lo-
an impervious layer. An impervious layer is necessary
cated in the foundation soils to maintain shaft move-
to install the bottom end of the casing.
ments within tolerable limits. This depth depends on
(a) Casing should not be pulled until the head of
the location and thickness of the expansive, compressi-
concrete is sufficient to balance the water head exter-
ble or other unstable soil, sand lenses or thin perme-
nal to the casing; otherwise, groundwater may mix
able zones, depth to groundwater, and depth to foun-
with the concrete.
dation soil of adequate bearing capacity. The design
(b) Squeezing or localized reduction in the bore-
depth may require modification to relocate the base in
hole diameter on removal of the casing can be mini-
the proper soil formation of adequate bearing capacity
mized by using a relatively high slump concrete with a
and below the active zone of heave. The purpose of lo-
sufficient pressure head.
cating the base of the shaft in the proper soil forma-
(c) Casing sometimes tends to stick in place dur-
tion should be emphasized to the inspector during the
ing concrete placement. If the concrete appears to be
first boring of the drilled shaft foundation. Under-
setting up, attempts to shake the casing loose should
reams may be bored in at least l.5-foot-diameter (pref-
be abandoned and the casing left in place to avoid the
erably 2.5-foot) dry or cases holes where inspections
formation of voids in the shaft when the casing is
are possible to ensure cleanliness of the bottom.
pulled.
(d) Steel reinforcement should be full length to
b. Minimization of problems, Long experience has
avoid problems in downdrag of the reinforcement
shown that drilled shaft foundations are reliable and
while the casing is pulled. The reinforcement cage
economical. Nevertheless, many problems are asso-
should also be full length if uplift forces are expected
ciated with these foundations and can occur from in-
on the drilled shaft from swelling soil.
adequate understanding of the actual soil profile and
(4) Problems with the slurry method. Slurry of
groundwater conditions, mistakes made while drilling,
sufficient viscosity is used to avoid problems with cav-
inadequate flow of concrete, and improper reinforce-
ing soils. A rough guide to appropriate slurry viscosi-
ment.
ties is given by a Marsh cone funnel test time of about
(1) Inadequate information.
30 seconds for sandy silts and sandy clays to 50 sec-
(a) Site conditions should be known to permit
onds for sands and gravels. The Marsh cone test time
optimum selection of equipment with the required mo-
is the time in seconds required to pour 1 quart of
bility.
slurry through the funnel. The workability of the
(b) Subsurface conditions should be known to
slurry should also be adequate to allow complete dis-
permit selection of equipment with adequate boring
placement of the slurry by the concrete from the
capacity.
perimeter of the borehole and steel of the rebar cage.
(c) Type of soil (e.g., caving and pervious strata)
(a) Slurries should be of sufficient viscosity to
may require slurry drilling. Specifications should per-
eliminate settling of cuttings. Loose cuttings adhering
mit sufficient flexibility to use slurry for those soil
to the perimeter of the hole can cause inclusions and a
conditions where it maybe needed.
defective shaft.
(d) Previously unnoticed sand lenses or thin
(b) The tremie sometimes becomes plugged,
permeable zones in otherwise impervious clay may
stopping the flow of concrete into the borehole. The
cause problems during construction of drilled shafts.
tremie should not be pulled above the concrete level in
Seepage through permeable zones may require casing
the shaft before the concrete placement is completed,
or slurry and may render construction of an under-
otherwise inclusions may occur in the shaft following
ream nearly impossible.
reinsertion of the tremie into the concrete.
(e) Overbreak or the loss of material outside of
(c) The reinforcement cage may move up if the
the nominal diameter of the shaft due to caving soil is
tremie is too deep in the concrete or the concrete is
a serious problem that can cause local cavities or de-
placed too rapidly.
fects in the shaft. The construction procedure (boring
dry, with casing, or using slurry) should be chosen to
c. Placement of concrete. Concrete strength of at
minimize overbreak.
least 3,000 pounds per square inch should be used and
(2) Problems with the dry method. Caving,
placed as soon as possible on the same day as drilling
squeezing soil, and seepage are the most common prob-
the hole. Concrete slumps of 4 to 6 inches and limited
8-2
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