TM 5-818-1 / AFM 88-3, Chap. 7
higher density than water. The reinforcing steel can be
11-6. Drilled piers. Drilled piers (also drilled shafts or
placed in the slurry-filled hole. Place concrete by tremie.
drilled caissons) are often more economical than piles
(3) Use of drilling mud and casing. The
where equipment capable of rapid drilling is readily
shaft is drilled using drilling mud, the casing is placed,
available, because of the large capacity of a pier as
and the drilling mud is bailed. Core barrels and other
compared with a pile.
special drilling tools are available to socket the pier shaft
a. Pier dimension and capacities. Drilled piers
into bedrock. With a good operator and a drill in good
can support large axial loads, up to 4, 000 kips or more,
shape, it is possible to place 30- to 36-inch cores into
although typical design loads are on the order of 600 to
solid rock at a rate of 2 to 3 feet per hour. Underreams
1, 000 kips. In addition, drilled piers are used under
are either hemispherical or 30- or 45-degree bell slopes.
lightly loaded structures where subsoils might cause
Underreaming is possible only in cohesive soils such that
building heaving. Shaft diameters for high-capacity piers
the underslope can stand without casing support, as no
are available as follows:
practical means currently exists to case the bell.
c. Estimating the load capacity of a drilled pier.
From 2- 1/2 feet
by 6-inch increments
Estimate the ultimate capacity, Q, of a drilled pier as
From 5 feet
by 1-foot increments
follows:
Also available are 15- and 2-foot-diameter shafts.
Commonly, the maximum diameter of drilled piers is
Qu =
Qus(skin resistance) + Qup(point)
(11-2)
under 10 feet with a 3- to 5-foot diameter very common.
Drilled piers can be belled to a maximum bell size of
The design load based on an estimated 1-inch
three times the shaft diameter. The bells may be
settlement is:
hemispherical or sloped. Drilled piers can be formed to a
maximum depth of about 200 feet. Low capacity drilled
Qd =
Qus + Qup
(11-3)
piers may have shafts only 12 to 18 inches in diameter
3
and may not be underreamed.
b. Installation. The drilled pier is constructed
(1) Drilled piers in cohesive soil. The skin
by drilling the hole to the desired depth, belling if
resistance can be computed from the following:
increased bearing capacity or uplift resistance is
H
required, placing necessary reinforcement, and filling the
Qus =αavg
∫
C cz dz
cavity with concrete as soon as possible after the hole is
0
drilled. The quantity of concrete should be measured to
where
ensure that the hole has been completely filled.
αavg
=
factor from table 11-2
Reinforcement may not be necessary for vertical loads;
H
=
shaft length
however, it will always be required if the pier carries
C
=
shaft circumference
lateral loads. A minimum number of dowels will be
cz
=
undrained shear strength at depth z
required for unreinforced piers to tie the superstructure to
Use table 11-2 for the length of shaft to be considered in
the pier.
Reinforcement should be used only if
computing H and for limiting values of side shear. The
necessary since it is a construction obstruction.
base resistance can be computed from the following:
Consideration should be given to an increased shaft
Qup =
Nc cB AB
(11-5)
diameter or higher strength concrete in lieu of
where
NC
=
bearing capacity factor of 9 (table 11-
reinforcement. In caving soils and depending on local
2)
experience, the shaft is advanced by:
cB
=
undrained shear strength for distance
(1) Drilling a somewhat oversize hole and
of two diameters below tip
advancing the casing with shaft advance. Casing may
AB
=
base area
be used to prevent groundwater from entering the shaft.
(2) Drilled piers in sand.
Compute skin
When drilling and underreaming is completed, the
resistance from the following:
reinforcing steel is placed, and concrete is placed
H
immediately. The casing may be left in place or
αavg C
∫
Pz tan φ dz
Qus =
(11-6)
withdrawn while simultaneously maintaining a head of
0
concrete. If the casing is withdrawn, the potential exists
where
∝avg
for voids to be formed in the concrete, and special
=
0.7, for shaft lengths less than 25
attention should be given to the volume of concrete
feet
∝avg
poured.
=
0.6, for shaft lengths between 25 and
(2) Use of drilling mud to maintain the shaft
40 feet
∝avg
cavity. Drilling mud may be used also to prevent water
=
0.5, for shaft lengths more than 40
feet
from entering the shaft by maintaining a positive head
differential in the shaft, since the drilling fluid has a
11-9
TM 5-818-1 / AFM 88-3, Chap. 7
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