01 Jul 97
other obstructions, the open-end pile permits inspection after
A special type of
removal of the plug material and ensures that the load will be
nondisplacement deep foundation is the uncased auger-placed
transferred directly to the load-bearing stratum. Splices are
grout shaft. This shaft is constructed by advancing a
commonly made by full penetration butt welds or fillet wells
continuous-flight, hollow-stem auger to the required depth and
(Figure 1-3b) or patented splicers.
filling the hole bored by the concrete grout under pressure as
the auger is withdrawn. Careful inspection is required during
(5) Compaction piles. These are sometimes driven with
installation, and shaft continuity should be verified by a
the objective of increasing the density of loose, cohesionless
combination of load tests and nondestructive testing as
soils and reducing settlement. Piles with a heavy taper are
described in Chapter 6.
often most effective in deriving their support from friction.
7. Selection of Deep Foundations
b. Nondisplacement piles. This pile consists of a drilled
shaft with a concrete cylinder cast into a borehole. Normally,
Deep foundations provide an efficient foundation system for
the drilled shaft does not cause major displacement of the
soils that do not have a shallow, stable bearing stratum.
adjacent ground surface. The hole is usually bored with a short
Selection of a deep foundation requires knowledge of its
flight or bucket auger. Loss of ground could occur if the
characteristics and capacity.
diameter of the hole is decreased because of inward
a. Characteristics. Information adequate for reaching
displacement of soft soil or if there is caving of soil from the
hole perimeter. Such unstable boreholes require stabilization
preliminary conclusions about types of driven piles or drilled
by the use of slurry or slurry and casing. Drilled shafts are not
shafts to be selected for a project is given in Table 1-4. This
subject to handling or driving stresses and therefore may be
table lists major types of deep foundations with respect to
designed only for stresses under the applied service loads.
capacity, application, relative dimensions, and advantages and
disadvantages. Refer to Foundations (Pile Buck Inc. 1992) for
Nondisplacement may be categorized as follows:
additional information. Information in the table provides
(1) Uncased shafts. Figure 1-4 illustrates a typical
general guidelines in the selection of a type of deep foundation.
uncased drilled shaft with an enlarged base. The base is not
Relevant codes and standards should be consulted with respect
perfectly flat because the shaft is drilled first, then the belling
to allowable stresses. A cost analysis should also be performed
tool rotates in the shaft. Uncased shafts may be constructed in
that includes installation, locally available practices, time
firm, stiff soils where loss of ground is not significant.
delays, cost of load testing program, cost of a pile cap, and
Examples of uncased shaft are given in the American Concrete
other elements that depend on different types of deep
Institute (ACI) Manual of Concrete Practice (1986). Other
terms used to describe the drilled shaft are "pier" or "caisson."
b. Capacity. Deep foundations transmit structural loads to
Large shafts greater then 36 inches in diameter are often called
caissons. The term "pile" is commonly associated with driven
deep strata that are capable of sustaining the applied loads.
deep foundations of relatively small diameter or cross section.
Accurate predictions of load capacity and settlement are not
always possible. Adequate safety factors are therefore used to
(2) Cased shafts. A cased shaft is made by inserting a
avoid excessive movement that would be detrimental to the
shell or casing into almost any type of bored hole that requires
structure that is supported and to avoid excessive stress in the
stabilization before placing concrete. Boreholes are caused
foundation. Driven piles or drilled shafts are often used to
where soil is weak and loose, and loss of ground into the
resist vertical inclined, lateral, or uplift forces and overturning
excavation is significant. The bottom of the casing should be
moments which cannot otherwise be resisted by shallow
pushed several inches into an impervious stratum to seal the
footings. These foundations derive their support from skin
hole and allow removal of the drilling fluid prior to completion
friction along the embedded length and by end bearing at the
of the excavation and concrete placement. If an impervious
tip (base). Both factors contribute to the total ultimate pile
stratum does not exist to push the casing into, the concrete can
capacity, but one or the other is usually dominant depending on
be placed by tremie to displace the drilling fluid.
the size, load, and soil characteristics. The capacity of deep
foundation is influenced by several factors:
(3) Drilling fluid shafts. Shafts can be installed in wet
sands using drilling fluid, with or without casing. This
(1) Design limits. The limiting design criterion is
procedure of installing drilled shafts can be used as an
normally influenced by settlement in soft and moderately stiff
alternative to the uncased and cased shafts discussed
soil, and bearing capacity in hard soil or dense sand, and by
pile or shaft structural capacity in rock.