Civil Engineering Reference
In-Depth Information
Design Criteria.
Several different items are used in the design and construction of piles:
1.
Engineering analysis:
Based on the results of engineering analysis, a deep foun-
dation could be designed and constructed such that it penetrates all the soil layers that are
expected to liquefy during the design earthquake. In this case, the deep foundation will
derive support from the unliquefiable soil located below the potentially troublesome soil
strata. However, the presence of downdrag loads as well as the loss of lateral resistance
due to soil liquefaction must be considered in the engineering analysis.
If a liquefiable soil layer is located below the bottom of the deep foundation, then Sec.
8.2.2 could be used to analyze the possibility of the deep foundation's punching into the
underlying liquefied soil layer. For end-bearing piles, the load applied to the pile cap can
be assumed to be transferred to the pile tips. Then based on the shear strength of the unliq-
uefiable soil below the bottom of the piles as well as the vertical distance from the pile tip
to the liquefiable soil layer, the factor of safety can be calculated using Eq. (8.1
b
). Note that
B
and
L
in Eq. (8.1
b
) represent the width and length, respectively, of the pile group.
2.
Field load tests:
Prior to the construction of the foundation, a pile or pier could be
load-tested in the field to determine its carrying capacity. Because of the uncertainties in
the design of piles based on engineering analyses, pile load tests are common. The pile load
test can often result in a more economical foundation then one based solely on engineering
analyses. Pile load tests can even be performed to evaluate dynamic loading conditions. For
example, ASTM provides guidelines on the dynamic testing of piles (for example, D 4945,
“Standard Test Method for High-Strain Dynamic Testing of Piles”). In this test method,
ASTM states:
This test method is used to provide data on strain or force and acceleration, velocity or dis-
placement of a pile under impact force. The data are used to estimate the bearing capacity and
the integrity of the pile, as well as hammer performance, pile stresses, and soil dynamics char-
acteristics, such as soil damping coefficients and quake values.
A limitation of field load tests is that they cannot simulate the response of the pile for
those situations where the soil is expected to liquefy during the design earthquake. Thus the
results of the pile load tests would have to be modified for the expected liquefaction con-
ditions.
3.
Application of pile driving resistance:
In the past, the pile capacity was estimated
based on the driving resistance during the installation of the pile. Pile driving equations,
such as the
Engineering News formula
(Wellington 1888), were developed that related the
pile capacity to the energy of the pile driving hammer and the average net penetration of
the pile per blow of the pile hammer. But studies have shown that there is no satisfactory
relationship between the pile capacity from pile driving equations and the pile capacity
measured from load tests. Based on these studies, it has been concluded that use of pile dri-
ving equations is no longer justified (Terzaghi and Peck 1967).
Especially for high displacement piles that are closely spaced, the vibrations and soil
displacement associated with driving the piles will densify granular soil. Thus the lique-
faction resistance of the soil is often increased due the pile driving (see compaction piles in
Sec. 12.3.3).
4.
Specifications and experience:
Other factors that should be considered in the deep
foundation design include the governing building code or agency requirements and local
experience. Local experience, such as the performance of deep foundations during prior
earthquakes, can be a very important factor in the design and construction of pile foundations.
The use of pile foundations is discussed further in Secs. 11.7 and 13.3.
