AASHTO (LRFD)
Specifications for
Nominal Driving
Resistance of Driven
Steel Piles
Discussions with dynamic test results
By Sujan K. Bhattacharya, Ph.D., P.E., F. ASCE, Golder Associates Inc.;
Matthew B. Adamson, P.E., Intertek PSI; and
Carlos Fuentes, P.E., North Perimeter Contractors LLC
The AASHTO (2014) LRFD specification
stipulates that driven piles
be designed considering hard driving
conditions during pile driving. The idea
is that during pile driving, the resistance of
the soil becomes greater than the resistance
to support the static axial load. But when
bedrock is encountered and piles are founded
on bedrock, the structural resistance of
steel controls the design (AASHTO 2014). In
Piedmont geology, before encountering bedrock,
piles are often driven through hard
Partially Weathered Rock (PWR), which forms
the upper relic of the bedrock. Based on several
PDA and CAPWAP results in Piedmont
geology, it can be shown that geotechnical
capacity of piles in hard PWR exceeds the
Maximum Factored Structural Resistance
(MFSR). Therefore, MFSR should control the
design for piles founded on hard PWR and the
current practice of choosing pile sizes using
the Nominal Driving Resistance (NDR) is a
redundant approach for piles on hard PWR,
which can be also explained in the light of
the theory of wave propagation. According to
the current AASHTO-LRFD specification and
some state DOT guidelines, a dynamic resistance
factor (jdyn ) is applied to the Maximum
Factored Strength Limit State Load (MFSLSL)
to obtain the NDR. As a result, two load factors
are applied to MFSLSL, which seems to be an
overly conservative approach for piles on hard
PWR. Therefore, two important questions pertinent
to driven piles in Piedmont geology
need to be understood: (1) Whether structural
resistance or geotechnical resistance controls
the design when piles are founded on hard
PWR, and (2) Is it reasonable to choose pile
sizes considering NDR when piles are tipped
on hard PWR? In this article, some light can
be shed to clarify these questions using five
PDA/CAPWAP results and discussing perceptions
of driving resistance based on the theory
of wave propagation.
TECHNICAL
1. Introduction
During the bidding phase (pre-design
phase) of a design-build (DB) project, pile
lengths and sizes can be estimated based
on anticipated structural load, whether
the piles are founded in hard Partially
Weathered Rock (PWR) or bedrock. The
strength of hard PWR in Piedmont geology
with N = 50/5" to N=50/1" can be considered
close to bedrock. This was evidenced
by Pile Driving Analyzer (PDA) tests and
CAPWAP analyses for piles in a project
site in the Atlanta area; discussions are
included in Section 5. Therefore, pile sizes
can be chosen based on their respective
Maximum Factored Structural Resistance
(MFSR, explained in Section 3) if piles are
founded in hard PWR or bedrock for which
structural resistance controls the design
(AASHTO 2014, Article 10.7.3.2.3).
For hard bearing materials such as
PWR and bedrock, designing piles
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