Evaluation of the Contribution of Component Stresses to the
Lateral Capacity of Bridge Piles
Thesis Proposal
Matthew Whelan
Advisor: Kerop Janoyan, Ph.D., P.E.
Clarkson University, Potsdam, New York
Abstract
The pile-supported bridge is a widespread and economical method for support of raised highway
infrastructure. When these structures are constructed in regions of seismic activity, they must be designed
to support the resultant lateral forces. The American Petroleum Institute (API) recommends the use of
empirical p-y curves to model the lateral soil-structure interaction in order to design for acceptable system
deformations during seismic events. However, recent studies suggest that the API p-y curves
underestimate of the actual strength of the soil-structure interaction for large diameter shafts by
underestimating the contribution of side friction. Correction of the current recommendations may lead to
a more cost effective design for large diameter shafts. This study aims at examining p-y curves for a
laboratory-scaled model pile-soil system and accounting for the effects of lateral loading, particularly
gapping, cyclic degradation, and interfacial slip. The results will be compared to software based
computational methods using nonlinear finite element analysis incorporating design curves.
Background
Tectonic activity along the Earth's plates propagates seismic waves throughout the soil that are transferred
to structures built in the area of seismic activity. Likewise, structures must be designed to withstand the
lateral forces induced by earthquakes. In order to evaluate the soil resistance along the length of a bridge
pile, McClelland and Focht (1958) developed the concept of p-y curves, which represent the lateral soil
reaction per unit length, p, versus the pile displacement at a specified depth, y. The curves developed
demonstrate the nonlinear nature of the soil reaction, since the soil modulus varies with deflection as well
as depth. The American Petroleum Institute has developed recommended p-y curves for the design of
piles subject to lateral loads based on a relatively small collection of available full-scale experimental
results (API, 1993). These p-y curves can be used to represent discreet nonlinear springs along the length
of the pile, which allows for the use of analytical models to determine the lateral capacity of a pile. In
particular, finite element analysis can be applied to the soil-structure interaction with a series of spring
elements representing the lateral soil reactions, skin friction, and tip resistance (Curras 2001).
Furthermore, software based computational methods have allowed for the inclusion of gapping, cyclic
degradation, and interfacial slip effects into the soil-structure interface analysis.
If the normal soil resistance accounted for predominantly all of the total soil resistance, then force and
moment equilibrium of the soil pressure cell readings down the length of the pile shaft should be satisfied.
However, a substantially large additional contribution would have to be present and concentrated in the
upper region of the shaft in order to satisfy this equilibrium (Bierschwale 1981). Therefore, in addition to
the passive soil resistance on the leading edge of the shaft, a significant surface shear stress must exist
(Fig. 1). Since soil pressure cells can only measure the normal force applied to their surface, previous
experimental research has relied on indirectly determining the contribution of side friction to the overall
resistance. One method implemented is the placement of several soil pressure cells along the
circumference of the shaft at the same depth and decomposition of pressure cell readings into shaft
normal and side shear components. Research has shown that the normal component of soil resistance
accounts for only roughly 20% of the total soil resistance at small load levels. This suggests that initially,
the side frictional resistance mobilizes rapidly and predominates over the normal stress. As the lateral
load is increased, the normal stress increases and thus accounts for a greater fraction of the overall soil
resistance (Fig. 2). In addition, full-scale experimental tests on large diameter drilled shafts indicate a
stiffer response than projected by the recommended API p-y curves, a phenomenon which is believed to
be attributed to the contribution of the increased side shear component over the larger circumference of
the shaft (Janoyan 2001). Increased understanding of the contributions of normal and side shear
components of soil resistance on pile shafts may lead to correction of the API recommended p-y curves
and a more cost-effective design approach for large diameter bridge piles.
Proposed Work
Testing of model piles is to be performed in a 2’ diameter cylindrical soil container that is currently being
constructed. The container has been designed with provisions, including a neoprene rubber membrane,
Kevlar band reinforcement, and ball-and-socket joint connected support rods, adopted from a similar
container at U.C. Berkeley's PEER Center, in order to appropriately simulate free-field conditions
(Meymand 1998). A traveling pluviation apparatus has been determined to be the most fitting method of
soil placement since it allows for developing an acceptable, and repeatable, dry density of the soil, high
uniformity throughout the specimen, and negligible layering effects on the mechanical properties (Fretti
1995). The soil medium has yet to be determined, although a sandy soil will most likely be chosen in
order to circumvent the time consuming consolidation process associated with clays. In-situ and
laboratory tests, in accordance with ASTM Standard testing procedures, will be performed on the
reconstituted specimens in order to properly classify the soil and quantify the mechanical properties and
relevant soil parameters. Model piles will be designed with the use of LPILE computational software in
order to ensure that test shafts will experience flexure during lateral loading. Proper instrumentation and a
data acquisition system will be selected in order to allow for development of p-y curves for the pile-soil
system and measurement of normal stress on the test pile, from which the side shear stress with be
indirectly determined. Of particular interest is the development or implementation of a sensor technology
capable of directly measuring the side shear resistance on the shaft. Reorientation and application of the
friction sleeve technology of an Electric Friction-Cone Penetrometer to the model pile in an area of
minimal normal shaft resistance could provide a means of attaining such side shear measurements.
References
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O'Neill, (editor)) New York: ASCE, 0-87262-285-1, p 98.
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