Interface Stresses on Laterally Loaded Piles
Matthew Whelan1, Department of Civil and Environmental Engineering
Piles are commonly selected as a cost effective option for the support of raised structures and highway
infrastructure. These structural members are often subjected to considerable lateral forces such as wind
loads in hurricane prone areas, earthquake loads in areas of seismic activity, and wave loads in offshore
environments. Soil-structure interaction is the mechanism that governs the pile response behavior and
ultimate capacity of the structure to these applied loads. A common approach to the analysis of laterally
loaded piles, the load-transfer approach, involves treating the soil as a series of discrete nonlinear springs
down the length of the pile and applying Bernoulli-Euler Classical Beam Theory. The spring
characteristics are defined by plots of the total lateral soil resistance at a specified depth, p, versus the
corresponding lateral pile displacement, y.
Typical lateral load tests of piles utilize the response of strain gauges instrumented at increments
down the length of the shaft to determine the bending curvature, , experienced in the pile. The lateral
soil reaction, p, corresponding to a given applied load can be calculated through double differentiation of
the bending moment diagram, M (M =  EI, where EI is the stiffness of the pile). Similar application of
beam theory allows for the pile deflection, y, diagram to be developed through double integration of the
bending curvature diagram. By correlating the soil reaction with the corresponding deflection for given
bending moments at a specified depth, p-y curves can be produced. The total lateral soil response, p, is
predominantly a result of two components; frictional resistance, F, produced by tangential interface
stresses, and frontal resistance, Q, produced by stresses normal to the pile cross-section (Fig. 1).
Side Shear, F
P
Applied
Lateral
Load
Total
Resistance, P
Normal
Stress, Q
Normal
Stress,
Q
Side
Shear, F
y
FIG. 1. Components of Lateral Soil Reaction
Class of ’04, Structural Engineering, Clarkson University, Honors Program
Research Mentor: Dr. Kerop D. Janoyan, P.E., Assistant Professor, Clarkson University,
Department of Civil and Environmental Engineering
Presentation Format: Oral
1
Due to the complex nonlinear nature of soil, truly representative p-y curves can only be produced
through field-testing under conditions identical to those used for design. Current methodologies for the
design of laterally loaded piles in sand are based on empirical correlations derived from a relatively small
library of field tests (Reese, 1974). However, recent studies suggest that the recommended design curves
underestimate of the actual strength of the soil-structure interaction by underestimating the contribution of
side friction, particularly for large diameter piles (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, resulting in a more cost-effective design approach.
A laboratory-scale test program will be undertaken in order to quantify the component soil
resistances on a free-head laterally loaded pile. A 24-inch diameter by 30-inch height soil container was
designed and fabricated specifically for the outlined testing procedure (Fig. 2). A cylindrical Neoprene
wall replicates the response of free-field soil and Kevlar bands provide sufficient lateral restraint while
allowing for free translational movement of the soil column for future dynamic testing. Model-prototype
scaling was implemented with a governing geometric scaling factor of 12 in order to develop a reasonable
soil and representative pile section. The preliminary model soil is ground silica sand that, when scaled,
replicates Ottawa Standard sand. Physical and mechanical property classification of the soil will be
achieved through grain size analysis and triaxial and simple shear box tests. Placement of the soil through
the use of a traveling pluviation apparatus will ensure uniform and repeatable in-situ density and
mechanical strengths. A miniature Cone Penetrometer will be used in order to verify the uniformity of
the soil and determine in-situ characteristics.
FIG. 2. Soil Test Container and Pile Instrumentation
(Pressure Transducers not shown; adjacent to strain gages)
The prototype pile is a 12-inch wide, .5-inch thick square steel concrete-filled pile designed to
undergo flexible behavior under typical working loads using the LPILE (2000) analytical software
package. In order to assume flexible behavior of the pile, the embedded length to pile diameter ratio was
set at 20, which dictates a prototype pile embedded length of 20 ft. Typical yield strength of 60 ksi and
Young’s modulus of 29,000 ksi were specified for the steel, resulting in a composite rigidity of 5.01107
k-in2. The model design adhered to a procedure outlined by Meymand (1998) for designing a model pile
in compliance with similarity requirements and verifying the selection. The composite rigidity of the pile
was calculated and scaled appropriately to obtain a target model rigidity value. Moment of inertia values
corresponding to available thin wall square tubing were calculated in order to determine a range of
acceptable modulus of elasticity values. These calculations dictated a model pile material selection of
aluminum 6061 T-6, with yield strength and Young’s modulus of 35 ksi and 10,000 ksi, respectively. In
order to most closely match the target flexural rigidity using commercially available products, a 1-inch
wide, 25-inch long, thin-walled tube of 0.04-inch thickness was selected as the model pile. By geometric
similarity, the model pile embedded depth is 20 inches into the soil. The model pile has been shown to
exhibits higher yield strength than the prototype, and thus the model pile will behave elastically under the
scaled working loads of the prototype pile, so the model pile can be deemed acceptable.
Displacement controlled lateral loading will be applied at the head of the pile under free-rotation
conditions. A 250 lb load cell will monitor the magnitude of the applied load. Six sets of temperature
compensated strain gages will be applied to the model pile at various depths and the lead wires will be fed
upward through the hollow shaft of the thin-walled tube thereby minimizing pile surface interference.
Wheatstone quarter-bridge completion of the gages will allow for calculation of the bending strains and
subsequent determination of bending moment diagrams for the pile from which the soil reaction will be
computed through double differentiation. Eight Linear Variable Differential Transducers (LVDTs) will
record both the ground line deflection and the below ground line deflections at the depths corresponding
to the placement of the strain gauges. With these measurements, pile displacement diagramscan be
approximated and then correlated with the soil reaction profile determined using the strain gages in order
to develop p-y curves.
Since physical restraints limit the use of standard soil pressure cells (SPCs) on a model-pile, low
profile surface mount pressure sensors will be used. The 5mm diameter and 1 mm thickness of these
pressure sensors offer the opportunity to directly measure soil resistance normal to the shaft surface at
several circumferential locations at various pile depths similarly to full-scale testing performed by
Janoyan (2001). Numerical integration of the pressures normal to the leading face of the shaft will be
used to approximate the contribution of passive frontal resistance to the total soil resistance at the pressure
sensor depths. By assuming negligible active soil resistance on the pile, the difference in the total soil
resistance to lateral loading and the computed normal resistance will be attributed to frictional stresses at
the interface. These calculated resistances will be correlated with pile displacements in order to develop
Q-y and F-y plots, which will be superimposed over the p-y curves in order to evaluate the soil-structure
interaction.
A second testing procedure consisting of pure torsional testing of equivalent aluminum pipe pile
will be performed in order to confirm the derivation of frictional resistance by the aforementioned
protocol. A torque-wrench will be used to apply a measurable increasing load on the pile. An LVDT will
be instrumented on the pile just above the ground line to measure circumferential displacement similar to
during the operation of a ring shear apparatus. These two measurements can be used to produce applied
torque versus circumferential displacement plots in order to deduce F-y curves at various depths for
comparison with those developed in the first testing procedure (Smith and Slyh, 1986).
ACKNOWLEDGEMENTS
Due appreciation is given to Dr. Kerop Janoyan for the extensive support and ingenuity, as well as
admirable commitment, that he has provided consistently throughout the course of the research.
Additional thanks is given to the skilled technicians at the Clarkson University machine shop for their
input on testing apparatus design and fabrication of necessary equipment. This research is part of an
ongoing effort to complete an undergraduate thesis under the requirements outlined by the Clarkson
University Honors Program.
REFERENCES
Janoyan, K. D. (2001). “Test Results for Full Scale Drilled Shafts Under Cyclic Lateral Loading.” PhD
Dissertation, Dept. of Civil & Envir. Engrg., University of California, Los Angeles, California.
LPILE Plus (2000). “A Program for Analyzing Stress and Deformation of a Pile or Drilled Shaft Under
Lateral Loading,” ENSOFT, Inc. Version 4.0.
Meymand, P. J. (1998). “Shaking Table Scale Model Tests of Nonlinear Soil-Pile-Superstructure
Interaction in Soft Clay.” PhD Dissertation, Dept. of Civil & Envir. Engrg., University of California,
Berkeley, California.
Reese, L.C., Cox, W.R., and Koop, F.D. (1974), “Analysis of Laterally Loaded Piles in Sand,” 6th
Offshore Technology Conference. OTC 2080, Vol 2, Houston, Texas. pp 473-485.
Smith, T. D., Slyh, R. (1986), “Side Friction Mobilization Rates for Laterally Loaded Piles from the
Pressuremeter,” The Pressuremeter and Its Marine Applications: Second International Symposium ASTM
STP 950, J.L. Briaud and J.M.E. Audibert, Eds., American Society for Testing and Materials.