Behavior of Fiber-Reinforced 

Polymer Composite Piles 

Under Vertical Loads

PUBLICATION NO. FHWA-HRT-04-107 AUGUST 2006

Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
FOREWORD 

A full-scale experiment on fiber-reinforced polymer (FRP) piles, including static and dynamic 

load tests, was conducted at a site provided by the Port Authority of New York and New Jersey 

at its Port of Elizabeth facility in New Jersey, with the cooperation and support of its engineering 

department and the New York State Department of Transportation. The engineering use of FRPbearing piles required field performance assessment and development and evaluation of reliable 

testing procedures and design methods to assess short-term composite material properties, loadsettlement response and axial-bearing capacity, drivability and constructability of composite 

piling, soil-pile interaction and load transfer along the installed piling, and creep behavior of FRP 

composite piles under vertical loads. 

Gary L. Henderson 

Director, Office of Infrastructure 

 Research and Development 

NOTICE 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

QUALITY ASSURANCE STATEMENT 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement. 
 Technical Report Documentation Page

1. Report No. 

FHWA-HRT-04-107

2. Government Accession No. 3. Recipient’s Catalog No. 

5. Report Date 

August 2006 

4. Title and Subtitle 

BEHAVIOR OF FIBER-REINFORCED POLYMER (FRP) COMPOSITE 

PILES UNDER VERTICAL LOADS 6. Performing Organization Code 

7. Author(s) 

Ilan Juran and Uri Komornik 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. 

Urban Utility Center 

Polytechnic University 

6 Metrotech Center 

Brooklyn, NY 11201

11. Contract or Grant No. 

DTFH61-99-X-00024 

13. Type of Report and Period Covered 

Final Report 

2000-2003 

12. Sponsoring Agency Name and Address 

Office of Infrastructure Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296

14. Sponsoring Agency Code 

15. Supplementary Notes 

Contracting Officer’s Technical Representative (COTR): Carl Ealy, HRDI-06 

16. Abstract 

Composite piles have been used primarily for fender piles, waterfront barriers, and bearing piles for light structures. In 1998, the Empire State Development 

Corporation (ESDC) undertook a waterfront rehabilitation project known as Hudson River Park. The project is expected to involve replacing up to 100,000 

bearing piles for lightweight structures. The corrosion of steel, deterioration of concrete, and vulnerability of timber piles has led ESDC to consider composite 

materials, such as fiber-reinforced polymers (FRP), as a replacement for piling made of timber, concrete, or steel. Concurrently, the Federal Highway 

Administration (FHWA) initiated a research project on the use of FRP composite piles as vertical load-bearing piles. 

A full-scale experiment, including dynamic and static load tests (SLT) on FRP piles was conducted at a site provided by the Port Authority of New York and 

New Jersey (PANY&NJ) at its Port of Elizabeth facility in New Jersey, with the cooperation and support of its engineering department and the New York State 

Department of Transportation (NYSDOT). The engineering use of FRP-bearing piles required field performance assessment and development and evaluation of 

reliable testing procedures and design methods to assess short-term composite material properties, load-settlement response and axial-bearing capacity, 

drivability and constructability of composite piling, soil-pile interaction and load transfer along the installed piling, and creep behavior of FRP composite piles 

under vertical loads. 

This project includes: 

• Development and experimental evaluation of an engineering analysis approach to establish the equivalent mechanical properties of the composite material. 

The properties include elastic modulus for the initial loading quasilinear phase, axial compression strength, inertia moment, and critical buckling load. The 

composite material used in this study consisted of recycled plastic reinforced by fiberglass rebar (SEAPILETM composite marine piles), recycled plastic 

reinforced by steel bars, and recycled plastic reinforced with randomly distributed fiberglass (Trimax), manufactured respectively by Seaward International 

Inc., Plastic Piling, Inc., and U.S. Plastic Lumber. 

• Static load tests on instrumented FRP piles. The instrumentation schemes were specifically designed for strain measurements. The experimental results were 

compared with current design codes as well as with the methods commonly used for evaluating the ultimate capacity, end bearing capacity, and shaft 

frictional resistance along the piles. As a result, preliminary recommendations for the design of FRP piles are proposed. 

• Analysis of Pile Driving Analyzer® (PDA) and Pile Integrity Tester (PIT) test results using the Case Pile Wave Analysis Program (CAPWAP)(1) and the GRL 

Wave Equation Analysis of Piles program GRLWEAP(2) to establish the dynamic properties of the FRP piles. The PDA also was used to evaluate the 

feasibility of installing FRP piles using standard pile driving equipment. Pile bearing capacities were assessed using the CAPWAP program with the dynamic 

data measured by the PDA, and the calculated pile capacities were compared to the results of static load tests performed on the four FRP piles. 

The dynamic and static loading test on instrumented FRP piles conducted in this project demonstrated that these piles can be used as an alternative engineering 

solution for deep foundations. The engineering analysis of the laboratory and field test results provided initial data basis for evaluating testing methods to 

establish the dynamic properties of FRP piles and evaluating their integrity and drivability. Design criteria for allowable compression and tension stresses in the 

FRP piles were developed considering the equation of the axial force equilibrium for the composite material and assuming no delamination between its basic 

components. However, the widespread engineering use of FRP piles will require further site testing and full-scale experiment to establish a relevant performance 

database for the development and evaluation of reliable testing procedure and design methods. 

17. Key Words 

Piles, fiber-reinforced polymers, static load tests, dynamic load 

tests 

18. Distribution Statement 

No restrictions. This document is available to the public through the 

National Technical Information Service, Springfield, VA 22161. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

97 

22. Price 

 Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized 
ii

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha 

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS 

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx 

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac 

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Appropria e te rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)
iii

TABLE OF CONTENTS 

CHAPTER 1. INTRODUCTION............................................................................................... 1

CURRENT STATE OF PRACTICE.......................................................................................... 2 

Mechanical Behavior of FRP Piles......................................................................................... 3 

Field Observations Use and Experimental Demonstration Projects....................................... 3 

RESEARCH AND DEVELOPMENT NEEDS ......................................................................... 6 

CHAPTER 2. MECHANICAL SHORT-TERM BEHAVIOR OF FRP COMPOSITE 

MATERIALS UNDER AXIAL COMPRESSION LOADS................................................. 9

SEAPILE COMPOSITE FRP MATERIAL............................................................................... 9 

MECHANICAL PROPERTIES OF THE FRP COMPOSITE AND THE COMPONENT 

MATERIALS ...................................................................................................................... 9 

ENGINEERING ANALYSIS OF THE AXIAL COMPRESSION TEST ON THE 

COMPOSITE PILE SAMPLE .......................................................................................... 14 

Elastic Modulus and Compressive Strength ......................................................................... 14 

Moment of Inertia ................................................................................................................. 17 

Critical Buckling Force......................................................................................................... 18 

PPI PILES................................................................................................................................. 20 

TRIMAX STRUCTURAL PLASTIC MATERIAL................................................................. 22 

Axial Compression Tests on the Trimax Pile Recycled Plastic............................................ 22 

CHAPTER 3. BEHAVIOR OF FRP COMPOSITE PILES UNDER VERTICAL 

LOADS.................................................................................................................................... 25

PILE MANUFACTURING AND INSTRUMENTATION..................................................... 26 

Lancaster Composite, Inc...................................................................................................... 28 

PPI......................................................................................................................................... 29 

SEAPILE Piles...................................................................................................................... 30 

FIELD TESTING PROGRAM................................................................................................. 31 

Full-Scale Experiment .......................................................................................................... 31 

GEOTECHNICAL SITE CONDITIONS................................................................................. 33 

STATIC LOAD TEST PROCEDURE..................................................................................... 34 

ENGINEERING ANALYSIS OF STATIC LOAD TEST RESULTS .................................... 37 

Ultimate Capacities of FRP Piles.......................................................................................... 37 

Analysis of the End Bearing and Shaft Friction Distribution............................................... 44 

Correlations With Standard Penetration Test (SPT) Data .................................................... 47 

Comparison Between Various Analysis Methods and Test Results..................................... 48 
iv

CHAPTER 4. EVALUATION OF FRP COMPOSITE PILING CAPACITY USING 

WAVE EQUATION ANALYSIS ...............................................................................................53 

TESTING PROGRAM AND SITE OBSERVATIONS .......................................................... 53 

Test Piles and Dynamic Instrumentation .............................................................................. 53 

Site Observations on Drivability and Durability During Pile Installation............................ 54 

DYNAMIC PROPERTIES OF THE FRP COMPOSITE PILE MATERIALS....................... 56 

Methods for Evaluating Dynamic Properties........................................................................ 56 

Dynamic Properties—Test Results....................................................................................... 57 

DYNAMIC PILE TESTING DATA ANALYSIS ................................................................... 58 

Case Method Analysis (PDA)............................................................................................... 58 

CAPWAP Analysis............................................................................................................... 59 

GRLWEAP Analysis ............................................................................................................ 63 

STATIC LOAD TEST RESULTS—CORRELATIONS WITH CAPWAP ANALYSIS ....... 66 

Pile Capacity and Setup ........................................................................................................ 66 

Correlations of CAPWAP Analysis With Static Load Test Results..................................... 67 

Pile Tension and Tensile Stress During Driving................................................................... 71 

CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS .......................................... 79

MECHANICAL SHORT-TERM BEHAVIOR OF FRP COMPOSITE MATERIALS 

UNDER AXIAL COMPRESSION LOADS..................................................................... 79 

BEHAVIOR OF FRP COMPOSITE PILES UNDER VERTICAL LOADS .......................... 80 

EVALUATION OF FRP COMPOSITE PILING CAPACITY, DRIVABILITY, AND 

CONSTRUCTABILITY ................................................................................................... 81 

Drivability and Integrity During Driving.............................................................................. 82 

Evaluation of Dynamic Testing Methods ............................................................................. 82 

CAPWAP Analysis............................................................................................................... 82 

Correlations Between CAPWAP Analysis and Static Load Tests........................................ 83 

Design Criteria and Allowable Stresses................................................................................ 83 

R&D NEEDS ASSESSMENT ................................................................................................. 83 

REFERENCES............................................................................................................................ 85

BIBLIOGRAPHY....................................................................................................................... 89
v

LIST OF FIGURES 

Figure 1. Photo. Marine borers (Limnoria) attacking untreated timber piles that support 

many of New York’s highways and harbor piers. ........................................................... 2 

Figure 2. Photo. Complete corrosion of steel H piles supporting a harbor pier 

(recently installed retrofit channels are already corroding). ............................................ 2 

Figure 3. Photo. Fender piles in the U.S. Naval Submarine Base, San Diego, CA........................ 4 

Figure 4. Photo. Fendering system in the U.S. Navy Pier 10, San Diego, CA............................... 4 

Figure 5. Photo. Fendering system, Nashville Avenue Marine Terminal, 

Port of New Orleans, LA. ................................................................................................ 5 

Figure 6. Photo. Floating dock project............................................................................................ 5 

Figure 7. Photo. Port of Elizabeth demonstration site. ................................................................... 6 

Figure 8. Illustration. Locations of pile manufacturers................................................................... 7 

Figure 9. Photo. SEAPILE composite marine piles...................................................................... 10 

Figure 10. Graph. Stress-strain relationship of 4.4-cm (1.75-inch) fiberglass bars...................... 11 

Figure 11. Photo. Fiberglass bar before axial compression test and disintegrated 

fiberglass parts. ........................................................................................................... 11 

Figure 12. Graph. Stress-vertical strain relationship of SEAPILE pile recycled plastic. ............. 12 

Figure 13. Graph. Stress-radial strain relationship of SEAPILE pile recycled plastic; 

rate 0.33 percent/min. ................................................................................................. 12 

Figure 14. Graph. Force-vertical strain relationship of SEAPILE pile sample. ........................... 13 

Figure 15. Graph. SEAPILE pile sample after axial compression test......................................... 14 

Figure 16. Equation. Applied load F............................................................................................. 14 

Figure 17. Equation. Young’s modulus E..................................................................................... 15 

Figure 18. Equation. σc/σb............................................................................................................. 15 

Figure 19. Equation. Ec/Eb. ........................................................................................................... 15 

Figure 20. Equation. Gc................................................................................................................. 16 

Figure 21. Equation. Rc/Rb............................................................................................................ 16 

Figure 22. Equation. FLF.............................................................................................................. 16 

Figure 23. Equation. Bending moment M. ................................................................................... 17 

Figure 24. Equation. Moment of inertia Ic(1). .............................................................................. 17 

Figure 25. Equation. Moment of inertia Ic(2). .............................................................................. 17 

Figure 26. Equation. Relative inertia moment coefficient λ......................................................... 17 

Figure 27. Equation. Ic/ΣIb............................................................................................................ 17 

Figure 28. Equation. Critical buckling force Pcr. .......................................................................... 18 

Figure 29. Equation. Pcr of an axially loaded bar.......................................................................... 18 

Figure 30. Equation. Equivalent critical buckling load for composite material. .......................... 18 

Figure 31. Equation. Critical buckling load factor BLF. .............................................................. 19 

Figure 32. Equation. Critical buckling load decomposed............................................................. 19 

Figure 33. Graph. Buckling force versus length for SEAPILE pile sample and 

fiberglass bars. ............................................................................................................ 20 

Figure 34. Photo. PPI pile............................................................................................................. 21 

Figure 35. Graph. PPI pile—stress-strain relationship. ................................................................ 21 

Figure 36. Photo. Trimax pile....................................................................................................... 22 

Figure 37. Graph. Trimax pile—vertical stress-strain curves at different rates............................ 23 
vi

Figure 38. Graph. Trimax pile—vertical stress-lateral strain curve; strain rate 0.33 percent 

per minute. .................................................................................................................. 23 

Figure 39. Photo. Port of Elizabeth demonstration site. ............................................................... 25 

Figure 40. Photo. Equipment used in the in-load tests. ................................................................ 26 

Figure 41. Illustration. Schematic of the equipment used in the in-load tests.............................. 26 

Figure 42. Illustration. Data acquisition system. .......................................................................... 27 

Figure 43. Photo. Strain gauges installation in pile of Lancaster Composite, Inc........................ 29 

Figure 44. Photo. Vibrating and foil strain gauges attached to steel cage in PPI pile. ................. 30 

Figure 45. Photo. Vibrating and foil strain gauges attached to SEAPILE composite 

marine pile. ................................................................................................................. 31 

Figure 46. Illustration. Schematic drawing of Port Elizabeth site. ............................................... 32 

Figure 47. Graph. Lancaster pile—settlement-time relationship.................................................. 35 

Figure 48. Graph. PPI pile—settlement-time relationship............................................................ 35 

Figure 49. Graph. SEAPILE pile—settlement-time relationship. ................................................ 36 

Figure 50. Graph. American Ecoboard pile—settlement-time relationship. ................................ 36 

Figure 51. Equation. Settlement, S. .............................................................................................. 37 

Figure 52. Graph. Lancaster Composite pile—Davisson criteria and measured 

load-settlement curve.................................................................................................. 39 

Figure 53. Graph. PPI pile—Davisson criteria and measured load-settlement curve. ................. 39 

Figure 54. Graph. SEAPILE pile—Davisson criteria and measured load-settlement curve. ....... 40 

Figure 55. Graph. American Ecoboard pile—Davisson criteria and measured 

load-settlement curve.................................................................................................. 40 

Figure 56. Graph. DeBeer criterion plotted for FRP piles............................................................ 41 

Figure 57. Chin-Kondner method plotted for FRP piles............................................................... 42 

Figure 58. Chin-Kondner method plotted for American Ecoboard pile. ...................................... 42 

Figure 59. Equation. Ultimate load capacity Puc........................................................................... 45 

Figure 60. Equation. Ultimate shaft friction in compression fs. ................................................... 45 

Figure 61. Equation. Ultimate end bearing resistance fb. ............................................................. 45 

Figure 62. Equation. Relationship between fs and in situ stresses................................................ 46 

Figure 63. Equation. Empirical correlations for shaft friction...................................................... 47 

Figure 64. Equation. Empirical correlation for end bearing resistance. ....................................... 47 

Figure 65. Graph. PPI pile, measured loads versus depth. ........................................................... 49 

Figure 66. Graph. SEAPILE pile, measured loads versus depth. ................................................. 50 

Figure 67. Graph. Lancaster Composite, Inc., pile, measured loads versus depth. ...................... 50 

Figure 68. Photo. PPI pile............................................................................................................. 54 

Figure 69. Photo. SEAPILE pile................................................................................................... 55 

Figure 70. Photo. Lancaster Composite, Inc., pile........................................................................ 55 

Figure 71. Photo. American Ecoboard pile................................................................................... 55 

Figure 72. Equation. Dynamic modulus E.................................................................................... 57 

Figure 73. Equation. Pile particle speed v. ................................................................................... 57 

Figure 74. Graph. American Ecoboard pile—blows per foot versus elastic modulus.................. 64 

Figure 75. Graph. PPI pile—blows per foot versus elastic modulus............................................ 65 

Figure 76. Graph. SEAPILE pile—blows per foot versus elastic modulus.................................. 65 

Figure 77. Equation. Selastic............................................................................................................ 67 

Figure 78. Graph. SLT and CAPWAP analysis—Lancaster Composite, Inc., pile...................... 68 

Figure 79. Graph. SLT and CAPWAP analysis—PPI pile........................................................... 69 
vii

Figure 80. Graph. SLT and CAPWAP analysis—SEAPILE pile................................................. 70 

Figure 81. Graph. SLT and CAPWAP analysis—American Ecoboard pile................................. 71 

Figure 82. Equation. Applied axial force F................................................................................... 72 

Figure 83. Equation. Equivalent axial stress σ

t

. ........................................................................... 72 

Figure 84. Graph. Stress versus penetration depth for Lancaster Composite, Inc., SLT pile....... 73 

Figure 85. Graph. Stress versus penetration depth for PPI SLT pile............................................ 74 

Figure 86. Graph. Stress versus penetration depth for SEAPILE SLT pile.................................. 75 

Figure 87. Graph. Stress versus penetration depth for American Ecoboard splice SLT pile. ...... 76 
viii

LIST OF TABLES

Table 1. Material properties—test results and model calculations............................................... 24 

Table 2. Selected design material properties (published by Lancaster Composite, Inc.). ............ 28 

Table 3. Compression strength testing of the concrete................................................................. 29 

Table 4. Testing program details, Port Elizabeth site. .................................................................. 33 

Table 5. Soil profile and soil properties at Port Elizabeth site...................................................... 34 

Table 6. Comparison of measured and calculated ultimate loads................................................. 43 

Table 7. Total stress analysis approaches for estimating fs.

(34) ..................................................... 46 

Table 8. Effective stress analysis approaches for estimating ultimate shaft friction.................... 47 

Table 9. Factor CN for base resistance.(43)..................................................................................... 48 

Table 10. Comparison between SLT results and several analysis methods and design codes. .... 51 

Table 11. Elastic modulus of FRP piles estimated from PIT and PDA tests................................ 58 

Table 12. PDA results................................................................................................................... 60 

Table 13. CAPWAP program calculation results. ........................................................................ 61 

Table 14. Quake and damping values recommended by GRL. .................................................... 62 

Table 15. Comparison of GRLWEAP results with measured elastic modulus, number of 

 blows, and energy. ........................................................................................................ 63 

Table 16. Comparison, static and dynamic elastic modulus of SEAPILE, PPI, and steel piles. .. 66 

Table 17. Comparison between CAPWAP analysis and static load test results........................... 68 

Table 18. Measured and allowable stresses for FRP piles............................................................ 77 
1

CHAPTER 1. INTRODUCTION 

Piles are mainly used to support structures under lateral and vertical loads. Vertical load-bearing 

capacity of piles depends mainly on site conditions, soil properties, pile material properties, pile 

dimensions, and method of pile installation. Pile types can be divided into two categories, 

according to their method of installation.(3) The first category consists of driven piles, which 

displace and disturb the soil, and the second category consists of piles that are installed without 

soil displacement. Steel, precast concrete, wood, and fiber-reinforced polymer piles belong to the 

first category. In the second category, soil is removed by drilling, and concrete is cast in the 

borehole to form the pile. 

Analytical methods(4) and several design codes(5,6,7) can be used to evaluate the axial pile 

capacity, end bearing, and shaft friction along the pile. Dynamic methods, based on pile driving 

measurements, can be used to predict the axial static capacity of driven piles and assess the 

drivability and durability of piles during installation.(8) However, before dynamic testing is 

conducted, several material properties and data must be determined; these include the pile length, 

cross-sectional area, specific weight, wave speed, and dynamic elastic modulus. The evaluation 

of the dynamic methods for predicting the load-set response and the ultimate static capacity of 

piles must be obtained by comparing the dynamic method analysis results and static load test 

(SLT) results. Considering the engineering use of fiber-reinforced composite piles, it is essential 

to perform the above-mentioned engineering testing procedure to evaluate the behavior of these 

piles under axial loads and to develop reliable design methods to assess the short- and long-term 

behavior of fiber reinforced polymer (FRP) composite piles. 

In 1998, a waterfront rehabilitation project known as the Hudson River Park began. One goal 

was the replacement of up to 100,000 bearing piles for lightweight structures. The corrosion of 

steel, deterioration of concrete, and vulnerability of timber piles to the marine environment led 

the developer to consider composite materials with high degradation resistance, such as FRPs, as 

a replacement for pilings made of timber, concrete, or steel. 

Concurrently, in 1998, the Federal Highway Administration (FHWA), with a long-term interest 

in the application of polymers in infrastructure, initiated a research project on the use of FRP 

pilings, 

The main purpose of this current research was to address the need for a feasibility assessment of 

FRP composite bearing piles for highway and related facilities substructures, replacing 

traditional materials such as timber, concrete, or steel, specifically in the construction of 

waterfront structures in hostile marine environments. The engineering application of FRP may 

present competitive alternatives to conventional pile materials, due to the accelerated degradation 

of these conventional materials in aggressive environments (figures 1 and 2). Further, chemical 

treatment of wood pilings has become more difficult, due to the toxic nature of typical chemical 

treatments. State departments of transportation (DOT) facing major rehabilitation projects also 

have become interested in assessing the feasibility of using FRP composite bearing piles for 

infrastructure applications. To assess the feasibility of using FRP composite piles as vertical 
2

load-bearing piles, laboratory tests and a full-scale experiment, including dynamic and static load 

tests on FRP piles were conducted. 

Figure 1. Photo. Marine borers (Limnoria) attacking untreated timber piles that 

support many of New York’s highways and harbor piers.(9)

Figure 2. Photo. Complete corrosion of steel H piles supporting a harbor pier 

(recently installed retrofit channels are already corroding).(9) 

CURRENT STATE OF PRACTICE 

Research on the engineering behavior of FRP piles has primarily been focused on these piles’ 

performance under lateral loading as fender piles. In general, current research and applications of 

FRP piles have addressed: 
3

• Mechanical behavior of FRP piles. 

• Field observations and experimental demonstration projects on the engineering 

performance of FRP piles. 

Mechanical Behavior of FRP Piles 

The research conducted thus far has involved primarily laboratory tests on the mechanical 

behavior, flexural behavior, and buckling resistance of FRP pile materials. 

 

Laboratory tests were performed on the Plastic Piling, Inc. (PPI), pile to determine the lateral 

flexural stiffness. The tested pile had a 38.7-centimeter (cm)- (15.25-inch-) diameter, and the 

steel reinforcement consisted of 16 25.4-millimeter (mm)- (1-inch-) diameter bars connected 

with a spiral. The free span between the two supports at both pile ends was 5.8 meters (m) 

(19 feet (ft)). During the testing, the load was applied cyclically at the middle of the free span. 

The test results show that loads exceeding 222 kilonewtons (kN) (50 kips) caused permanent 

deflections in the pile. At these loads, yielding of the steel bars was initiated, inducing slippage 

in the recycled plastic. The ultimate lateral load-bearing capacity was 311 kN (70 kips), at which 

point the pile underwent fully plastic behavior. Further, the pile’s response was ductile up to this 

load, and the recycled plastic did not appear to crack during loading. 

Two axial laboratory tests were performed to obtain the buckling loads of Lancaster Composite, 

Inc., piles.(10) The pile samples had diameters of 25.4 and 30.5 cm (10 and 12 inches) and lengths 

of 9 and 11 m (29.52 and 36.08 ft), respectively. Two loading cycles were applied to the 30.5-

cm- (12-inch-) diameter pile sample to a maximum load of 2113 kN (475 kips) and 2228 kN 

(501 kips); the latter load caused the sample to experience failure. A third applied loading cycle 

caused this pile sample to buckle at 1841 kN (414 kips). The maximum applied load on the 25.4-

cm- (10-inch-) diameter pile sample at which it experienced failure was 1392 kN (313 kips). 

Reviewing the current state-of-practice within the framework of the current research with the 

manufacturers of the selected FRP piles illustrated the need for both laboratory and field axial 

loading tests to assess the engineering performance of these piles as axial load-bearing piles. 

Field Observations Use and Experimental Demonstration Projects 

The current state-of-practice related to the engineering use of FRP piles(9,11) indicates that FRP 

composite piles primarily have been used experimentally for fender piles, waterfront barriers, 

and bearing piles for light structures.(12-13) 

Figure 3 shows fender piles manufactured by Lancaster Composite, Inc., in the U.S. Naval 

Submarine Base, San Diego, CA. Pier 5000, located in Point Loma at the southern tip of the San 

Diego Bay, is one of four major piers that serve as home base for several attack submarines.(14)
4

Figure 3. Photo. Fender piles in the U.S. Naval Submarine Base, 

San Diego, CA. 

Figure 4 shows fender piles manufactured by PPI in U.S. Navy Pier 10 in San Diego, CA. This 

pier berths U.S. Navy vessels. 

Figure 4. Photo. Fendering system in the U.S. Navy Pier 10, 

San Diego, CA. 

Figure 5 shows the Nashville Avenue Marine Terminal at Port of New Orleans, LA. This 

terminal contains 40.6-cm (16-inch) SEAPILE® piles and a 30.5- by 30.5-cm (12- by 12-inch) 

SEATIMBER® fendering system that was manufactured by Seaward International in 1996. 
5

Figure 5. Photo. Fendering system, Nashville Avenue Marine 

Terminal, Port of New Orleans, LA. 

Figure 6 shows a floating dock project that consists of piles manufactured by American 

Ecoboard. The piles are 30.5 cm (12 inches) in diameter and 13.7 m (45 ft) long. 

Figure 6. Photo. Floating dock project. 

The current practice illustrates the lack of mechanical laboratory test results that would help 

assess engineering behavior of the FRP composite materials under axial loading, as well as the 

lack of SLT results and comparisons with dynamic experimental test results. The engineering 

applications of the FRP composite piles as bearing piles raise the following research and 

development (R&D) needs. 
6

RESEARCH AND DEVELOPMENT NEEDS 

The engineering use of FRP bearing piles requires assessing their field performance, as well as 

developing and evaluating reliable testing procedures and design methods to assess: 

• Drivability and constructability of composite piling. 

• Quality control during manufacturing and during pile installation. 

• Load-settlement response and axial bearing capacity. 

• Soil-pile interaction and load transfer along the installed piling. 

• Short-term composite material properties. 

• Creep and time effects on the long-term pile performance. 

• Physical, mechanical, and chemical material degradation mechanisms. 

To address these R&D needs and assess the feasibility of using FRP composite piles as vertical 

load-bearing piles, a full-scale experiment including dynamic and static load tests on FRP piles 

was conducted at a site provided by the Port Authority of New York and New Jersey 

(PANY&NJ) at their Port of Elizabeth facility in New Jersey. Figure 7 shows the Port of 

Elizabeth demonstration site. The full-scale experiment was conducted with the cooperation and 

support of the PANY&NJ engineering department and the New York State Department of 

Transportation (NYSDOT). 

Figure 7. Photo. Port of Elizabeth demonstration site. 

The scope of this demonstration project included: 

• Manufacturing and instrumentation of selected FRP composite piles. 

• Field and laboratory soil investigations. 

• Laboratory tests on full-scale recycled plastic and composite material samples under axial 

compression loads. 
7

• Driving and monitoring 11 FRP piles and a reference steel pile at Port of Elizabeth, NJ. 

• Performing four full-scale SLTs at Port of Elizabeth, NJ. 

To select the piles for this demonstration project, a team of experts prepared a request for 

qualification and disseminated the request to composite piling manufacturers; these 

manufacturers were invited to submit their qualifications and products for consideration. The 

items in the request for qualifications were: contact information, general description of piling 

system, material properties, drivability, durability, creep, quality assurance/quality control, 

experience, field records, and specimens. The selected FRP composite piles consisted of recycled 

plastic reinforced by fiberglass rebar (SEAPILE composite marine piles), recycled plastic 

reinforced by steel bars, recycled plastic with no reinforcement rebar, recycled plastic with 

randomly distributed fiberglass fibers (Trimax), and concrete-filled plastic shell manufactured, 

respectively, by Seaward International Inc., PPI, American Ecoboard, U.S. Plastic Lumber 

(USPL), and Lancaster Composite, Inc. Figure 8 indicates the locations of the pile 

manufacturers. 

Figure 8. Illustration. Locations of pile manufacturers. 

Pile instrumentation was performed at the manufacturing plants and included installing strain 

gauges at several levels in the piles. The manufacturers delivered these piles to the site, and the 

installation of the piles was conducted with the full support and cooperation of the 

manufacturers. All the manufacturers also attended the SLTs. 

This research report summarizes the results of this full-scale experiment and the companion 

laboratory tests. 
8

Chapter 2 presents an engineering analysis approach for establishing the equivalent mechanical 

properties of the composite material, including elastic modulus for the initial loading quasilinear 

phase, axial compression strength, inertia moment, and critical buckling load. The composite 

material used in this study consisted of recycled plastic reinforced by fiberglass rebar (SEAPILE 

composite marine piles), recycled plastic reinforced by steel bars, and recycled plastic reinforced 

with randomly distributed fiberglass (Trimax), manufactured by Seaward International Inc., PPI, 

and USPL, respectively. 

Chapter 3 describes the results of the SLTs, along with the instrumentation schemes that were 

specifically designed for strain measurements in the FRP piles. The experimental results are 

compared with current design codes and with the methods commonly used for evaluating the 

ultimate capacity, end bearing capacity, and shaft frictional resistance along the piles. This 

engineering analysis leads to preliminary recommendations for the design of FRP piles. 

In chapter 4, Pile Driving Analyzer® (PDA) and Pile Integrity Tester (PIT) test results were 

analyzed using the Case Pile Wave Analysis Program (CAPWAP)(1) and the GRL Wave 

Equation Analysis of Piles program (GRLWEAP)(2) to establish the dynamic properties of the 

FRP piles. The PDA also was used to evaluate the feasibility of installing FRP piles using 

standard pile driving equipment. Pile bearing capacities were assessed using the CAPWAP 

program with the dynamic data measured by the PDA, and the calculated pile capacities were 

compared with the results of SLTs performed on the four FRP piles. 

Chapter 5 summarizes the main conclusions and recommendations of this project.
9

CHAPTER 2. MECHANICAL SHORT-TERM BEHAVIOR OF FRP 

COMPOSITE MATERIALS UNDER AXIAL COMPRESSION LOADS 

The engineering use of FRP piles on a widespread basis requires developing and assessing 

reliable testing procedures and data analysis methods to establishing the material properties of 

the FRP composite piles. The results can be used to determine if these piles offer an alternative 

for deep foundation construction, especially in waterfront environments and aggressive 

soils.(15-20) To address these engineering issues and assess the mechanical behavior of these 

materials under axial compression loads, FRP composite material specimens were tested in the 

laboratory. The composite material used in this study consisted of recycled plastic reinforced by 

fiberglass rebar (SEAPILE composite marine piles), recycled plastic reinforced by steel bars, and 

recycled plastic reinforced with randomly distributed fiberglass (Trimax). 

SEAPILE COMPOSITE FRP MATERIAL 

In this chapter, the results of the laboratory compression tests performed on full-scale FRP 

composite material samples are discussed. The composite material used in this study consisted of 

recycled plastic reinforced by fiberglass rebar (SEAPILE composite marine piles) manufactured 

by Seaward International, Inc. The tests were performed on both the composite material 

specimens and each of the component materials, including the fiberglass bars and the recycled 

plastic. 

The authors present an engineering analysis approach for establishing the equivalent mechanical 

properties of the composite material, including elastic modulus for the initial loading quasilinear 

phase, axial compression strength, inertia moment, and critical buckling load. The equivalent 

mechanical properties of the composite material are related to the mechanical properties of the 

component materials, assuming strain compatibility between the plastic and the fiber 

reinforcement during the axial compression loading. The results of this analysis illustrate that the 

main effect of the recycled plastic on the response of the composite material is to increase the 

buckling resistance. For the tested SEAPILE sample, the recycled plastic appears to prevent 

buckling of the bars but does not effectively prevent the peripheral disintegration of the 

fiberglass bars; therefore, it makes only a limited contribution to the axial compression strength 

of the composite material. 

MECHANICAL PROPERTIES OF THE FRP COMPOSITE AND THE COMPONENT 

MATERIALS 

 

SEAPILE composite marine piles consist of an extruded recycled plastic material reinforced by 

fiberglass bars (figure 9). The fiberglass rebar are manufactured in a pultrusion process, using 

fiberglass strands and polyester resin. The recycled plastic is composed mainly of high, medium, 

and low density polypropylene (HDPE, MDPE, and LDPE). It also contains a small percentage 

of polypropylene made from post-consumer waste (e.g., blow-molded bottles, plastic barrels, 

plastic bags, and TV cable jackets) and industrial waste (e.g., scrap pipes, telephone cables, film 

auto carpets, diaper trim, and auto insulation). The composite pile material consists of an inner 

core, foam that contains the fiberglass bars, and a skin. The core and the foam are made of 
10

recycled plastic, and the skin is made of a solid polyethylene compounded with pigments, 

ultraviolet (UV) absorbers, and antioxidants. The skin is relatively smooth, giving a friction 

coefficient of approximately 0.21 (wet) and 0.22 (dry), when tested in accordance with American 

Society for Testing and Materials (ASTM) F489.(21,22)

Figure 9. Photo. SEAPILE composite marine piles. 

To establish the mechanical properties of the fiberglass bars, two axial compression tests were 

performed on 4.4-cm- (1.75-inch-) diameter bars. The maximum load reached 591.6 kN 

(133 kips) and, at this point, the bars displayed a brittle failure. Figure 10 illustrates the stressstrain relationship of the fiberglass bars. Young’s modulus of the fiberglass bars is 1.6x107 kPa 

(2,320 ksi), and the maximum stress at failure is 3.63x105 kPa (52.6 ksi). Figure 11 shows the bar 

both before and after parts of it disintegrated upon test completion. It illustrates that the failure 

mechanism involves peripheral disintegration by peeling fiberglass from the 9-cm- (3.54-inch-) 

long bar, which did not buckle during the loading. Considering the elastic properties of the bar, 

its buckling free length was calculated (according to Euler’s equation) to be 22.5 cm (8.9 inches) 

for this failure load. 
11

Figure 10. Graph. Stress-strain relationship of 4.4-cm (1.75-inch) fiberglass bars. 

Figure 11. Photo. Fiberglass bar before axial compression test and 

disintegrated fiberglass parts. 

Axial compression tests were performed on the recycled plastic foam. The sample’s dimensions 

were 15.2 cm (6 inches) in diameter and 30.4 cm (12 inches) long. Figure 12 shows the stressstrain relationships of the recycled plastic material obtained for strain rates of 0.1 cm (0.04 inch) 

per minute (min) (0.33 percent/min) and for 0.025 cm (0.01 inch) per min (0.083 percent/min). 

The results illustrate that the nonlinear response of this recycled plastic material to the axial 

loading is strain-rate dependent. Young’s modulus of the recycled plastic obtained for the linear 

portion of the stress-strain curves are 3 x 105 kilopascals (kPa) (43.5 ksi) and 1.93 x 105 kPa 

(28 ksi), respectively, for the strain rates of 0.33 percent/min and 0.083 percent/min. Figure 13 

shows the axial stress versus radial strain relationships obtained for the recycled plastic foam. 
12

The calculated Poisson’s ratio is equal to 0.37, and the corresponding shear modulus value for 

the linear portion of the stress-strain curve is 109,489 kPa (15.9 ksi). 

Figure 12. Graph. Stress-vertical strain relationship of SEAPILE pile recycled plastic. 

Figure 13. Graph. Stress-radial strain relationship of SEAPILE pile recycled plastic; 

rate 0.33 percent/min. 

To establish the composite material properties, a compression axial test was performed on a 

SEAPILE pile sample 40.6 cm (16 inches) in diameter and 81 cm (32 inches) long. The sample 
13

contained 16 fiberglass reinforcing rods each 7.6 cm (1.75 inches) in diameter. Because the top 

end of the sample was not leveled, Hydrostone® grout was placed between the pile upper surface 

and the loading steel plate. Three Geokon vibrating wire strain gauges were installed in the pile 

section on the fiberglass rebar, and four additional strain gauges were glued into holes on the 

outer skin. After the initial seating load was applied, the nominal loading rate was 0.25 mm 

(0.01 inch) per min. 

Figure 14 shows the force-strain relationship obtained for the FRP composite material under the 

axial compression test. The maximum load reached was 9661 kN (2172 kips), at which point the 

sample experienced a brittle failure. The Young’s modulus of the composite material was 3.38 x 

106 kPa (490.23 ksi). Figure 15 shows the post-failure, full-scale sample section. It illustrates that 

the bars did not experience any buckling during loading, and only the upper section of the 

sample was damaged. 

Figure 14. Graph. Force-vertical strain relationship of SEAPILE pile sample. 
14

Figure 15. Graph. SEAPILE pile sample after axial compression test. 

ENGINEERING ANALYSIS OF THE AXIAL COMPRESSION TEST ON THE 

COMPOSITE PILE SAMPLE 

Elastic Modulus and Compressive Strength 

The main purpose of the engineering analysis is to establish the equivalent mechanical properties 

of the composite material, including the elastic modulus for the initial loading phase, the axial 

compression strength, the inertia moment, and the critical buckling load. The equivalent material 

properties of the composite are related to the mechanical properties of the component materials, 

assuming strain compatibility between the plastic and the fiber reinforcement during the axial 

compression loading. The following assumptions were used in this analysis. 

For the composite material section, the equilibrium equation under the applied load, F, can be 

written as in figure 16. 

 

Figure 16. Equation. Applied load F. 

Where:

σc = Stress of the total section of the sample. 

Ac = Cross-sectional area of the sample. 

n = Number of bars in the composite section. 

σb = Stress of the fiberglass bar. 

Ab = Section area of the fiberglass bar. 

σp = Stress of the plastic. 

Ap = Section area of the plastic. 
15

For axial compression, assuming elastic materials and strain compatibility between the plastic 

and the fiber reinforcement during the axial compression loading implies the relationships in 

figure 17. 

Figure 17. Equation. Young’s modulus E. 

Where:

E = Young’s modulus. 

Eb = Young’s modulus of the bar. 

Ep = Young’s modulus of the plastic. 

Ec = Young’s modulus of the total section of the sample. 

Solving the equations in figures 16 and 17 yields the equation in figure 18. 

Figure 18. Equation. σc/σb. 

 

By making the following substitutions, 

α = nAb/A, where α is defined as the area replacement factor, 

β = Ep/Eb, where β is the relative axial stiffness coefficient,

Ac = A, where Ac is the section area of the composite sample, 

the equation in figure 18 can be written in the form of the equation in figure 19. 

Figure 19. Equation. Ec/Eb. 

The equation in figure 19 yields the equivalent Young’s modulus of the composite material as a 

function of the replacement factor α, the relative stiffness coefficient β, and the Young’s 

modulus of the fiberglass bar Eb. For the SEAPILE composite material, equation 4 yields an 

equivalent Young’s modulus value of Ec= 3.05 x 106 kPa (442.36 ksi); the difference between 

this calculated Ec value and the experimental result obtained for the composite sample Ec = 3.38 

x 106 kPa (490.23 ksi) is less than 10 percent of the experimental value. 

The equivalent shear modulus, Gc, of the composite material can be calculated from the theory of 

elasticity assuming that the FRP composite is an isotropic elastic material. This assumption does 

not take into account the anisotropy of the composite material induced by the preferential 

direction of the fiber reinforcement. Accordingly the equivalent shear modulus, Gc can be 

calculated by the equation in figure 20. 
16

Figure 20. Equation. Gc. 

Where: 

υc and υp are the Poisson’s ratio values of the composite material and its recycled plastic 

component, respectively. 

The equivalent compression strength, Rc, of the composite material is given by the equation in 

figure 21. 

Figure 21. Equation. Rc/Rb. 

Where: 

η = Rp/Rb, the compression strength ratio of the component materials. 

Rp = Compression strength of the fiberglass bar. 

Rb = Compression strength of the recycled plastic mobilized at the failure strain of the fiberglass 

bar (ε = 2.6 percent). 

For the SEAPILE composite material, the equation in figure 21 yields an equivalent compression 

strength Rc = 7.4 x 104 kPa (10.73 ksi); the difference between this calculated Rc value and the 

experimental result obtained for the composite sample, Rc = 6.8 x 104 kPa (9.86 ksi), is about 

8.8 percent of the experimental value. 

The effect of the recycled plastic on the behavior of the composite material can be analyzed 

considering the failure load factor (FLF), defined as the ratio of the failure load Fc of the 

composite material to the failure load nFb, defined as the failure load of the bar multiplied by the 

number of bars in the composite material. The FLF can be calculated by the equation in figure 

22. 

Figure 22. Equation. FLF. 

Figure 14 illustrates the experimental relationships between the applied axial load and the axial 

strain for the group of 16 bars and the composite material. The results show that, at failure by 

axial compression, the recycled plastic has only a limited effect on the load-bearing capacity of 

the composite material. The equation in figure 22 yields an FLF of 1.068, which is quite 

consistent with the experimental results (FLF = 1). 
17

Moment of Inertia 

Bending moment, assuming elastic materials and strain compatibility between the plastic and the 

fiber reinforcement during bending, implies the relationships in figure 23. 

Figure 23. Equation. Bending moment M. 

Where: 

M = Bending moment. 

y′′ = Second derivative of the displacement variation with depth. 

ΣIb = Sum of the moments of inertia of all the fiberglass bars with respect to the axis of 

 symmetry of the composite material section. 

 Ip = Moment of inertia of the recycled plastic section with respect to the axis of symmetry of 

 the composite material section. 

The moment of inertia (Ic) of the composite material can be derived by rearranging the equation 

in figure 23 to give the equation in figure 24. 

Figure 24. Equation. Moment of inertia Ic(1). 

By substituting β for Ep/Eb, the equation in figure 24 can be rewritten as the equation in figure 

25. 

Figure 25. Equation. Moment of inertia Ic(2). 

The relative inertia moment coefficient λ is given by the equation in figure 26. 

Figure 26. Equation. Relative inertia moment coefficient λ. 

Preceding equations can be combined and rearranged to give the equation in figure 27. 

Figure 27. Equation. Ic/ΣIb. 
18

Critical Buckling Force 

The critical buckling force, Pcr, can be calculated according to Euler’s equation (figure 27).: 

Figure 28. Equation. Critical buckling force Pcr. 

Where: 

E = Young’s modulus. 

l = Length of the bar. 

I = Moment of inertia. 

For the fiberglass bar sample (E = 1.6 x 107 kPa (2,320.6 ksi); l = 9 cm (3.5 inches); I = 

19.16 cm4

 (0.46 inch4)), the equation in figure 28 yields the calculated critical buckling load of 

Pcr = 3,736 kN (839.8 kips). For a given fixed boundary condition at the top and at the bottom of 

the bar, k = 0.5, Pcr = 1,868 kN (419.9 kips). This buckling load exceeds the compression failure 

load of 591.6 kN (133 kips), causing the bar to collapse by peripheral disintegration. 

The equivalent critical buckling load, Pcr, of an axially loaded bar confined in a low shear 

modulus material such as the recycled plastic is given by the equation in figure 29.(23)

Figure 29. Equation. Pcr of an axially loaded bar. 

The term Po

cr in figure 29 is the equivalent critical buckling load calculated for the composite 

material according to Euler’s equation (figure 30).

Figure 30. Equation. Equivalent critical buckling load for composite material. 

The terms Ec and Ic in figure 30 are the elastic Young’s modulus and the moment of inertia of the 

composite material, respectively. 

The effect of the recycled plastic on the buckling failure load of the composite material can be 

analyzed by considering the critical buckling load factor (BLF), defined as the ratio of the critical

buckling load Pc

cr of the composite material to the critical buckling load of the bar, multiplied by 

the number of bars in the composite material ΣPb

cr. The BLF can be calculated by the equation in 

figure 31. 
19

Figure 31. Equation. Critical buckling load factor BLF. 

Substituting λo = Ic /n Ib as a relative inertia moment index, the equation in figure 31 can be 

written as the equation in figure 32. 

Figure 32. Equation. Critical buckling load decomposed. 

Figure 16 illustrates the variation of the critical buckling load with the length of the sample for 

the composite material and for the group of fiberglass bars. The critical buckling load of the 

composite material is calculated according to the equations in figures 29 and 30. The critical 

buckling load for the group of 16 bars is calculated from the sum of the buckling loads of the 

individual bars: ΣPb

cr = n*Pbcr, where the critical buckling load of each bar, Pbcr, is calculated 

from the equation in figure 28. The results show that the recycled plastic has a significant effect 

on the critical buckling load. 

The failure load of the sample is equal to the smaller load that can cause either buckling or 

peripheral disintegration of the pile matrix. It was observed that, for the failure load of 9,661 kN 

(2,172 kips), the composite material sample (40 cm (16 inches) in diameter and 80 cm 

(32 inches) in height) did not experience any buckling failure (figure 15). As illustrated in 

figure 33, under a critical buckling load which is equal to the compressive strength of the group 

of 16 bars, the calculated maximum free length is about 22 cm (8.7 inches), which is 

significantly smaller than the height of the composite sample. However, the maximum free 

length of the composite sample calculated from the equation in figure 29 for the composite 

material reaches about 150 cm (59 inches), illustrating that the recycled plastic appears to 

prevent buckling of the bars in the composite material. The critical BLF calculated from the 

equation in figure 31 for the composite sample reaches about 38, indicating the significant effect 

of the recycled plastic on the buckling resistance of the composite material. 
20

Figure 33. Graph. Buckling force versus length for SEAPILE pile 

sample and 16 fiberglass bars. 

 

PPI PILES 

PPI manufactures recycled plastic piles reinforced by steel bars (figure 34). The company 

obtains recycled plastic pellets from recycling firms that acquire their plastic from curbside 

collection and industrial waste. HDPE and LDPE are required to develop a cohesive and 

durable product. The formulation of the recycled plastic includes a mixture of approximately 

30 percent HDPE and 70 percent LDPE. Carbon black material is added to the mix at the rate 

of 4 percent by weight of plastic to serve as a UV inhibitor. Celogen AZ130® is added at the 

rate of 0.5 percent by weight of plastic to encourage uniformity of porosity and minimize the 

size of voids. Calcium carbonate can be added to the mix to make the plastic material heavier 

when it is necessary to avoid a product that is lighter than water. The outer surface of the 

recycled plastic yields a material friction coefficient of approximately 0.20 (wet) and 0.22 

(dry), when tested in accordance with ASTM F489.(24)
21

Figure 34. Photo. PPI pile. 

To establish the composite material properties, a compression axial test was performed on a PPI 

pile sample containing 16 steel reinforcing rods of 2.54-cm (1.0-inch) nominal diameter. The 

sample was 38.7 cm (15.25 inches) in diameter and 80 cm (31.5 inches) long. The results 

illustrate that this composite material has a behavior similar to that of steel (figure 35) and yields 

a compressive strength of 45,190 kPa (6.55 ksi). The Young’s modulus of the composite material 

obtained for the linear portion of the stress-strain curve is 7.84 x 105 kPa (113.7 ksi). 

Figure 35. Graph. PPI pile—stress-strain relationship. 
22

TRIMAX STRUCTURAL PLASTIC MATERIAL 

Trimax structural plastic lumber material consists of recycled plastic, fiberglass, and selected 

additives (figure 36). The plastic raw material used in Trimax is derived from post-consumer 

bottle waste such as milk and detergent bottles. This material is compounded into a consistent, 

reinforced plastic timber product using reactive compatibilizers, which create a stable 

plastic/fiber matrix.(25)

Figure 36. Photo. Trimax pile. 

Axial Compression Tests on the Trimax Pile Recycled Plastic 

Axial compression tests were performed on the Trimax pile recycled plastic. The sample’s 

dimensions were 25.4 cm (10 inches) in diameter and 50.8 cm (20 inches) long. Figure 37 shows 

the vertical stress-strain relationships obtained for several strain rates. The results illustrate that 

the nonlinear response of this recycled plastic material to the axial loading is strain-rate 

dependent. The Young’s modulus of this material, obtained for the linear portion of the stressstrain curve at the strain rate of 1.7 mm (0.07 inch) per min (0.33 percent/min), is 370,000 kPa 

(53.7 ksi). Figure 38 shows the axial compression stress versus radial strain relationships 

obtained for strain rates of 0.33 percent/min. Poisson’s ratio calculated for this strain rate at 

1000 kPa (0.145 ksi) is equal to 0.35, and the corresponding shear modulus value for the linear 

portion of the stress-strain curve is 1.37 x 105

 kPa (19.9 ksi). 
23

Figure 37. Graph. Trimax pile—vertical stress-strain curves at different rates. 

Figure 38. Graph. Trimax pile—vertical stress-lateral strain curve; 

strain rate 0.33 percent per min. 
24

Table 1 summarizes the comparison between the predicted and experimental values of the 

SEAPILE composite material properties. 

Table 1. Material properties—test results and model calculations. 

Material 

Property 

Recycled 

Plastic 

16 Fiberglass 

Bars 

Composite Material 

Test Results 

Model 

Prediction 

Elastic modulus, 

kPa (ksi) 

300,000 

(43.5) 

1.6 x 107 

(2,320) 3.38 x 106

 (490.23) 3.05 x 106 (442.36) 

Compressive strength, 

kPa (ksi) 

1.169 x 104

(1.69) 

3.63 x 105

(51.9) 6.8 x 104

 (9.86) 7.4 x 104 (10.73) 

Moment of inertia, 

m4 (inch4) 

1.6 x 10-3

(3,844) 

118.98 x 10-6

(285.8) – 0.77 x 10-3 (1850) 

Critical buckling load, 

kN (kips) (80 cm) 

sample 

– 757 (170) 9661 (2172) 

(no buckling observed) 29,066 (6,534) 
25

CHAPTER 3. BEHAVIOR OF FRP COMPOSITE PILES UNDER 

VERTICAL LOADS 

The engineering use of FRP piles on a widespread basis requires developing and evaluating 

reliable testing procedures and design methods that will help evaluate the load-settlement curve 

of these composite piles and their static bearing capacity. In particular, full-scale loading tests on 

FRP piles must be conducted to evaluate the behavior of these types of piles under vertical loads. 

The main objective of the study described in this chapter was to conduct a full-scale experiment 

(including dynamic and static load tests) on FRP piles, to address these engineering needs, and 

assess the feasibility of using FRP composite piles as vertical load-bearing piles. The full-scale 

experiment, described in this chapter, was conducted in a selected site of the Port Authority of 

New York and New Jersey at Port of Elizabeth, NJ. It included dynamic pile testing and analysis 

on 11 FRP driven piles and a reference steel pile, as well as SLTs on four FRP composite piles. 

Figure 39 shows the Port of Elizabeth demonstration site. 

 

In this chapter, the results of SLTs are presented along with the instrumentation schemes that 

were specifically designed for strain measurements in the FRP piles. The experimental results are 

compared with current design codes and with the methods commonly used for evaluating the 

ultimate capacity, end bearing capacity, and shaft frictional resistance along the piles. This 

engineering analysis leads to preliminary recommendations for the design of FRP piles. 

Figure 39. Photo. Port of Elizabeth demonstration site. 
26

PILE MANUFACTURING AND INSTRUMENTATION

The FRP test piles were manufactured and instrumented to allow for the measurement of the load 

and settlements at the pile top, the displacements of the reaction dead load, and the axial strain at 

several levels in the pile to obtain the axial force variation with depth along the piles. The pile 

top settlements were measured by four linear variable differential transformers (LVDTs) (Model 

10,000 hollow cylinder apparatus (HCA)), and the dead load displacements were measured by a 

LVDT (Model 5,000 HCA). Additional digital dial gauge indicators were also used to measure 

displacements. Vibrating wire and foil strain gauges were used to determine the axial force at 

several levels in the pile. The foil strain gauges were connected to a National Instruments data 

acquisition system, and the instrumentation of the pile head and the loading system was 

connected to a Geokon data acquisition system. The instrumentation and data acquisition 

systems (figures 40 through 42) allowed for online monitoring during the SLTs of: the pile top 

displacement versus the applied load; the applied load; and the stresses at different levels in the 

pile. 

The manufacturers calibrated data acquisition with the LVDTs, the load cell, and the strain 

gauges before performing the field tests. All the strain gauges were installed in the FRP pile 

during the manufacturing process before the piles were delivered to the site. Pile manufacturing 

methods and instrumentation details are briefly described in the following paragraphs. 

Figure 40. Photo. Equipment used in the 

in-load tests. 

Figure 41. Illustration. Schematic of the equipment 

used in the in-load tests. 
27

Figure 42. Illustration. Data acquisition system. 
28

Lancaster Composite, Inc. 

Lancaster Composite, Inc., piles are composed of a hollow FRP pipe that is filled before 

installation with an expanding concrete and is coated with a durable corrosion-resistant coating 

layer (figure 43). The hollow pipe is produced from unsaturated polyester or epoxy reinforced 

with reinforcement rovings (E-glass) and appropriate filler material to form a rigid structural 

support member. E-glass is incorporated as continuous rovings and is set in resin under pressure 

during the fabrication process. Table 2 shows the design material properties. 

Table 2. Selected design material properties (published by Lancaster Composite, Inc.). 

Material Properties—FRP Shell for 16.5-inch outer diameter 

Young’s modulus—axial (tensile) (psi) 2.79 x 106 

Tensile strength—axial direction (psi) 50,400 

Young’s modulus—axial (compression) (psi) 1.9 x 106

Compressive strength—axial direction (psi) 30,000 

Tensile strength—hoop direction (psi) 35,000 

Elastic modulus—hoop direction (psi) 4.5 x 108

Material Properties—Concrete Core for 16.5-inch outer diameter 

Concrete property (28 days)—fc prime (psi) 6,000 

Expansion—confined permanent positive stress (psi) 25 

1 psi (lbf/in2) = 6.89 kPa 

1 inch = 2.54 cm 

The precast FRP composite pile (C40) is produced in two stages. The first stage consists of 

fabricating the hollow FRP tube, which is produced using the continuous filament winding 

method. The tube is given its final protective coating in the line during the winding process at the 

last station of the production line. During the second stage of the process, the tube is filled with 

an expanding cementious material at the plant. 

Two vibrating wire strain gauges (model 4200VW) are installed 0.25 m (9.84 inches) from the 

bottom of the pile. The strain gauges were installed in the piles before the tube was filled with 

the cementious material. The installation procedure included casting strain gauges in the concrete 

(figure 43), inserting the strain gauge wires into the 1.9-cm- (0.75-inch-) diameter hollow 

fiberglass pipe, and locating the pipe in the center of the hollow FRP tube using steel spacers. At 

the end of the pile manufacturing process, concrete was cast into the FRP tube. To detect any 

zero drifts, strain readings were taken before strain gauge installation, after casting the concrete, 

before pile driving, and before performing the SLT. 

Table 3 shows compressive laboratory test results performed on two samples that were taken 

from a concrete pile. The compressive strengths of the concrete after 28 days were 41,693 kPa 

(6,047 psi) and 43,609 kPa (6,325 psi), which are greater than the manufacturer’s strength 

recommended values, by 0.8 percent and 5.4 percent, respectively. 
29

Figure 43. Photo. Strain gauges installation in pile of Lancaster Composite, Inc. 

Table 3. Compression strength testing of the concrete. 

Material Properties—Concrete Core for 16.5-inch O.D. 

Test 

Number 

Compression after 

7 days, kPa (psi) 

Compression after 

28 days, kPa (psi) 

1 36,756 (5331) 43,609 (6325) 

2 35,660 (5172) 41,693 (6047) 

1 inch = 2.54 cm 

PPI 

PPI piles are composed of steel reinforcing bars that are welded to a spiral-reinforcing cage and 

encapsulated with recycled polyethylene plastic (figure 44). The manufacturing process uses a 

steel mold pipe and involves a combination of extrusion and injection molding, into which the 

plastic marine products are cast. Mold pipes for plastic pilings range in diameter from 20.3 cm 

(8 inches) to 121.9 cm (48 inches). The structural steel cage core is held at the center of the 

molded pile while the plastic is injected into the mold (figure 44). A centering apparatus is 

connected to the steel rebar cage so that it can be removed after the pile has been formed. 

Plastic is then fed into the extruder hopper through a mixing chamber, ensuring mixing with the 

carbon black material and the Celogen AZ130. The extruder temperature is set at 308 °C 

(525 °F). Plastic pressures range from 1.38 to 3.45 kPa (200 to 500 psi) throughout the mold 

while the plastic is being injected. After the extrusion process is complete, the mold is dropped 

into chilled water for a cooling period of several hours. 
30

Figure 44. Photo. Vibrating and foil strain gauges attached to steel cage in PPI pile. 

The strain gauges were installed in the piles at the company site before extruding the plastic 

material; therefore, the gauges and the cables had to resist high temperatures. Six vibrating wire 

strain gauges (model 4911-4HTX) were installed in the pile at 0.8 m (2.6 ft), 9.8 m (32.1 ft), and 

18 m (59 ft) from the bottom. Two strain gauges were installed at each level. The strain gauges 

were attached to steel rebar and calibrated. Two additional foil strain gauges (model N2K-06-

S076K-45C) were installed 0.8 m (2.6 ft) from the bottom of the pile. The foil strain gauges were 

attached to the steel rebar and sealed with epoxy glue to avoid moisture penetration. They then 

were assembled as a half-bridge and connected with the strain gauges located outside of the pile, 

creating a full Winston bridge. The installation process in the factory included welding the rebar 

with the strain gauges to the pile’s steel reinforcement cage (figure 44). The strain gauges were 

connected to the data acquisition system to collect data before welding them to the steel cage, 

before extruding the hot plastic, and after cooling the pile down to detect any drifts of the zero 

reading. At the Port of Elizabeth, NJ, site, strain readings also were taken before pile driving and 

conducting the SLT. 

SEAPILE Piles 

SEAPILE composite marine piles contain fiberglass bars in recycled plastic material 

(figure 45). The manufacturing process of the SEAPILE composite marine piles consists of 

extruding the plastic material around the fiberglass bars using a blowing agent to foam to a 

density of 6.408 kN/m3 (40 lbf/ft3) at 217.5 °C (380 °F) and adding a black color for UV 

protection. The extrusion is followed by cooling the product and cutting to the desired length. 
31

Figure 45. Photo. Vibrating and foil strain gauges attached to SEAPILE 

composite marine pile. 

The strain gauges were installed at the factory after the pile was manufactured and the recycled 

plastic was cooled. Six vibrating wire strain gauges (model VK-4150)—two at each level—

were installed in the pile 1 m (3.3 ft), 9.5 m (31.2 ft), and 18 m (59 ft) from the bottom. Two 

additional foil strain gauges (model EP-O8-250BF-350) were installed 1 m (3.3 ft) from the 

bottom of the pile. Each of these foil strain gauges was attached to plastic pieces by heating and 

sealing, using epoxy glue to avoid moisture penetration. The strain gauges were assembled to 

create a full Winston bridge. The installation procedure for the gauges included routing two 

grooves in the plastic material along the pile, providing an access to two rebar, and attaching 

the strain gauges and covering them by PC7 heavy duty epoxy paste. The wires in the groove 

were covered using hot, liquefied, recycled plastic. 

The strain gauges were connected to the data acquisition system, and data were collected before 

connecting the gauges to the pile and after covering them with epoxy paste to detect any drift of 

the zero reading. Onsite strain readings also were taken before pile driving and before 

performing the SLT. 

FIELD TESTING PROGRAM 

Full-Scale Experiment 

Figure 46 shows the schematic drawing of the Port Elizabeth site, and table 4 provides the 

testing program details, including pile structure, diameter, length, and the pile driving order. 

The test pile types included: 

• A reference steel closed-end pipe pile, 40.6 cm (16 inches) in diameter with a wall 

1.27 cm (0.5 inch) thick; the pile was 20 m (65.6 ft) long and furnished with a 45-

degree conical shoe; the pipe pile was manufactured using A252, Grade 2 steel, which 

has a minimum yield strength of 241,316.5 MPa (35 ksi). 
32

• Two Lancaster Composite, Inc., piles, 41.9 cm (16.5 inches) in diameter and 18.8 m 

(61.7 ft) long, and a spliced pile with a total length of 29.6 m (97 ft) that was driven to 

refusal. 

• Three PPI piles, 38.7 cm (15.25 inches) in diameter and 19.8 m (65 ft) long; these piles 

were constructed from solid polyethylene and reinforced with 16 steel reinforcing bars, 

each 2.54 cm (1 inch) in diameter. 

• Three SEAPILE piles, 20 m (65.6 ft) long and 42.5 cm (16.75 inches) in diameter; these 

piles were solid polyethylene reinforced with 16 fiberglass bars, each 4.4 cm 

(1.75 inches) in diameter. 

• Two American Ecoboard solid polyethylene piles; these piles were delivered in 6.1-m 

(20-ft) sections and were 41.9 cm (16.5 inches) in diameter; the total length of one pile, 

after splicing, was 11.8 m (38.75 ft); the splice was made with steel pipe sections 

connected with bolts through the pile. 

Figure 46. Illustration. Schematic drawing of Port Elizabeth site. 
33

Table 4. Testing program details, Port Elizabeth site. 

1

 A is area in square meters; E is Young’s modulus in kilopascals, which is kilonewtons per square meter. 

GEOTECHNICAL SITE CONDITIONS 

Port Elizabeth was constructed over a tidal marsh deposit consisting of soft organic silts, clays, 

and peats extending from mean high water to a depth of 3 to 9 m (10 to 30 ft). The site was 

reclaimed by placing fill over the marsh and surcharged to consolidate the compressible soils. 

The depth of the water table at the site is 1.7 m (5.5 ft) below the surface of the ground. Borings 

B-1 and B-2 indicated that the moisture content of the organic deposits ranged from 60 percent to 

131 percent. The organic deposits are underlaid by silty fine sand. Below these materials are 

glacial lake deposits that generally range from sandy silt to clay and are sometimes varved. The 

glacial lake deposit at this site is overconsolidated. The consolidation test performed on a sample 

that was taken from this layer indicated a preconsolidation load of approximately 23 metric tons 

(t)/m2

 (4.7 kips/ft2) and an overburden pressure of approximately 21.5 t/m2 (4.4 kips/ft2). The site 

is underlaid by shale rock. Two soil borings were performed: one near the steel pile, and the 

other near the American Ecoboard pile (figure 46). Soil laboratory tests show that the soil 

profiles can be roughly stratified into the layers in table 5. 

Pile Type Pile Structure Driving Order 

Number 

Ref. Number 

of Tested 

(SLT) Piles 

Diameter 

cm (inch) 

Length 

m (ft) AE1

kN (kips) 

Steel Pile Steel 1 — 41.9 (16.5) 20 (65.6) — 

Lancaster Fiberglass cased 

concrete 2,3,4 3 41.9 (16.5) 18.8 (61.7) 4.050 x 106

(9.1 x 105)

PPI 

16 steel bars 2.54 cm 

(1inch) in diameter 

in recycled plastic 

5,6,7 6 38.7 (15.25) 19.8 (65.0) 0.890 x 106

(2 x 105)

SEAPILE 

16 fiberglass bars 

4.5 cm (1.75 inches) 

in diameter in 

recycled plastic 

8,9,10 9 42.5 (16.75) 20 (65.6) 0.745 x 106

(1.67 x 105)

American 

Ecoboard Recycled plastic 11,12 12 41.9 (16.5) 6.1 (20) 11.8 (38.8) — 
34

Table 5. Soil profile and soil properties at Port Elizabeth site. 

Depth, 

m (ft) 

Soil 

Classification

Total Unit 

Weight, 

kN/m3 (lb/ft3)

Effective 

Cohesion,

kN/m2

(psf) 

Effective 

Friction Angle, 

Degrees 

0–4.6 

(0–15) 

SW-SC–Fill 

material 20 (125) 6 (125) 35 

4.6–6.7 

(15–22) 

OL–Organic 

clay with peat 14.9 (93) 1 (21) 22 

6.7–12.2 

(22–40) 

SM–Fine sand 

with silt 20(125) 6 (125) 37 

12.2–23.2 

(40–76) 

CL–Silt and 

clay 19.4 (121) 1 (21) 23 

23.2–25.9 

(76–85) 

Weathered 

shale 

– – – 

25.9–29.7 

(85–97.5) Red shale – – – 

STATIC LOAD TEST PROCEDURE 

Four SLTs were performed on the FRP piles 6 months after they were driven. This procedure 

allowed for dissipation of the excess pore water pressure that was generated in the clay layers 

during pile driving. The SLTs on the selected FRP piles were performed according to ASTM 

D-1143. Figures 47–50 illustrate the settlement-time records obtained during the SLTs. The 

load at each increment was maintained until the measured settlement was less than 0.001 mm 

(0.000039 inch). At each stage, an increment of 10 t (22 kips) were applied until the test ended 

or failure occurred. 

Two loading cycles were performed in the SLTs on Lancaster Composite, Inc., and PPI piles. 

After loading to 100 t (220 kips) at the end of the first cycle, or to the maximum applied load, 

the piles were unloaded to zero load at 25-t (55-kip) increments. 
35

Figure 47. Graph. Lancaster pile—settlement-time relationship. 

Figure 48. Graph. PPI pile—settlement-time relationship. 
36

Figure 49. Graph. SEAPILE pile—settlement-time relationship. 

Figure 50. Graph. American Ecoboard pile—settlement-time relationship. 
37

ENGINEERING ANALYSIS OF STATIC LOAD TEST RESULTS 

Vertical load-bearing capacity of piles depends mainly on site conditions, soil properties, method 

of pile installation, pile dimensions, and pile material properties. Full-scale SLTs (ASTM D1143), analytical methods(26,27,28) based on pile and soil properties obtained from in situ or 

laboratory tests, and dynamic methods(8) based on pile driving dynamics are generally used to 

evaluate the static pile capacity. Testing procedures and design methods to determine the FRP 

composite pile capacities have not yet been established. For the purpose of this study, several 

analysis methods commonly used to design steel and concrete piles have been considered for 

evaluating the maximum load, end bearing, and shaft friction distribution along the FRP 

composite piles. 

Ultimate Capacities of FRP Piles 

The Davisson Offset Limit Load 

The offset limit method proposed by Davisson defines the limit load as the load corresponding to 

the settlement, S, resulting from the superposition of the settlement Sel due to the elastic 

compression of the pile (taken as a free standing column) and the residual plastic settlement Sres

due to relative soil pile shear displacement.(26) The total settlement S can be calculated using the 

empirically derived Davisson’s equation (figure 51). 

Figure 51. Equation. Settlement, S. 

Where: 

Sel = PL/EA. 

Sel = The settlement due to the elastic compression of a free standing pile column. 

D = Pile diameter (m). 

l = Pile length (m). 

A = Pile cross section area (m2

). 

e = Pile Young’s modulus (kPa). 

P = Load (kN). 

The limit load calculated according to this method is not necessarily the ultimate load. The offset 

limit load is reached at a certain toe movement, taking into account the stiffness, length, and 

diameter of the pile. The assumption that the elastic compression of the pile corresponds to that 

of a free-standing pile column ignores the load transfer along the pile and can, therefore, lead to 

overestimating the pile head settlement, particularly for a friction pile. Because of the load 

transfer along the pile, the settlement corresponding to the elastic compression generally does not 

exceed 50 percent of the elastic settlement for a free-standing column. 

 
38

Pile data used to calculate the ultimate capacities according to the Davisson offset limit load 

method for FRP piles are summarized in table 6. The elastic properties of pile materials were 

determined by laboratory compression tests on full-section composite samples. 

Figures 52–55 show the load versus pile head settlement curves measured during the SLTs. As 

illustrated in these figures, Davisson’s limit method is used to establish the limit loads under the 

following assumptions: 

• The settlement due to the elastic compression corresponds to the settlement of an elastic 

free-standing pile column and can, therefore, be calculated from the equation in figure 51. 

• The settlement due to the elastic compression corresponds to the elastic compression of a 

friction pile, assuming a constant load transfer rate along the pile, and can, therefore, be 

estimated as 50 percent of the elastic compression of a free-standing pile column. 

• The settlement due to the elastic compression corresponds to the pile load movement 

during an unloading-reloading cycle, and the equivalent pile elastic modulus can, 

therefore, be derived directly from the slope of the load-settlement curve obtained for the 

unloading-reloading cycle. This procedure incorporates the field conditions and the effect 

of the load transfer along the pile. 

Comparing the results obtained with the Davisson’s procedure for these three assumptions and 

load-settlement curves obtained from the static loading tests, it can be concluded that for the 

highly compressible FRP piles, the assumption that the pile elastic compression is equivalent to 

that of a free-standing column leads to greatly overestimating the pile head settlement. The 

assumption that an equivalent elastic modulus of the composite pile can be derived from the 

slope of the load-settlement curve that is defined from an unloading-reloading cycle leads to 

settlement estimates that appear to be quite consistent with the experimental results. Further, 

these settlements are also quite close to the settlements, due to the elastic compression calculated 

for the friction piles, assuming a constant load transfer rate along the pile (i.e., about 50 percent 

of the elastic compression of a free-standing pile column). For the American Ecoboard pile that 

was driven to the sand layer, the load-settlement curve did not reach any plunging failure. For 

this pile, the Davisson’s procedure that considers the equivalent elastic modulus derived from the 

unloading-reloading load-settlement curve yields estimates of the limit load (about 50 t) and pile 

head settlement (about 6 cm (2.4 inches)), which appear to be consistent with the experimental 

results. 
39

Figure 52. Graph. Lancaster Composite pile—Davisson criteria and 

measured load-settlement curve. 

Figure 53. Graph. PPI pile—Davisson criteria and measured load-settlement curve. 
40

Figure 54. Graph. SEAPILE pile—Davisson criteria and measured load-settlement curve. 

Figure 55. Graph. American Ecoboard pile—Davisson criteria and 

measured load-settlement curve. 
41

DeBeer Yield Load 

The DeBeer ultimate capacity is determined by plotting the load-settlement curve on a log-log 

scale diagram.(27) As illustrated in figure 56, on the bilinear plot of the load versus settlement, the 

point of intersection (i.e., change in slope) corresponds to a change in the response of the pile to 

the applied load before and after the ultimate load has been reached. The load corresponding to 

this intersection point is defined by DeBeer as the yield load. Figure 56 shows the DeBeer 

criteria as plotted for the FRP piles. The DeBeer yield loads are 110 t (242 kips) for the 

Lancaster Composite, Inc., and PPI piles and 80 t (176 kips) for the SEAPILE pile. The yield 

load for the American Ecoboard pile could not be defined. This pile did not experience a 

plunging failure during the SLT. 

Figure 56. Graph. DeBeer criterion plotted for FRP piles. 

Chin-Kondner Method 

Chin(28) proposed an application of the Kondner(29) method to determine the failure load. As 

illustrated in figure 57, the pile top settlement is plotted versus the settlement divided by the 

applied load, yielding an approximately straight line on a linear scale diagram. The inverse of the 

slope of this line is defined as the Chin-Kondner failure load. The application of the ChinKondner method for the engineering analysis of the SLTs conducted on the FRP piles, illustrated 

in figure 57, yields the ultimate loads of 161 t (354 kips), 96 t (211 kips), and 125 t (275 kips) for 

the Lancaster Composite, Inc., SEAPILE, and PPI piles, respectively. 
42

Figure 57. Chin-Kondner method plotted for FRP piles. 

Applying the Chin-Kondner method for the American Ecoboard pile, illustrated in figure 58, 

yields a failure load of approximately 102 t (224 kips). 

Figure 58. Chin-Kondner method plotted for American Ecoboard pile. 
43

The application of the Chin-Kondner method yields a failure load that is defined as the 

asymptotic ultimate load of the load-settlement curve. It therefore yields an upper limit for the 

failure load leading in practice to overestimating the ultimate load. 

However, if a distinct plunging ultimate load is not obtained in the test, the pile capacity or 

ultimate load is determined by considering a specific pile head movement, usually 2 to 

10 percent of the diameter of the pile, or a given displacement, often 3.81 cm (1.5 inches). As 

indicated by Felenius, such definitions do not take into account the elastic shortening of the pile, 

which can be substantial for composite long piles such as FRP piles.(30) 

Table 6. Comparison of measured and calculated ultimate loads. 

Static Load 

Test Results 

Davisson 

Offset Limit 

Load 

DeBeer Yield 

Load 

ChinKondner

Davisson Criteria 

Un/reloading 

Slope Pile 

Manufacturer 

Load 

t 

Settlement 

cm 

Load

t 

Settlement

cm 

Load

t 

Settlement 

cm 

Load

t 

Settlement 

cm 

Load 

t 

Settlement

cm 

Lancaster 

Composite1 128 1.73 122 1.31 110 0.96 161 – 119 1.3 

PPI 115 1.64 121 3.40 110 0.96 125 – 114 1.5 

SEAPILE 90 1.16 92 4.60 80 0.98 96 – 85 1.8 

American 

Ecoboard1 60 9.34 – – – – 102 – 50 6.0 

1

 The pile did not experience plunging failure during the static load test. 1 cm = 0.39 inch; 1 t = 2.2 kips 

Table 6 summarizes the maximum loads applied during the SLTs and the ultimate loads 

calculated using the Davisson offset limit load, DeBeer yield load, and the Chin-Kondner 

methods. Distinct plunging failure occurred during the SLTs on PPI and SEAPILE piles as the 

applied loads reached 115 and 90 t (253 and 198 kips) and the measured pile top settlements 

were 1.64 cm (0.65 inch) and 1.16 cm (0.46 inch), respectively. The Lancaster Composite, Inc., 

pile did not experience a distinct plunging failure, and as the maximum load applied on the pile 

reached 128 t (282 kips), the measured settlement was 1.73 cm (0.68 inch). The maximum load 

applied on the American Ecoboard pile was 60 t (132 kips). At this load, the pile top settlement 

was 9.34 cm (3.7 inches), and no distinct plunging was observed. The pile top settlements of this 

pile, which contains only recycled plastic with no reinforcement bars, were significantly greater 

than the settlements measured during the tests on the other FRP piles (figures 52 through 57). 

The Davisson’s offset limit is intended primarily to analyze test results from driven piles tested 

according to quick testing methods (ASTM D-1143) and, as indicated by Fellenius, it has gained 

widespread use with the increasing popularity of wave equation analysis of driven piles and 

dynamic testing.(31) This method allows the engineer, when proof testing a pile for a certain 
44

allowable load, to determine in advance the maximum allowable movement for this load 

considering the length and dimensions of the pile.(32) The application of the Davisson’s offset 

limit load method illustrates that for the compressible FRP piles, the effect of load transfer along 

the piles must be considered in estimating the maximum allowable pile movement. The 

equivalent elastic modulus of the composite pile derived from the measured pile response to the 

unloading-reloading cycle leads to settlement estimates that are quite consistent with the 

experimental results. 

The DeBeer yield load method was proposed mainly for slow tests (ASTM D-1143).(33) In 

general, the loads calculated by this method are considered to be conservative. The calculated 

loads for the Lancaster Composite, Inc., and PPI piles (110 t (242 kips)), and for the SEAPILE 

pile, (80 t (176 kips)), are smaller than the maximum load applied on these piles by 14 percent, 

4 percent, and 11 percent, respectively. For these calculated loads, the calculated settlements for 

the Lancaster Composite, Inc., PPI, and SEAPILE piles are smaller than the measured 

settlements by 44.5 percent, 41.5 percent, and 15.5 percent, respectively. 

The Chin-Kondner method is applicable for both quick and slow tests, provided constant time 

increments are used. As indicated by Fellenius,(30) during a static loading test, the Chin-Kondner 

method can be used to identify local low resistant zones in the pile, which can be of particular 

interest for the composite FRP piles. 

As an approximate rule, the Chin-Kondner failure load is about 20 to 40 percent greater than the 

Davisson limit.(30) As shown in table 6, all the loads calculated by the Chin-Kondner method 

were greater than the maximum load applied on the piles in the field test. The calculated loads 

for the Lancaster Composite, Inc., PPI, SEAPILE, and American Ecoboard piles, 161, 125, 96, 

and 102 t (354, 275, 211, and 224 kips), are greater than the Davisson offset limit load by 

32 percent, 3 percent, 4 percent, and 30 percent, respectively. 

In general, the ultimate loads calculated for FRP piles using the Chin-Kondner method are 

greater than the maximum loads applied in the field test. The DeBeer yield load method yields 

conservative loads in comparison to the maximum loads obtained at the SLTs. The Davisson 

offset limit load method, using the equivalent elastic modulus obtained from the pile response to 

an unloading-reloading cycle, yields ultimate loads that are within the range of loads obtained 

with the above-mentioned methods and allowable settlements that are close to the settlements 

reached with maximum loads applied in the field tests. 

Analysis of the End Bearing and Shaft Friction Distribution 

Axial load-bearing capacity, end bearing, and shaft friction along the pile can be evaluated using 

analytical methods based on pile and soil properties obtained from in situ and/or laboratory tests. 

For the purpose of the SLT engineering analysis, the experimental results were compared with 

several commonly used methods, including the “alpha” total stress analysis and the “beta” 

effective stress analysis. A state-of-the-art review on foundations and retaining structures 

presented by Poulos, et al., (briefly summarized below) relates to axial capacity of piles.(34) An 

estimate of a pile’s ultimate axial load-bearing capacity can be obtained by the superposition of 

the ultimate shaft capacity, Psu, and the ultimate end bearing capacity, Pbu. The weight, Wp, of the 
45

pile is subtracted for the compressive ultimate load capacity. For compression, then, the ultimate 

load capacity, Puc, is given by the equation in figure 5. 

Figure 59. Equation. Ultimate load capacity Puc. 

Where: 

fs = Ultimate shaft friction resistance in compression (over the entire embedded length of the 

pile shaft). 

C = Pile perimeter. 

dz = Length of the pile in a specific soil layer or sublayer. 

fb = Ultimate base pressure in compression. 

Ab = Cross-sectional area of the pile base. 

Alpha Method—Total Stress Analyses for Pile Capacity Evaluation in Clay Soils 

Commonly, the alpha method is used to estimate the ultimate shaft friction in compression, fs, for 

piles in clay soils. The equation in figure 60 relates the undrained shear strength su to fs. 

Figure 60. Equation. Ultimate shaft friction in compression fs. 

Where: 

α = Adhesion factor. 

The ultimate end bearing resistance fb is given by the equation in figure 61. 

Figure 61. Equation. Ultimate end bearing resistance fb. 

Where: 

Nc = Bearing capacity factor. 

Several common approaches used for estimating the adhesion factor α for driven piles are 

summarized in table 7. Other recommendations for estimating α, for example, in Europe, are 

given by De Cock.(35) For pile length exceeding about three to four times the diameter, the 

bearing capacity factor Nc is commonly taken as 9. 

A key difficulty in applying the total stress analysis is estimating the undrained shear strength su. 

It is now common to estimate su from in situ tests such as field vane tests or cone penetration 

tests, rather than estimating it from laboratory unconfined compression test data. Values for su
46

can vary considerably, depending on the test type and the method of interpretation. Therefore, it 

is recommended that local correlations be developed for α in relation to a defined method of 

measuring su.

(34) 

Table 7. Total stress analysis approaches for estimating fs.

(34) 

Pile Type Remarks Reference 

Driven 

α = 1.0 (su ≤ 25 kPa) 

α = 0.5 (su ≥ 70 kPa) 

Interpolate linearly between 

API(6)

Driven 

α = 1.0 (su ≤ 25 kPa) 

α = 0.5 (su ≥ 70 kPa) 

Linear variation between 

length factor applies for L/d ≥ 50 

Semple and Rigden(36)

Driven 

α = (su/σ′v) 0.5 (su/σ′v) –0.5 

for (su/σv’) ≤ 1 

α = (su/σ′v) 0.5 (su/σ′v) –0.25 

for (su/σv’) ≥ 1 

Fleming, et al.(37)

Beta Method—Effective Stress Analysis 

For any soil type, the beta effective stress analysis may be used for predicting pile capacity. The 

relationship between fs and the in situ effective stresses may be expressed by the equation in 

figure 62. 

Figure 62. Equation. Relationship between fs and in situ stresses. 

Where: 

Ks = Lateral stress coefficient. 

δ = Pile-soil friction angle. 

σ′v = Effective vertical stress at the level of point under consideration.

A number of researchers have developed methods of estimating the lateral stress coefficient Ks. 

Table 8 summarizes several approaches, including the commonly used approaches of Burland(38)

and Meyerhof.(4) 
47

Table 8. Effective stress analysis approaches for estimating ultimate shaft friction. 

Pile Type Soil Type Details Reference 

Driven 

and Bored Sand 

Ks = A + BN

where N = SPT (standard penetration test) value 

A = 0.9 (displacement piles) 

Or 0.5 (nondisplacement piles) 

B = 0.02 for all pile types 

Go and Olsen(39) 

Driven 

and Bored Sand 

Ks = Ko (Ks/Ko) 

where Ko= at-rest pressure coefficient 

(Ks/Ko) depends on installation method; range is 0.5 for jetted piles to up 

to 2 for driven piles 

Stas and 

Kulhawy(40)

Driven Clay 

Ks = (1 – sin φ′) (OCR) 0.5

where φ′ = effective angle of friction 

OCR = overconsolidation ratio; 

Also, δ = φ′

Burland(38)

Meyerhof(4)

Correlations With Standard Penetration Test (SPT) Data 

SPT test result data imply the empirical correlations expressed in figures 63 and 64. 

Figure 63. Equation. Empirical correlations for shaft friction. 

Where: 

AN and BN = Empirical coefficients. 

N = SPT blow count number at the point under consideration. 

Figure 64. Equation. Empirical correlation for end bearing resistance. 

Where: 

CN = Empirical factor. 

Nb = Average SPT blow count within the effective depth of influence below the pile base 

(typically one to three pile base diameters). 

Meyerhof recommended using AN = 0, BN = 2 for displacement piles and 1 for small 

displacement piles, and CN = 0.3 for driven piles in sand.(41) Limiting values of fs of about 

100 kPa (14.5 psi) were recommended for displacement piles and 50 kPa (7.25 psi) for small 

displacement piles. Poulos summarized other correlations, which include several soil types for 

both bored and driven piles.(42) Decourt’s recommendations included correlations between fs and 

SPT, which take into account both the soil type and the methods of installation.(43) For 

displacement piles, AN = 10 and BN = 2.8, and for nondisplacement piles, AN = 5–6 and 

BN = 1.4–1.7. Table 9 shows the values of CN for estimating the end bearings. Rollins et al. 
48

discussed the correlations for piles in gravel.(44) The correlations with SPT must be treated with 

caution, as they are inevitably approximate, and are not universally applicable.(34)

Table 9. Factor CN for base resistance.(43)

Soil Type Displacement Piles Nondisplacement Piles 

Sand 0.325 0.165 

Sandy silt 0.205 0.115 

Clayey silt 0.165 0.100 

Clay 0.100 0.080 

Comparison Between Various Analysis Methods and Test Results 

SLT results were compared with several design codes(5-7) and methods commonly used(4) for 

evaluating the end bearing and shaft friction along the pile. Figures 65, 66, and 67 show the loads 

versus depth measured in the SLTs for PPI, SEAPILE, and Lancaster Composite, Inc., piles, 

respectively. 
49

Figure 65. Graph. PPI pile, measured loads versus depth. 
50

Figure 66. Graph. SEAPILE pile, measured loads versus depth. 

Figure 67. Graph. Lancaster Composite, Inc., pile, measured loads versus depth. 
51

As illustrated in these figures, the axial force distribution measured along the piles indicates that 

the FRP piles at the Port Elizabeth site were frictional piles, with maximum end bearing 

capacities, which were equal to 1.6 percent, 1.0 percent, and 8.4 percent of the total applied load 

for the PPI, SEAPILE, and the Lancaster Composite, Inc. piles, respectively. 

Table 10. Comparison between SLT results and several analysis methods and design codes. 

Static Load Test Results 

PPI SEAPILE Lancaster 

Composite 

Go 

and 

Olsen(39)

Stas 

and 

Kulhawy(40) 

FHWA

(5)

API 

(6)

Meyerhof 

(4)

AASHTO

(7)

End 

bearing 

capacity, t 

1.9 0.9 10.8 – – 73.5 6.6 75.4 – 

Total shaft 

friction, t 113.1 89.1 117.7 11.3 7.33 49.7 81 4.3 16.4 

Shaft 

friction 

depth 0 to 

10 m, t/m2

 6.0 5.6 – 2.9 2.0 2.4 4.0 4.5 7.3 

Shaft 

friction 

depth 10 to 

20 m, t/m2

 3.15 1.65 – 9.1 5.8 2.6 8.8 6.8 1.2 

Shaft 

friction 

depth 0 to 

20 m, t/m2

 4.5 3.5 4.7 4.5 2.6 4 6.4 5.6 4.2 

Total 

capacity, t 115 90 128.5 – – 123.2 87.6 79.7 – 

End 

bearing 

capacity 

Total load, 

% 

1.6 1.0 8.4 – – 59.7 7.5 94.6 – 

1 t = 2.2 kips; 1 m = 3.28 ft 

Table 10 summarizes the comparison of the average measured shaft friction along the FRP piles 

and the shaft friction values calculated from several design codes and methods of analysis 

commonly used. The engineering analysis of the shaft friction distribution takes into 

consideration two soil layers corresponding to the upper soil layers from 0 to 10 m (32.8 ft), 

which consists of fill material, a soft clay layer, and a sandy layer, and the lower soil layer from 

10 to 20 m (32.8 to 65.6 ft), which consists of a 3-m (9.8-ft) sand layer overlaying a 7-m (23-ft) 

clay and silt layer. Average values of the shaft friction also are obtained, taking into account the 

axial force measured at the top and the bottom of the soil. For the PPI and SEAPILE piles, the 

average shaft friction values obtained at the upper 10-m (32.8-ft) layer were greater than the 

shaft friction values measured at the lower clay layer. The Burland and Meyerhof methods and 

the AASHTO code appear to yield the best correlation for shaft friction values in the upper layer. 

The FHWA code appears to yield the best correlation for the shaft friction values obtained in the 

lower clay and silt layer. 

53

CHAPTER 4. EVALUATION OF FRP COMPOSITE PILING CAPACITY 

USING WAVE EQUATION ANALYSIS 

The engineering use of FRP piles on a widespread basis presently raises the need to establish 

testing procedures to determine the dynamic properties of FRP composite materials. In 

particular, PDA tests and full-scale loading tests on FRP piles must be conducted to establish site 

correlations for evaluating the use of wave equation analysis in predicting static piling capacity 

and evaluating pile drivability and integrity. The main objective of the study described in this 

chapter was to conduct a full-scale experiment including dynamic and static load tests on FRP 

piles to address these engineering needs. 

PDA and PIT test results were analyzed using CAPWAP(1,45-46) and GRLWEAP(47) programs to 

establish the dynamic properties of the FRP piles. The PDA was also used to evaluate the 

feasibility of installing FRP piles using standard pile driving equipment. Pile-bearing capacities 

were assessed using the CAPWAP program with the dynamic data measured by the PDA, and 

the calculated pile capacities were compared to the results of SLTs performed on the four FRP 

piles. 

TESTING PROGRAM AND SITE OBSERVATIONS 

Test Piles and Dynamic Instrumentation 

The test piles were driven with an ICE 70 single-acting, variable stroke hydraulic hammer. 

According to the manufacturer’s literature, this hammer has a 31.1-kN (7.0-kip) ram with a 

maximum rated energy of 28.5 kJ (21 ft-kips) at a maximum stroke of 91.4 cm (3 ft). This 

hammer has a pump-controlled stroke, and during the initial driving and restrike, the stroke was 

varied from 30.5 to 91.4 cm (1 to 3 ft). Plywood cushions were used to protect the pile top. In 

most cases during driving and restriking, the cushion was nominally 24.8 cm (9.75 inches) thick. 

15.2-cm- (6-inch-) thick cushions were used to drive the Lancaster piles. 

Dynamic measurements of strain and acceleration were taken 91.4 cm to 1.5 m (3 to 5 ft) below 

the head of the test piles. During initial drive, two strain transducers and two accelerometers 

were bolted at opposite sides of the pile to monitor strain and acceleration. During redrive, four 

strain transducers and four accelerometers were attached to the pile. Strain and acceleration 

signals were conditioned and converted to forces and velocities by the PDA. During driving, the 

PDA calculated values of the maximum transferred hammer energy and the maximum 

compression stress at the gauge location, and an estimate of the pile capacity by the Case 

Method.(8) These results were displayed on the PDA monitor for every blow. 

Force and velocity records from the PDA were also displayed on a graphic liquid crystal display 

(LCD) screen to evaluate data quality, soil resistance distribution, and pile integrity. The force 

and velocity records were also digitally stored on a disc for subsequent laboratory analysis. 
54

Site Observations on Drivability and Durability During Pile Installation 

Figures 68 through 71 show the FRP piles before and after driving. Site observations on 

drivability and durability during installation of the test piles are summarized below. 

The steel pile was driven with the lowest blow counts (36 blows/m (11 blows/ft)) and the highest 

transfer energy of 23.1 kJ (17 ft-kips); this pile, however, was driven without a plywood pile 

cushion and with a helmet specifically designed for steel pipe piles. Adding 15.2 to 24.8 cm (6 to 

9.75 inches) of pile cushion may explain some of the energy losses and increased blow counts 

observed with the other piles. 

Among the FRP piles, the SEAPILE pile was driven with the lowest blow counts (62 to 

118 blows/m (19 to 36 blows/ft)), and showed relatively high transfer energies of 13.6 to 17.6 kJ 

(10 to 13 ft-kips) at the end of driving. The pile cushion for these piles showed very little 

degradation after driving. 

The PPI piles were driven with blow counts ranging between 1 to 1.4 blows/mm (27 to 

37 blows/inch), and transfer energies at the gauge location of 8.1 to 10.8 kJ (6 to 8 ft-kips), 

significantly lower than those obtained for the SEAPILE and Lancaster Composite, Inc., piles. 

After driving, it was observed that the PPI piles experienced severe pile top degradation; the 

plastic matrix covering the rebar had melted and some pieces of the steel rebar had sheared off. 

The heat and pile top damage could have been the reason for the losses in the transfer energy. 

After driving, the upper foot of each PPI pile was removed, and no damage or separation 

between bars and the plastic material was observed. 

Figure 68. Photo. PPI pile. 
55

Figure 69. Photo. SEAPILE pile. 

Figure 70. Photo. Lancaster Composite, Inc., pile. 

Figure 71. Photo. American Ecoboard pile. 

At the end of installation (EOI), the Lancaster Composite, Inc., piles had blow counts ranging 

from 154 to 305 blows/m (47 to 93 blows/ft), with calculated transfer energy values of 12.2 to 

17.6 kJ (9 to 13 ft-kips) at the end of drive. It should be noted that to reduce the estimated 

tension stresses, the Lancaster Composite, Inc., SLT pile was driven with a reduced stroke of 
56

61.0 cm (2 ft) for the last few feet of penetration; this driving process resulted in a higher blow 

count of 305.1 blows/m (93 blows/ft). By the EOI of these piles, it was observed that the pile 

cushion had experienced initial burning. 

The Lancaster Composite, Inc., pile that was driven to refusal (almost 0.4 blows/mm (10 blows/ 

inch)) into the weathered shale at the depth of 26.5 m (87 ft) below grade was spliced. Driving 

resistance ranged from 164 to 299 blows/m (50 to 91 blows/ft) for a penetration of 18.8 to 

26.5 m (60 to 87 ft). 

The American Ecoboard piles were driven into the upper sand layer at approximately 9.1 m 

(30 ft) below the ground surface. They were spliced at the depth of 6 m (19.7 ft). After splicing, 

the American Ecoboard dynamic testing results evidenced a toe signal, which indicated that the 

stress wave propagated through the splice. A significant impedance increase followed by a sharp 

decrease was observed at the splice location. This wave reflection is likely to be the result of the 

additional steel bolts and sleeve, as well as of a slight gap between the two sections. 

DYNAMIC PROPERTIES OF THE FRP COMPOSITE PILE MATERIALS 

Methods for Evaluating Dynamic Properties 

Before dynamic testing, several material properties at the gauge location have to be determined. 

These data include the pile length, cross-sectional area, specific weight, wave speed, and elastic 

modulus. Testing procedures to establish the dynamic elastic modulus of FRP composite piles 

have not yet been established or evaluated. For the purpose of this study, the dynamic testing 

procedures commonly used for steel and concrete piles have been followed to evaluate the 

dynamic properties of the composite piles, including: 

• PIT—low-strain integrity testing (ASTM D5882).(48)

• PDA testing—high strain (ASTM D4945).(49)

• PDA testing—considering the initial portion of the high-strain records 

(ASTM D4945).(49)

These testing procedures are briefly summarized below. 

Pile Integrity Tester (PIT) 

Before initial installation, low-strain integrity testing was performed with a PIT. The PIT testing 

(ASTM D5882) consists of attaching a small accelerometer to the pile top sample and using a 

small handheld hammer to lightly impact the pile sample top. This impact induces a low-strain 

stress wave in the pile, which travels to the pile toe and reflects back toward the pile top. With 

the length of the pile sample known, a wave speed, c, can be calculated from the time measured 

between the impact and the toe response. The wave speed can, in turn, be used with a known 

material density, ρ, to calculate the low-strain dynamic modulus using the relationship in 

figure 72. 
57

Figure 72. Equation. Dynamic modulus E. 

PDA Testing—High Strain 

PDA testing yields wave speed following a procedure similar to that used for low-strain PIT 

tests. For a specified length of the pile, the time between impact and toe response can be used to 

estimate an overall wave speed. The dynamic modulus, E, can be determined using the equation 

in figure 72. 

PDA Testing—Initial Portion of the High-Strain Records 

This method involves the initial portion of the high-strain records and the expected 

proportionality between measured strain and wave speed. Assuming that for low strains, the FRP 

piles behave as a linear elastic material and no changes in resistance or pile properties occur 

during the pile installation for the initial part of the high-strain record, this assumption implies 

that the wave speed is directly proportional to the measured strain and can, therefore, be 

calculated from the equation in figure 73. 

Figure 73. Equation. Pile particle speed v. 

Where: 

v = Pile particle speed calculated from the acceleration record. 

ε = Measured strain.

c = Material wave speed. 

The dynamic modulus, E, then can be determined using the equation in figure 72. 

Dynamic Properties—Test Results 

PIT tests were performed before driving on American Ecoboard, Lancaster Composite, Inc., PPI, 

and SEAPILE full-scale piles. For the FRP piles, the foam (i.e., outer layer) is stiffer than the 

core (i.e., inner layer). Therefore, for the American Ecoboard piles, the PIT tests were conducted 

on the outer layer, and for the SEAPILE and PPI piles, tests were conducted on the fiberglass 

bars and the reinforced foam, respectively. 

 

In a linearly elastic pile material, such as steel or concrete, the wave speed determined from the 

proportionality condition (figure 73) is equal to the overall wave speed calculated from the PDA 

results. In the case of FRP piles, however, the overall wave speeds obtained with PIT, cl, PDA, 

ch, and the proportional wave speed, cp, values were consistently different. Table 11 summarizes 

these results. 
58

Table 11. Elastic modulus of FRP piles estimated from PIT and PDA tests. 

Specific 

Weight1 Low Strain 

(PIT) 

High Strain 

(PDA)

Pile Type

kN/m3

 (lb/ft3) 

Measured 

Wave Speed 

(Test I) 

cl , m/s 

(ft/s) 

Estimated 

Elastic 

Mod.El, kPa 

(ksi) 

Proportional

Wave Speed

(Test II) 

cp, m/s 

(ft/s) 

Estimated 

Elastic Mod. 

Ep, kPa (ksi) 

Overall 

Wave Speed

(Test III) 

ch, m/s 

(ft/s) 

Estimated 

Elastic Mod.

Eh, kPa 

(ksi) 

American 

Ecoboard 

7.9 

(50) 

1524 

(5000) 

1.86 x 106 

(270) 

1828.8 

(6000)

2.68 x 106 

(388) 

1371.6 

(4500)

1.50 x 106 

(218)

Lancaster 

Composite 

22.0 

(140) 

4114.8 

(13500) 

37.94 x 106 

(5503) 

4175.76 

(13700) 

39.07 x 106 

(5667) 

4023.36 

(13200) 

36.27 x 106 

(5261)

PPI 8.0 

(51) 

3505.2 

(11500) 

10.03 x 106 

(1455) 

3810 

(12500) 

11.85 x 106 

(1719) 

3322.32 

(10900) 

9.01 x 106 

(1307)

SEAPILE 8.5 

(54) 

2743.2 

(9000) 

6.50 x 106 

(943) 

3048 

(10000) 

8.03 x 106 

(1165) 

2529.84 

(8300) 

5.53 x 106 

(802) 1

 Reported by the manufacturers. 

DYNAMIC PILE TESTING DATA ANALYSIS 

The PDA data analysis was conducted to obtain the dynamic properties, including Smith 

damping and soil quake, and evaluate the load-set response and the static bearing capacity of the 

FRP piles. The dynamic analysis involved: 

• Case method analysis (PDA)—for onsite measurement of the transfer energy and 

evaluation of the driving system performance, pile stresses, structural integrity, and static 

capacity. 

• CAPWAP analysis—using the force and velocity data recorded in the field to obtain the 

dynamic properties, including Smith damping and soil quake, and to evaluate the load-set 

pile response. 

• GRLWEAP—using CAPWAP analysis results to evaluate the dynamic Young’s modulus 

of each FRP composite pile. 

The dynamic analysis results were compared to the results of SLTs performed on four FRP piles. 

Case Method Analysis (PDA) 

During the PDA testing, the Case Method is used to interpret the measured dynamic data to 

assess the hammer and driving system performance, determine pile head compression stresses, 

evaluate structural integrity, and estimate the static pile capacity. Table 12 summarizes the PDA 

dynamic test results, including the maximum measured pile top strain, maximum compressive 

stress, maximum estimated tensile stress, transfer energy to pile, hammer operating rate, and the 

pile capacities calculated by the case method and by CAPWAP analysis. The maximum 

compressive and tensile stresses were calculated using the dynamic elastic modulus obtained 
59

with the PDA testing, considering the wave speed-strain proportionality condition of the initial 

portion of the high-strain records. 

CAPWAP Analysis 

The CAPWAP Version 2000-1 was used with the data recorded in the field to evaluate the 

ultimate pile capacity of the tested piles. CAPWAP analyses compute soil resistance forces and 

their approximate distribution using the velocity and speed data recorded in the field during the 

dynamic pile testing. The CAPWAP results include an evaluation of the soil resistance 

distribution, soil quake and damping factors, and a simulated static load-set graph. The static 

load-set simulation is based upon the CAPWAP calculated static resistance parameters and the 

elastic compression characteristics of the pile. For each pile, the three elastic modulus values 

calculated from PDA and PIT tests as reported in table 11 were considered. The CAPWAP 

analyses using the elastic modules calculated from PDA testing and considering the initial 

portion of the high-strain records (1) yielded the most reasonable energy values for the ICE 70 

hammer and (2) predicted load-set curves that yielded the best correlation with SLT results. 

CAPWAP static capacities calculated using the dynamic elastic modulus values are reported in 

table 13. 
60

Table 12. PDA results. 

Pile 

Designation 

Test 

Date 

Pile 

Penetration,1

m (ft) 

Blow 

Count,1

blows/m 

(blows/ft) 

Average 

Maximum

Measured

Strain 

(με) 

Average 

Maximum 

Compressive

Stress,2

kPa (ksi) 

Maximum

Estimate 

Tensile 

Stress,3

kPa (ksi) 

Average 

Transfer

Energy,2

kJ 

(ft-kips) 

Hammer 

Rate, 

blows/ 

min 

Case 

Method 

Capacity,4

kN (kips)

CAPWAP

Estimated

Capacity, 

kips

Remarks5

11/14/01 18.9 (62) 36.1 (11) 714 147,547.8 

(21.4) NA 23.1(17) 47 413.7 (93) 489.3 (110) Steel EOI 

Reference 5/24/02 18.6 (61) 0.31 b/mm 

(8/inch) 499 103,421.4 (15.0 ) NA 13.6 (10) 45 1267.7 (285) 

1156.5 

(260) BOR 

Ecoboard 5/22/02 8.8 (29) 98.4 (30) 1540 4,136.9 (0.6) 689.5 (0.1) 12.2 (9) 76 435.9 (98) 382.5 (86)

EOI, 

Section 2; 

9.75-inch 

cushion 

11/14/01 17.7 (58) 210.0 (64) 392 151,684.7 (2.2) 3447.5 (0.5) 14.9 (11) 50 186.8 (42) – 

ID, Section 

1; 9.75-inch 

cushion 

11/14/01 26.5 (87) 298.6 (91) 523 199,948.0 (2.9) 5516 (0.8) 17.6 (13) 49 1450 (326) – 

EOI, 

Section 2; 

7.5-inch 

cushion 

Lancaster 1 

5/24/02 26.2 (86) 1.5 b/mm 

(77/ 0.5 inch) 448 16,547.4 (2.5) 4137 (0.6) 16.3 (12) 43 2375.2 (534) 

2090.6 

(470) EOR 

11/15/01 17.7 (58) 154.2 (47) 377 14,479.0 (2.1) 5516 (0.8) 12.2 (9) 52 346.9 (78) – EOI; 6-inch 

cushion Lancaster 4 

5/24/02 17.1 (56) 0.96 b/mm 

(25/inch) 324 12411 (1.8) 1379 (0.2) 9.5 (7) 52 1072 (241) – BOR 

11/15/01 17.7 (58) 154.2 (93) 399 15858.5 (2.3) 8274 (1.2) 13.6 (10) 56 480.4 (108) 516 (116)

EOI; 6-inch 

cushion; 2-ft 

stroke Lancaster 3 

(SLT) 

5/24/02 17.1 (56) 1.20 b/mm 

(31 / inch) 309 11721.5 (1.7) 1379 (0.2) 9.5 (7) 44 1049.7 (236) 

1000.8 

(225) BOR 

11/15/01 19.2 (63) 121.4 (37) 436 5171.25 (0.75) 2068.5 (0.3) 9.5 (7) 49 453.7 (102) – EOI; 9.75-

inch cushion PPI 5 

5/23/02 17.7 (58) 0.94 b/mm 

(24 / inch) 514 6205.5 (0.9) 689.5 (0.1) 6.8 (5) 45 756.2 (170) – BOR 

11/15/01 19.05 (62.5) 111.5 (34) 535 6205.5 (0.9) 2068.5 (0.3) 10.9 (8) 48 484.8 (109) – EOI; 9.75-

inch cushion PPI 7 

5/23/02 17.7 (58) 0.82 b/mm 

(21 / inch) 529 6205.5 (0.9) 1379 (0.2) 6.8 (5) 44 791.7 (178) – BOR 

11/15/01 18.6 (61) 88.6 (27) 373 4137 (0.6) 2068.5 (0.3) 8.1 (6) 47 387 (87) 409.2 (92) EOI; 9.75-

inch cushion PPI 6 (SLT) 

5/24/02 17.7 (58) 0.86 b/mm 

(22 / inch) 654 7584.5 (1.1) 1379 (0.2) 10.9 (8) 44 938.5 (211) 978.6 (220) BOR 

11/16/01 18.6 (61) 78.7 (24) 875 6895 (1.0) 2758 (0.4) 13.6 (10) 57 382.5 (86) – EOI; 9.75-

inch cushion Seaward 8 

5/22/02 17.7 (58) 0.86 b/mm 

(22 / inch) 858 6895 (1.0) 1379 (0.2) 12.2 (9) 56 885.2 (199) – BOR 

11/16/01 18.6 (61) 62.3 (19) 855 6895 (1.0) 2758 (0.4) 17.6 (13) 54 516 (116) – EOI; 9.75-

inch cushion Seaward 10 5/22-

23/02 17.7 (58) 0.39 b/mm (10 / inch) 926 7584.5 (1.1) 1379 (0.2) 16.3 (12) 43 978.6 (220) – BOR 

11/16/01 18.7 (61.3) 118.1 (36) 758 6205.5 (0.9) 2758 (0.4) 12.2 (9) 57 516 (116) 520.4 (117) EOI; 9.75-

inch cushion Seaward 9 

(SLT) 5/23/2002 17.7 (58) 0.51 b/mm 

(13 / inch) 1074 8274 (1.2) 1379(0.2) 14.9 (11) 45 978.6 (220) 1067.5 (240)

BOR–

Original 

CW 1

 Pile penetrations from ground surface for initial installation and bottom of excavation for restrike. 1 inch = 2.54 cm, 1 ft = 0.305 m 2

 Measured at the location of the transducers. 

3

 Maximum tension stress estimated over all blows. 

4

 Using RX9 (maximum case method with Jc = 0.90) equation. 5

 Unless otherwise noted, 3-ft hammer stroke. 

BOR—Beginning of redrive. 

EOI—End of installation.
61

CAPWAP analyses were performed on the records obtained for both the EOI and the beginning 

of restrike (BOR) of each SLT pile, the steel reference pile, and the Lancaster Composite, Inc., 

pile (No. 1), which was driven to refusal. The results are summarized in table 13. 

Table 13. CAPWAP program calculation results. 

BOR—Beginning of restrike. 

EOI—End of installation. 

EOR—End of redrive.

CAPWAP 

Pile Static Capacity Estimate Smith Damping Soil Quakes

Designation 

Penetration 

Depth in 

Soil, 

m (ft) 

Driving 

Resistance, 

blows/set Shaft, 

kN (kips) 

Toe, 

kN (kips) 

Total, 

kN (kips) 

Shaft, 

s/m (s/ft) 

Toe, 

s/m (s/ft) 

Shaft, 

mm (inch) 

Toe, 

mm (inch) 

Remarks

18.9 

(62) 11/ft 369.2 

(83) 

120.1 

(27) 

489.3 

(110) 

0.26 

(0.08) 

0.16 

(0.05) 

2.8 

(0.11) 

26.9 

(1.06) EOI Steel Pile 

18.3 

(60) 8/inch 1049.7 (236) 

106.8 

(24) 

1156.5 

(260) 

0.69 

(0.21) 

0.66 

(0.2) 

2.3 

(0.09) 

6.9 

(0.27) BOR 

American 

Ecoboard (SLT) 

8.8 

(29) 30/ft 102.3 (23) 

280.2 

(63) 

382.5 

(86) 

0.95 

(0.29) 

1.05 

(0.32) 

2.8 

(0.11) 

16.0 

(0.63) EOI; after 

splicing 

Lancaster 1 25.9 

 (85) 77/0.5 inch 702.8 (158) 

1387.8 

(312) 

2090.6 

(470) 

0.66 

(0.2) 

1.05 

(0.32) 

2.5 

(0.10) 

1.27 

(0.05) EOR; pile 

driven to refusal

17.7 

(58) 93/ft 240.2 (54) 

275.8 

(62) 

516 

(116) 

0.66 

(0.2) 

0.1 

(0.03) 

3.0 

(0.12) 

18.3 

Lancaster 3 (0.72) EOI 

(SLT) 18.3 

(60) 32/inch 836.2 (188) 

164.6 

(37) 

1000.8 

(225) 

1.12 

(0.34) 

1.25 

(0.38) 

2.0 

(0.08) 

3.8 

(0.15) BOR 

18.6 

(61) 27/ft 329.2 

(74) 

80.1 

(18) 

409.2 

(92) 

0.39 

(0.12) 

0.69 

(0.21) 

3.6 

(0.14) 

16 

(0.63) EOI PPI 6 (SLT) 17.7 

(58) 22/inch 818.4 

(184) 

160.1 

(36) 

978.6 

(220) 

0.75 

(0.23) 

1.28 

(0.39) 

3.3 

(0.13) 

3.8 

(0.15) BOR 

18.7 

(61.3) 36/ft 400.3 (90) 

120.1 

(27) 

520.4 

(117) 

0.66 

(0.2) 

0.79 

(0.24) 

2.3 

(0.09) 

15.5 

(0.61) EOI 

17.4 

(57) 130 969.7 (218) 

97.9 

(22) 

1067.5 

(240) 

0.98 

(0.3) 

1.25 

(0.38) 

2.5 

(0.10) 

2.8 

(0.11) BOR 

Seaward 9 

(SLT) 

17.4 

(57) 130 809.5 (182) 

84.5 

(19) 

889.6 

(200) 

0.89 

(0.27) 

1.45 

(0.45) 

2.5 

(0.10) 

2.0 

(0.08) BOR; revised 

with SLT 
62

Table 14 shows the quake and damping values recommended by the consulting firm GRL 

Engineers, Inc., for various types of soils and piles. 

Table 14. Quake and damping values recommended by GRL. 

Comparison of the CAPWAP analysis results reported in table 13 and the GRL 

recommendations summarized in table 14 leads to the following conclusions: 

• The recommended soil shaft quake values appear to be consistent with the CAPWAP 

results obtained at the beginning of restriking for all the piles tested, with the exception of 

the PPI piles. The soil quake values obtained for the PPI piles are about 30 percent 

greater than those recommended by GRL. As soil shaft quake values are generally 

assumed to be independent of the pile type, the values obtained for PPI piles are most 

probably affected by local damages at the pile head observed during the pile driving. 

• For the soil type of the site, submerged soils, recommendations yield soil toe quake 

values of D/60 = 6.85 mm (0.27 inch), where D is the pile diameter. This value agrees 

with the CAPWAP analysis results obtained at the beginning of restriking for the 

reference steel pile. The soil toe quake values obtained for the Lancaster Composite, Inc., 

PPI, and SEAPILE piles are significantly lower (about 45 percent) than the values 

recommended by GRL. 

• Comparing the values of the soil toe quake obtained for all the test piles at the EOI with 

those obtained at the BOR, the dissipation of excess pore water pressure generated during 

pile driving resulted in a significant decrease (up to 80 percent) of the soil toe quake 

values. 

• The Smith shaft damping values recommended by GRL for the soil profile at this test 

site, which consists of cohesive and noncohesive soil layers, are within the range of 0.16 

to 0.65 second (s)/m (0.05 to 0.20 s/ft). These values appear to be consistent with the 

CAPWAP results obtained at the EOI for all the piles tested. It should, however, be noted 

that the Smith shaft damping values obtained at the EOI for the FRP piles are about 250 

percent greater than those obtained for the reference steel pile. 

• For all soil types, GRL recommendations yield a toe damping value of 0.49 s/m 

(0.15 s/ft). This value is consistent with the CAPWAP analysis results obtained at the 

beginning of restriking for the reference steel pile, indicating a toe damping value of 

 Soil Type Pile Type or Size Quake 

mm (in)

Damping 

Factors/m (s/ft)

Shaft Quake All soil types All Types 2.5 (0.10) –

All soil types, soft rock Open ended pipes 2.5 (0.10) –

In dry soils, or in very 

dense or hard soils

Displacement piles of 

diameter D or width D D/120 – Toe Quake

In submerged soils or in 

loose or soft soils

Displacement Piles of 

diameter D or width D D/60 –

Noncohesive soils – – 0.16 (0.05) Shaft 

Damping Cohesive soils – – 0.65 (0.20) 

Toe 

Damping

In all soil types – – 0.50 (0.15) 
63

0.65 s/m (0.2 s/ft). The toe damping values obtained for the American Ecoboard, 

Lancaster Composite, Inc., PPI, and SEAPILE piles are consistently greater (about 250 

percent) than the values recommended by GRL. 

• Comparing the values obtained for all the test piles at EOI with those obtained at the 

BOR, the dissipation of excess pore water pressure generated during pile driving resulted 

in a significant increase (up to about 100 percent) of the shaft and toe Smith damping. 

GRLWEAP Analysis 

To evaluate the dynamic testing procedures used to obtain the dynamic elastic modulus of the 

FRP piles (i.e., PIT, the PDA testing—high-strain record, and the PDA testing considering the 

proportionality condition), a parametric study was conducted using the GRLWEAP Version 

2002-1 program. GRLWEAP computes the number of blows per foot and the energy at the pile 

top. The parametric study consisted of calculating, for each FRP pile, the transfer energy 

obtained by PDA, the static capacity calculated by CAPWAP, and the number of blows per foot 

using the different elastic modulus values obtained with these testing procedures (table 15). The 

calculated transfer energy was compared to the PDA results, and the calculated blow counts were 

compared to the measured number of blows per inch at the beginning of restriking of each pile. 

CAPWAP results, including ultimate pile capacity, soil quake, and toe and shaft damping are 

used as GRLWEAP input data (table 13). 

Table 15. Comparison of GRLWEAP results with measured elastic modulus, 

number of blows, and energy. 

Pile 

Manufacturer 

Measured Elastic Modulus, 

kPa (ksi) 

Measured 

Number of

Blows, 

blows/m 

(blows/ft) 

Calculated 

Number of Blows 

(GRLWEAP), 

kN/m (kips/ft) 

Measured 

Energy 

(PDA), 

kN/m (kips/ft) 

Calculated 

Energy 

(GRLWEAP), 

kN/m (kips/ft) 

PIT—low strain 6.5 x 106

(943) 18427.2 (1263) 145.9 (10) 

PDA—high strain 5.5 x 106

(802) 4537.5 (311) 145.9 (10) SEAPILE 

PDA—early portion 

of high-strain records 

8 x 106

(1165) 

515.1 (157) 

2290.6 (157)

160.5 (11) 

145.9 (10)

PIT—low strain 10 x 106

(1455) 21899.6 (1501) 116.7 (8) 

PDA—high strain 9 x 106

(1307) PPI 7893.2 (541) 116.7 (8) 

PDA—early portion 

of high-strain records

11.9 x 106

(1719)

872.7 (266) 

3881 (266) 

116.7 (8)

116.7 (8)

PIT—low strain 1.9 x 106

(270) 525.2 (36) 189.7 (13) 

PDA - high strain 1.5 x 106

(218) 481.5 (33) 189.7 (13) American 

Ecoboard 

PDA—early portion 

of high-strain records 

2.7 x 106

(388) 

98.4 (30) 

437.7 (30) 

131.3 (9) 

189.7 (43) 
64

The results of this parametric study (figures 74, 75, and 76) illustrate (for the same transfer 

energy level) the effect of the elastic modulus on the number of blows per foot for the American 

Ecoboard, PPI, and SEAPILE piles. The GRLWEAP calculations show that the elastic modulus 

obtained from PDA testing—considering the proportionality condition for the initial portion of 

the high-strain records—yields the best agreement between (1) the calculated values of the 

transfer energy level and the energy measured by PDA and (2) the calculated blow count and the 

measured field records. Figure 74 shows that for the American Ecoboard pile, the effect of 

Young’s modulus on the blow count was relatively small. This may be because Young’s 

modulus values measured by PDA and PIT tests for the American Ecoboard pile were relatively 

small, compared to those obtained for the PPI and SEAPILE pile materials. 

Figure 74. Graph. American Ecoboard pile—blows per foot versus elastic modulus. 
65

Figure 75. Graph. PPI pile—blows per foot versus elastic modulus. 

Figure 76. Graph. SEAPILE pile—blows per foot versus elastic modulus. 
66

In general, materials exhibit stiffer behavior as the rate of loading increases. Thus, when 

comparing dynamic test results with those obtained from static laboratory or field load tests, the 

dynamic modulus are generally greater than the static elastic modulus.

In table 16, the dynamic elastic modulus, calculated from PDA testing—early portion of the 

high-strain records, are compared with static elastic modulus for steel and FRP piles, including 

the PPI and SEAPILE piles, obtained from laboratory compression tests. For steel, which is a 

rigid material compared to the FRP materials, the difference between static and dynamic 

modulus is 3 percent. The ratio of dynamic-to-static elastic modulus for FRP materials is 1.64 

and 2.38 for the PPI plastic with steel bars and SEAPILE plastic with fiberglass bars, 

respectively. 

Table 16. Comparison, static and dynamic elastic modulus of SEAPILE, PPI, and steel piles. 

STATIC LOAD TEST RESULTS—CORRELATIONS WITH CAPWAP ANALYSIS 

Pile Capacity and Setup 

One of the main objectives of the field demonstration project was to evaluate the dynamic 

method currently used for predicting the load-set response and ultimate static capacity of FRP 

piles. To simulate the field conditions as consistently as possible, the static load results were 

used to evaluate the maximum ratio of the toe bearing capacity to the shaft resistance as an input 

data for the CAPWAP analysis. The CAPWAP analysis results for the total static capacity, the 

shaft resistance, and the toe bearing capacity for all the test piles at the EOI and BOR are 

indicated in table 13. 

 

The PDA and CAPWAP analysis results summarized in tables 12 and 13 show that all the piles 

experienced a long-term increase in pile capacity from the EOI to the BOR. The capacity gains 

ranged from 311.4 to 854.1 kN (70 to 192 kips) for the piles installed to 17.7-m (58-ft) 

penetrations. For most of the tested piles, the measured setup factor, defined as a ratio of the 

BOR to EOI capacities, is in excess of 2.0. For all the SLT piles, the CAPWAP capacities ranged 

from 938.6 to 1,267.7 kN (211 to 285 kips) during the BOR and from 186.8 to 556.0 kN (42 to 

125 kips) at the EOI. 

In general, the CAPWAP analysis showed that at the BOR for the piles driven to penetrations of 

17.7 m (58 ft), most of the ultimate capacity was due to the shaft resistance, which ranged from 

827.4 to 1,049.8 kN (184 to 236 kips). The toe resistance ranged from 97.9 to 164.6 kN (22 to 37 

kips). For the American Ecoboard pile, which was driven to a penetration depth of 8.8 m (29 ft), 

the analysis yielded a shaft resistance of 102.3 kN (23 kips) and a toe resistance of 280.2 kN 

Pile 

Manufacturer 

Static Elastic 

Modulus, 

kPa (ksi) 

Dynamic Elastic 

Modulus, 

kPa (ksi) 

Dynamic Elastic Modulus/ 

Static Elastic Modulus 

SEAPILE 3.38 x 106 (490) 8.03 x 106 (1165) 2.38 

PPI 7.22 x 106 (1048) 1.185 x 107 (1719) 1.64 

Steel 1.999 x 108 (29000) 2.068 x 108 (30000) 1.03 
67

(63 kips). The Lancaster Composite, Inc., pile, which was driven to refusal at a depth of 26.5 m 

(87 ft), had a mobilized ultimate capacity of 2,090.7 kN (470 kips) at the end of redrive, with 

1,387.8 kN (312 kips) in toe resistance. 

Correlations of CAPWAP Analysis With Static Load Test Results 

Figures 78 through 81 illustrate the comparison between the load-set curves calculated by the 

CAPWAP program for the BOR and the SLT results for the Lancaster Composite, Inc., PPI, 

SEAPILE, and American Ecoboard piles. Table 17 summarizes the main experimental and 

dynamic analysis results, including the maximum applied or ultimate loads and the 

corresponding pile top settlements. These settlements are also compared with the elastic 

settlements of the piles due to their elastic compression, which are calculated assuming that the 

Lancaster Composite, Inc., PPI, and SEAPILE piles are frictional piles. For frictional piles, 

assuming a constant load transfer rate along the pile, the settlement due to the elastic 

compression, Selastic, can be approximately calculated by the equation in figure 77. 

Figure 77. Equation. Selastic. 

Where: 

Pmax = Maximum applied or ultimate load. 

E = Elastic modulus of the pile. 

A = Pile section area. 

L = Pile length. 

The elastic modulus values indicated in table 17 were obtained from laboratory static 

compression tests conducted on FRP pile samples, taking into consideration the initial 

quasilinear portion of the stress-strain curves. 
68

Table 17. Comparison between CAPWAP analysis and static load test results. 

CAPWAP Static Load Test 

Pile Manufacturer Pmax, 

kN (kips) 

Slimit, 

mm 

(inches) 

Pmax, 

kN 

(kips) 

Slimit, 

mm 

(inches) 

EA, 

kN (kips) 

Selastic, 

mm 

(inches) 

Lancaster Composite 

(end of first loading 

cycle) 

1000 

(224.8) 

5 

(0.2)

1000 

(224.8) 

7.5 

(0.3) 

4.1 x 106 

(0.9 x 106) 

2.3 

(0.1) 

PPI 980 

(220.3)

12 

 (0.5) 

1100 

(247.3) 

9.6 

(0.4) 

0.9 x 106 

(0.2 x 106) 

11 

(0.4) 

SEAPILE 900 

(202.3) 

15 

(0.6)

800 

(179.8) 

9.8 

(0.4) 

0.745 x 106

(1.67 x 105

)

16.8 

(0.7) 

600 

(134.9) 

93.0 

(3.6) American Ecoboard 382 

(85.9) 

30 

(1.2) 400 

(89.9) 

40 

(1.6) 

– – 

The comparison between the CAPWAP analysis and the SLT results for the FRP piles leads to 

the following observations. 

For the Lancaster Composite, Inc., SLT pile, the CAPWAP analysis yields a load-set curve, 

which is quite consistent with the results of the first static load cycle. However, it leads to 

underestimating the total bearing capacity of the pile reached in the second load cycle. The 

difference between the CAPWAP analysis and the SLT results is likely due to the very high blow 

counts observed during the restrike (approximately 1 blow/mm (32 blows/inch)). With such low 

sets, the pile displacement under the dynamic load appears to be too small to fully mobilize the 

soil resistance and to obtain reliable measurements of the velocity and the force waves as input 

data for the CAPWAP analysis. 

Figure 78. Graph. SLT and CAPWAP analysis—Lancaster Composite, Inc., pile. 
69

Figure 79. Graph. SLT and CAPWAP analysis—PPI pile. 

For the PPI SLT pile, the CAPWAP analysis yields an ultimate capacity that corresponds to 

about 90 percent of the plunging failure load observed during the SLT. The predicted limit 

settlement at failure is about 25 percent higher than the measured settlement, indicating that the 

CAPWAP analysis underestimates the stiffness of the pile response to static loading. The 

calculated elastic settlement due to pile compression is quite close to the limit settlement 

calculated by CAPWAP and slightly exceeds the experimental value. This seems to indicate that 

for the PPI pile, due to its low stiffness compared with the Lancaster Composite, Inc., pile, the 

settlement at failure is mainly the result of the elastic compression of the pile during loading. 
70

Figure 80. Graph. SLT and CAPWAP analysis—SEAPILE pile. 

For the SEAPILE SLT pile, the CAPWAP analysis yields an ultimate capacity that corresponds 

to about 112.5 percent of the plunging failure load observed during the SLT. Similar to the PPI 

pile, the predicted limit settlement at failure is higher (about 25 percent under the experimental 

ultimate load) than the experimental settlement, indicating that the CAPWAP analysis 

underestimates the stiffness of the pile response to static loading. Consistent with the results 

obtained for the PPI pile, the calculated elastic settlement due to pile compression is quite close 

to the limit settlement calculated by CAPWAP and exceeds the experimental value. This seems 

to confirm that for the SEAPILE and PPI tested piles, the settlement at failure is mainly the result 

of the elastic compression of the pile during loading. 
71

Figure 81. Graph. SLT and CAPWAP analysis—American Ecoboard pile. 

For the American Ecoboard SLT pile, the CAPWAP analysis showed a good correlation with the 

SLT up to 2 cm (0.8 inch) settlement at a load of 25 t (5.6 kips). CAPWAP analysis to predict 

the behavior of piles during SLTs is limited to settlements that do not exceed approximately 

5 cm (2 inches). This might explain the difference between the SLT behavior and CAPWAP 

prediction curve for the low stiffness American Ecoboard pile, which reached a settlement of 

96 mm (3.8 inches) under the static loading of 600 kN (134.9 kips). In addition, the American 

Ecoboard pile was driven to the sandy layer to assess its behavior primarily as an end bearing 

pile. The experimental results illustrated that no distinct plunging occurred under the applied 

static loading. As indicated in table 13, for this pile, the CAPWAP analysis predicted a ratio of 

end bearing to shaft resistance of 270 percent, which is substantially higher than that predicted 

for all the FRP piles driven to the clay layer at the depth of about 20 m (65.6 ft). 

Pile Tension and Tensile Stress During Driving 

In situ evaluation of the integrity of FRP piles during driving requires: (1) reliable data 

monitoring for determining the compression and tension stresses in the pile, and (2) development 

of appropriate design criteria for the allowable dynamic stresses in the composite pile material 

and its basic components. 

Figures 84 through 87 show typical pile stress versus penetration depth profiles obtained for the 

Lancaster Composite, Inc., PPI, SEAPILE, and American Ecoboard piles from the field PDA 

testing. The dynamic elastic modulus used for this analysis was determined from the proportional 

condition of the initial portion of the high-strain records. The results show that the calculated 

tensile stresses did not exceed 6,894.8 kPa (1 ksi); calculated compressive stresses at the pile top 

ranged from 4,136.9 to 15,857.9 kPa (0.6 to 2.3 ksi) during the driving of all FRP piles. 
72

The design criteria for allowable compression and tension stresses in the FRP piles were 

primarily developed considering the equation of the axial force equilibrium for the composite 

material and assuming no delamination between its basic components (figure 82). 

Figure 82. Equation. Applied axial force F. 

Where: 

F = The applied axial force. 

At = Total pile cross section area. 

Ar

 = Total cross section area of the reinforcement. 

Ap = Total cross section area of the plastic or concrete materials. 

σ

t

 = Equivalent axial stress, tension or compression, acting on the pile section. 

σr

allowable = Allowable axial stress, tension or compression, acting in the pile reinforcements. 

σp

allowable = Allowable axial stress, tension or compression, acting in the plastic material or the 

 concrete for the Lancaster Composite, Inc., pile. 

The equation in figure 82 can be solved for the equivalent allowable axial stress in the composite 

pile section (figure 83). 

Figure 83. Equation. Equivalent axial stress σ

t

. 

Table 18 summarizes the main criteria used for establishing the allowable stresses for the FRP 

pile installation. It also shows, for each pile, the actual stresses measured by PDA during pile 

driving. 
73

Figure 84. Graph. Stress versus penetration depth for Lancaster Composite, Inc., SLT pile. 
74

Figure 85. Graph. Stress versus penetration depth for PPI SLT pile. 
75

Figure 86. Graph. Stress versus penetration depth for SEAPILE SLT pile. 
76

Figure 87. Graph. Stress versus penetration depth for American Ecoboard splice SLT pile. 
77

Table 18. Measured and allowable stresses for FRP piles. 

Type of Stress Lancaster 

Composite 

Lancaster 

Composite; 

Driven to the 

Bedrock 

Layer 

Seaward 

International PPI American Ecoboard Steel Pile 

Compressive 

stress, kPa (ksi) 

85% of the 

concrete 

compressive 

strength 

85% of the 

concrete 

compressive 

strength 

σm

allowable=0

Ar/At=0.175 

σr=1.81x105

(26) 

σm

allowable=0

Ar/At=0.011 

σr=2.89x105

(42) 

– 

90% of 

yield 

strength for 

steel piles 

Tension stress 

σm

allowable=0

Ar/At=0.036 

σr=3.47x105 

(50.3) 

σm

allowable=0

Ar/At=0.036 

σr=3.47x105 

(50.3)

σm

allowable=0

Ar/At=0.175 

σr= NA 

σm

allowable=0

Ar/At=0.011 

σr=2.89x105

(42) 

– 

90% of 

yield 

strength 

for steel 

piles 

Allowable 

compressive 

stress, kPa (ksi) 

37233 (5.4) 37233 (5.4) 31027.5 (4.5) 3171.7 (0.46) NA – 

Allowable tension 

stress, kPa (ksi) 

12617.9 

(1.83) 

12617.9 

(1.83) NA 3171.7 (0.46) NA – 

Maximum 

measured 

compressive 

stress, kPa (ksi) 

15858.5 (2.3) 19995.5 (2.9) 8274 (1.2) 7584.5 (1.1) 4895.5 

(0.71) 

183407 

(26.6) 

Maximum 

measured tension 

stress, kPa (ksi) 

7998.2 (1.16) 5791.8 (0.84) 2827 (0.41) 2206.4 (0.32) 689.5 (0.1) 9790.9 

(1.42) 

Maximum 

measured 

compressive stress 

during restrike, 

kPa (ksi) 

14824.3 

(2.15) 16548 (2.4) 8549.8 (1.24) 8205.1 (1.19) – 238567 (34.6) 

Maximum 

measured tension 

stress during 

restrike, kPa (ksi) 

1448 (0.21) 4137 (0.60) 3378.6 (0.49) (0.18) – (9.2) 

During pile driving, the measured compression and tension stresses did not exceed the allowable 

stresses, except for PPI pile. During and after pile driving and restriking of PPI and SEAPILE 

piles, no damage or separation between the bars and the recycled plastic material was observed, 

except at the upper foot of each PPI pile. In the case of the Lancaster Composite, Inc., piles, no 

damage or separation between the concrete and the FRP shell piles was observed (figures 68 

through 71). Following these site observations, in the absence of experimental data, the 

maximum stresses obtained for the FRP piles can be used to establish allowable tension and 

compression stresses for pile driving. 

79

CHAPTER 5. CONCLUSIONS AND RECOMMENDATIONS 

The main purpose of this project was to address the need for a feasibility assessment of FRP 

composite-bearing piles for highway and related facilities substructures, replacing traditional 

materials such as timber, concrete, or steel, specifically in the construction of waterfront 

structures in hostile marine environments. The engineering use of FRP composite-bearing piles 

raised the need to investigate their field performance and to develop and evaluate reliable testing 

procedures and design methods to assess: 

• Mechanical short-term behavior of FRP composite materials under axial compression 

loads. 

• Behavior of FRP composite piles under vertical loads. 

• Evaluation of FRP composite piling capacity, drivability, and constructability. 

This project report summarizes the results of the full-scale experiment conducted at Port 

Elizabeth, NJ, and the companion laboratory tests. The main conclusions are summarized below. 

MECHANICAL SHORT-TERM BEHAVIOR OF FRP COMPOSITE MATERIALS 

UNDER AXIAL COMPRESSION LOADS 

Chapter 2 presented an engineering analysis approach for establishing the equivalent mechanical 

properties of the composite material, including elastic modulus for the initial loading quasilinear 

phase, axial compression strength, inertia moment, and critical buckling load. The conclusions of 

this chapter were as follows: 

• The laboratory axial compression tests conducted in this study on the SEAPILE 

composite sample and its component materials (the fiberglass bars and the recycled 

plastic), illustrated that the recycled plastic has a significant effect on the composite 

material stability regarding buckling. However, due to its low stiffness and compressive 

strength compared with the reinforcing fiberglass bars, the recycled plastic does not 

appear to effectively prevent the peripheral disintegration of the fiberglass bars and, 

therefore, has a limited contribution to the axial compression strength of the composite 

material. No delamination between the rebar and the plastic was observed during the 

axial compression loading. 

• The experimental results obtained for the SEAPILE composite materials illustrated that 

the nonlinear response of the recycled plastic to the axial loading is strain-rate dependent. 

However, because the quasilinear response of the fiberglass bars to the axial loading does 

not seem to be strain-rate dependent, the strain-rate effect on the mechanical behavior of 

the SEAPILE composite is expected to be relatively small. 

• A composite material model has been developed for establishing the equivalent 

mechanical properties of the SEAPILE composite material, including elastic modulus for 

the initial quasilinear loading phase, axial compression strength, moment of inertia, and 

critical buckling load. The equivalent material properties of the composite are related to 

the mechanical properties of the component materials, assuming strain compatibility 
80

between the plastic and the fiber reinforcement bars during the axial compression 

loading. 

• As illustrated in table 1, which summarizes the comparison between the predicted and 

experimental values of the SEAPILE composite material properties, the proposed model 

appears to predict the experimental results of the axial loading tests on the composite 

material sample fairly well. Further, this model seems to provide an effective framework 

for analyzing the effect of the recycled plastic on the mechanical behavior of the FRP 

composite. 

• Further research is required to better understand the failure mechanisms involved in the 

SEAPILE composite material. Both laboratory and full-scale loading tests are required to 

provide a relevant database to evaluate and develop reliable design methods for the 

engineering applications of composite construction materials in load-bearing piles that 

are used for waterfront and highway structures. 

• A compression axial test was performed on a 38.7-cm- (15.25-inch-) diameter, 80-cm- 

(31.5-inch-) long PPI pile sample containing 16 steel reinforcing rods of 2.54-cm 

(1.0-inch) nominal diameter to establish the composite material properties. The test 

results on the PPI sample showed that this material has a behavior similar to that of steel. 

• Axial compression tests were performed on the Trimax pile recycled plastic at several 

strain rates. The dimensions of the sample were 25.4 cm (10 inches) in diameter and 

50.8 cm (20 inches) long. The test results showed that the nonlinear response of the 

Trimax material to the axial loading is strain-rate dependent. The Young’s modulus of 

this material obtained for the linear portion of the stress-strain curve at the strain rate of 

1.7 mm (0.07 inch) per min (0.33 percent/min) is 370,000 kPa (53.7 ksi). Poisson’s ratio 

calculated for this strain rate at 1000 kPa (0.145 ksi) is equal to 0.35, and the 

corresponding shear modulus value for the linear portion of the stress-strain curve is 

1.37 x 105 kPa (19.9 ksi). 

BEHAVIOR OF FRP COMPOSITE PILES UNDER VERTICAL LOADS 

The testing program included four SLTs on instrumented piles, which were driven in the selected 

site at Port Elizabeth, NJ. In chapter 3, the experimental results were compared with the methods 

commonly used for evaluating the ultimate capacity, end bearing capacity, and shaft frictional 

resistance along the piles. This engineering analysis led to the following conclusions: 

• The full-scale experiment confirmed that the FRP composite piles can be used effectively 

as load-bearing piles and represent an alternative for deep foundation construction, 

especially in waterfront environments and aggressive soils. 

• Distinct plunging failure occurred during the SLTs on PPI and SEAPILE piles as the 

applied loads reached 115 and 90 t (253 and 198 kips) and the measured pile top 

settlements were 1.64 cm (0.65 inch) and 1.16 cm (0.46 inch), respectively. 

• The Lancaster Composite, Inc., pile did not experience a distinct plunging failure, and the 

maximum load applied on the pile reached 128 t (282 kips) with a measured settlement of 

1.73 cm (0.68 inch). 

• The maximum load applied on the American Ecoboard pile, which was driven to the 

sandy layer, was 60 t (132 kips). At this load, the pile top settlement was 9.34 cm 

(3.7 inches), and no distinct plunging failure was observed. The pile top settlements of 
81

this pile, which contained only recycled plastic with no reinforcement bars, were 

significantly greater than the settlements measured during the tests on the other FRP 

piles. 

• Several methods commonly used for evaluating the ultimate capacity were compared 

with the SLT results. In general, the loads calculated for FRP piles using the ChinKondner method are greater than the maximum loads that were applied at the field test. 

The DeBeer yield load method yields conservative loads compared to the maximum loads 

applied at the SLTs. The Davisson offset limit load method, using an equivalent elastic 

modulus that is experimentally derived from the quasilinear load-settlement relationship 

of the unloading-reloading cycle, yields limit loads that are within the range of loads 

obtained with the above-mentioned methods and settlement estimates that are consistent 

with the field test results. 

• The experimental results are compared with several codes(5-7) and analytical 

methods(4,39-40) commonly used for evaluating the shaft friction and end bearing capacity 

of the piles. The maximum end bearing capacities measured at the Port Elizabeth site 

were relatively small in comparison to the applied loads, indicating that the piles were 

frictional piles. The American Petroleum Institute (API) method yields end bearing 

capacities that are significantly lower than the FHWA and Meyerhof methods, and are in 

fairly good agreement with the end bearing capacity measured in the field tests. 

• Several methods commonly used for evaluating the shaft friction along the piles were 

compared with the experimental results. Burland and Meyerhof’s methods and the 

AASHTO code yield shaft friction values that are in good agreement with the average 

shaft friction measured on the PPI and SEAPILE piles in the upper soil (fill and sand) 

layer. The FHWA method yields the best correlation for the shaft friction values obtained 

in the lower silt and clayey soil layer. 

• The engineering use of FRP piles on a widespread basis requires developing and 

evaluating design methods through extensive comparison of predictions with reliable data 

measured during full-scale loading tests. For this purpose, the FRP piles were 

instrumented by strain gauges that were specifically designed for strain measurements in 

these piles. This instrumentation allowed measuring the shaft friction distribution along 

the piles and the end bearing capacities in the saturated soils of the selected site during 

the SLTs on the FRP piles. 

• This full-scale experiment demonstrated the feasibility of effectively using FRP piles as 

vertical load piles. However, because soil-pile interaction depends largely on prevailing 

soil profile and site conditions, further full-scale testing in different soil profiles is 

required to establish a reliable database for developing and evaluating codes and methods 

of analysis for designing FRP piles. 

EVALUATION OF FRP COMPOSITE PILING CAPACITY, DRIVABILITY, AND 

CONSTRUCTABILITY 

In chapter 4, the authors described the full-scale experiment, the dynamic pile testing results, and 

the engineering analysis of the SLTs on the FRP piles. The main objectives of this demonstration 

project were to: 

• Assess the drivability and durability of FRP piles during installation using PDA. 
82

• Evaluate the currently available dynamic testing methods for establishing the dynamic 

properties of FRP piles and the dynamic soil-pile interface parameters during driving. 

• Evaluate the dynamic methods currently used in predicting the load-set response and 

ultimate static capacity of FRP piles and, more specifically, the correlations between 

CAPWAP analysis and SLT results. 

• Develop appropriate design criteria for the allowable dynamic stresses in the composite 

pile material and its basic components during pile driving. 

• Obtain dynamic data records of the FRP pile driving that, in the absence of design criteria 

and field data for these piles, can be used to establish allowable tension and compression 

stresses for performing dynamic analyses and evaluating drivability and integrity during 

driving.

The engineering analysis of the dynamic pile testing results and the SLTs led to several 

conclusions. 

Drivability and Integrity During Driving 

• PDA during pile driving and restriking of the PPI and SEAPILE piles showed no damage 

or separation between the bars and the recycled plastic material, except at the upper foot 

of each PPI pile. Similarly, in the case of the Lancaster Composite, Inc., piles, no damage 

or separation between the concrete and the FRP shell piles was observed. 

• The PDA and CAPWAP analysis results showed that all the piles experienced a longterm increase in pile capacity from the EOI to the BOR. For most of the tested piles, the 

measured setup factor, defined as a ratio between the BOR and EOI capacities, was in 

excess of 2.0. The dissipation of excess pore water pressure generated during pile driving 

resulted in a significant increase, up to about 100 percent, of the shaft and toe Smith 

damping. 

 

Evaluation of Dynamic Testing Methods 

• The dynamic testing procedures commonly used for steel and concrete piles (i.e., PIT, 

PDA testing—high-strain, and PDA testing—early portion of the high-strain records) 

were conducted to evaluate the dynamic properties of the composite piles. The parametric 

study conducted with CAPWAP and GRLWEAP showed that the elastic modulus 

obtained from PDA testing—early portion of the high-strain records yields the best 

correlations between (1) the calculated values of the transfer energy level and the energy 

measured by PDA, and (2) the calculated blow count and the measured field records. 

CAPWAP Analysis 

• Comparison between the GRL recommendations and the dynamic soil properties 

obtained from the CAPWAP analysis leads to the following results: 

o The soil shaft quake values recommended by GRL appear to be consistent with 

the CAPWAP results obtained for all the piles tested with the exception of the PPI 

piles. 
83

o The soil toe quake values obtained for the FRP test piles are significantly lower 

(about 45 percent) than the values recommended by GRL. 

o The toe damping values obtained for the FRP test piles are consistently greater 

(about 250 percent) than the values recommended by GRLl. 

o The shaft Smith damping values recommended by GRL appear to be consistent 

with the CAPWAP results obtained for all the piles tested. 

• For all SLT piles, CAPWAP capacities ranged from 938.6 to 1,267.7 kN (211 to 

285 kips) during the BOR. In general, CAPWAP analysis showed most of the ultimate 

capacity came from shaft resistance when the piles were driven to penetrations of 17.7 m 

(58 ft). 

Correlations Between CAPWAP Analysis and Static Load Tests 

• The CAPWAP analysis yielded load-set curves and ultimate capacities, which correspond 

fairly well to the SLT results. 

• For the SEAPILE and PPI piles, the CAPWAP analysis seemed to indicate that the 

settlement at failure was close to elastic compression settlement during loading. 

• For the low stiffness American Ecoboard pile that was driven to the sandy layer, the 

settlement, reaching 96 mm (3.7 inches) under a 60-t (132-kips) load, appeared to be due 

mainly to the elastic compression. The CAPWAP analysis was limited to the settlement 

range of about 20 mm (0.78 inch). For this range, it seemed to be quite consistent with 

SLT results. 

Design Criteria and Allowable Stresses 

• Design criteria for allowable compression and tension stresses in the FRP piles were 

developed considering the equation of the axial force equilibrium for the composite 

material and assuming no delamination between its basic components. 

• During pile driving, with the exception of the PPI piles, the measured compression and 

tension stresses did not exceed the allowable stresses. In the absence of design criteria 

and field data for FRP plies, the maximum stresses obtained for these piles in the site 

observations can be used to establish allowable tension and compression stresses for pile 

driving. 

R&D NEEDS ASSESSMENT 

The dynamic and static loading tests on instrumented FRP piles conducted in this project 

demonstrated that these piles can be used as an alternative engineering solution for deep 

foundations. However, their widespread use will require further site testing and full-scale 

experiments to establish a relevant performance database to develop and evaluate reliable testing 

procedures and design methods. 

The time-dependent stress-deformation behavior of composite recycled plastic materials is of 

concern, because the FRP piles may undergo an excessive deformation due to an applied 

sustained loading. The engineering use of FRP piles on a widespread basis requires developing 
84

and evaluating reliable testing procedures and design methods to determine the long-term 

behavior of these composite piles. 

Further, research is now required to evaluate the effect of environmental conditions (i.e., soil 

confinement, groundwater, etc.) on the long-term behavior of recycled plastic composite 

materials as well as the combined effects of chemical and mechanical degradation processes. 

Both laboratory and full-scale loading tests are required to provide a relevant database to develop 

and evaluate the assessment of the long-term performance of composite, time-dependent FRP 

piles and to determine the limit creep load for their engineering use in waterfront and highway 

structures. 
85

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HRDI-06/08-06(1M)E

Recycled

Recyclable