A Case History of Pile Freeze Effects in Dense Florida Sands

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1 A Case History of Pile Freeze Effects in Dense Florida Sands By Ching Kuo 1, Guoan Cao 2, Amy L. Guisinger 3, and Paul Passe 4 Submission Date: August 1, ,804 Words 1 Table 5 Figures 1 Professional Service Industries, Inc., 5801 Benjamin Center Drive, Suite 112, FL 33634; PH (813) ; FAX (813) ; ching.kuo@psiusa.com 2 Professional Service Industries, Inc., 5801 Benjamin Center Drive, Suite 112, FL 33634; PH (813) ; FAX (813) ; guoan.cao@psiusa.com 3 Professional Service Industries, Inc., 5801 Benjamin Center Drive, Suite 112, FL 33634; PH (813) ; FAX (813) ; amy.guisinger@psiusa.com 4 Professional Service Industries, Inc., 5801 Benjamin Center Drive, Suite 112, FL 33634; PH (813) ; FAX (813) ; paul.passe@psiusa.com

2 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 1 ABSTRACT Increase in pile capacity from the freeze/setup effect after initial driving has been observed and studied by a number of researchers. This phenomenon not only occurs in clays but also in sands. On many occasions, the pile freeze effect was also observed in dense to very dense sands. This paper presents the Ernest Lyons Bridge project, in Stuart, Florida, as a case history to demonstrate pile freeze effects observed in very dense cemented sands. In this project 24-inch square prestressed concrete piles were driven over 25 feet into very dense cemented sands. A comparison of estimated pile capacities at the end of initial driving (EOD) and the beginning of redrive (BOR) obtained by the Pile Driving Analyzer (PDA) with that predicted by the Program SPT-97 are discussed. Although it is well known that setup occurs primarily from side shear along the pile shaft, for this particular project the majority of the pile capacities were mobilized along the bottom part of the pile. It is difficult to separate side shear from end bearing in a CAPWAP analysis because of the quality of the PDA data. Therefore, in this paper, the total pile capacity including side shear and end bearing were used to evaluate freeze effects. A conservative Setup Factor, A, which is the change in capacity normalized by the initial capacity per log cycle of time, of 0.2 was derived for the project site. INTRODUCTION Owing to relatively loose to very loose surficial sands and high ground water tables encountered throughout Florida, the Florida Department of Transportation (FDOT) has widely used driven piles as the preferred bridge foundation alternative for many decades. To aid the design of driven piles, the University of Florida (UF) has developed a design methodology and Computer Program SPT-97 through a research grant from the FDOT (Schmertmann, 1967). With a better understanding of the load transfer mechanism of driven piles and the demand of bigger bridge structures, geotechnical engineers have designed higher and higher pile capacities, from a service load of 45 tons prior to 1970 to 100 tons currently, for an 18-inch square prestressed concrete pile. Bigger hammers with higher transfer energies have also been required to drive piles to required capacities. Consequently, in many instances, SPT-97 greatly overpredicts pile capacities obtained from the Pile Driving Analyzer (PDA) at the end of initial driving. However, at the time of redriving the pile, after splicing occurs, a significant increase in capacity from pile freeze effects is observed. Many researches have documented significant increases in pile capacity with time (Bartolomey and Yushkov, 1985; Lukas and Bushell, 1989; Schmertmann, 1991; Majboor, 1996). Although early studies mainly focused on clays, this phenomenon is also observed in sands including dense to very dense sand (Axelsson, 1998; Chow, et al. 1996; York, et al. 1994). UF has conducted research, sponsored by the FHWA and FDOT, on pile freeze effects (McVay, Schmertmann and Bullock, 1999). The research concluded that for practical purposes, the Setup Factor, A, which is the change in capacity normalized by the initial capacity per log cycle of time, of 0.2 for all pile lengths and in all soils in North Florida, in design is conservative.

3 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 2 This paper presents a case history of the Ernest Lyons Bridge project in Stuart, Florida in order to evaluate pile freeze effects in areas other than North Florida which was the focus of the study by UF mentioned above. In addition, this paper demonstrates pile freeze effects observed in dense sands which were encountered at the Lyons Bridge project. PROJECT DESCRIPTION The Florida Department of Transportation (FDOT) District VI planned to replace the existing bridge structures of A1A with precast segmental bridges, approximately 4,650 feet in length, over the Intracoastal Waterway in Stuart, Florida. The proposed main bridge consisted of two end bents and 30 intermediate piers, supported by 24-inch square prestressed concrete piles with a nominal bearing capacity ranging from 658 to 902 kips per pile. A total of 24 test piles were driven and monitored with PDA and subsequent CAPWAP analysis ran to aid in the determination of production pile lengths and pile driving criteria. Subsequently, verification tests with PDA monitoring were performed on selected production piles for QA/QC purposes. GEOTECHNICAL SETTING Based on 29 borings completed for the project, subsurface conditions can generally be characterized as fine sand with trace to some silt, trace to few shells and slightly silty, silty and clayey sands in composition with silts, shell and cemented sand (i.e. SP, SP-SM, SM and SC materials). The soils are generally loose to medium dense to an approximate elevation of -30 feet, National Geodetic Vertical Datum, 1929 (NGVD); grading to medium dense, dense and very dense at elevations deeper than -60 feet, NGVD which were considered as the primary bearing layer with SPT N-values ranging from 50 blows over a foot to 50 blows over one inch. Figure 1 shows general site conditions of SPT N-values versus elevation. Exceptions to this general characterization include lenses with clay, significant amounts of calcareous cemented sands and isolated areas where the bearing layers of refusal materials were not encountered to the termination of the soil borings at an approximate elevation of -100 feet, NGVD.

4 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 3 SPT-N Value (Blows/ft) Elevation (ft, NGVD) FIGURE 1 General Site Conditions DESIGN AND CONSTRUCTION CONTROL OF DRIVEN PILES During the design phase of the Ernest Lyons Bridge project, SPT-97 analyses were performed to estimate the required pile lengths for each bridge pier according to the soil boring information and the required design loads ranging from 658 to 902 kips per 24-inch square concrete pile. SPT-97 is considered the primary design tool for driven piles in Florida, especially for FDOT projects. The design methodology was established through extensive research including static pile load testing and correlating the pile skin friction and end bearing to the SPT N-values (Schmertman, 1967). In general, pile load tests are performed several days after pile installation because of the time required to set up the load test equipment. Therefore, the pile capacity obtained during the static load test might be significantly different from that at the end of initial pile driving because of pile freeze or relaxation effects. Since SPT-97 was developed based on the static load test data, it is reasonable to believe that the pile capacity prediction from SPT-97 should include pile freeze effects, if present. According to FDOT design guidelines, the test pile length is determined based on the required pile capacity and the anticipated pile length from the SPT-97 analysis results plus 10 to 15 feet. A total of 24 test piles were selected at different pier locations, ranging from 85 to 110 feet. During construction, an APE D46x32 hammer was utilized to drive piles, and dynamic load testing with Pile Driving Analyzer (PDA) monitoring was performed on every test pile. However, lower than estimated pile capacities at the end of initial driving were observed during the test pile installation, therefore, a series of setcheck/redrives subjected to PDA monitoring at different time periods were performed to evaluate pile freeze effects at the project site to validate the SPT-97 predictions. The following section will discuss the pile capacities estimated from the SPT-97 analysis and that monitored by PDA at different time periods after initial pile driving.

5 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 4 COMPARISON OF PILE CAPACITY PREDICTED BY SPT-97 AND PDA MONITORING RESULTS A total of 70 dynamic load tests with PDA monitoring were performed on 34 piles. Table 1 summarizes the pile capacities estimated by PDA monitoring and the corresponding hammer transfer energy during driving at different time periods. The SPT-97 predicted pile capacity corresponding to the final pile tip elevation is included, in addition to the required load for each pier. TABLE 1 Summary of Pile Data Pile Tip Time Transfer Required Pier Pile Elevation Date Elapsed of Energy PDA SPT-97 Capacity No. No. (feet) Days Driving (k-ft) (kips) (kips) (kips) /28/2005? BOR /15/2005? BOR /27/2005? BOR /15/ EOD /10/ BOR /27/2005? BOR /14/2005? BOR /14/2005? BOR /19/2004? BOR /20/ EOD /30/ BOR /27/ EOD /28/ BOR /30/ BOR /4/ BOR /27/ BOR /25/ EOD /30/ BOR /12/ EOD /15/ BOR /29/ EOD /30/ BOR /3/ BOR /6/ BOR /27/ BOR /15/ EOD /16/ BOR /21/ BOR /17/ EOD /21/ BOR /19/ EOD /21/ BOR /30/ BOR /23/ EOD

6 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 5 Pile Tip Time Transfer Required Pier Pile Elevation Date Elapsed of Energy PDA SPT-97 Capacity No. No. (feet) Days Driving (k-ft) (kips) (kips) (kips) 3/30/ BOR /25/ EOD /30/ BOR /29/ EOD /11/ BOR /1/ EOD /5/ BOR /11/ BOR /4/2005? BOR /7/ EOD /11/ BOR /31/ BOR /9/ EOD /11/ BOR /3/ EOD /5/ BOR /5/ EOD /8/ BOR /8/ EOD /12/ BOR /14/ BOR /10/ EOD /16/ BOR /12/ EOD /14/ BOR /14/2006? BOR /27/ EOD /8/ BOR /9/ BOR /16/ EOD /20/ BOR /13/ EOD /14/ BOR /20/ BOR /20/ EOD /23/ BOR /11/2006? BOR As shown in the table, the majority of pile tip elevations ranged from 85 feet to 100 feet, NGVD with an average elevation of 92 feet, NGVD. Each pile penetrated into the dense sand layer encountered at depths greater than 25 feet. Figure 2 demonstrates the relationship between the pile capacities predicted by SPT-97 analyses and that determined by PDA monitoring. In general the pile capacities determined by PDA at the end of initial driving (EOD) ranged from about 25% to 75%, with an average of 57% and Standard Deviation of 15%, of that predicted by SPT-97. However, the pile capacities determined by PDA at the beginning of

7 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 6 redrive (BOR) ranged from about 60% to 150%, with an average of 97% and Standard Deviation of 39%, of that predicted by SPT-97. It should be noted that because of the construction schedule, the waiting period for pile redrive ranged from 2 to 51 days. Nevertheless, the results indicated that the SPT-97 analyses gave a good prediction of pile capacity at the beginning of redrive in terms of the average value. It also indicated that every pile exhibited some degree of pile setup even when driven into dense sands. PDA Capacity (kips) EOD BOR PDA/SPT-97 = SPT- 97 Prediction (kips) FIGURE 2 Pile Capacity Comparison (SPT-97 vs. PDA) Figure 3 illustrates the relationship of pile capacities determined by PDA at the EOD and BOR conditions. Figure 3 was re-plotted as the ratio of pile capacities at BOR and EOD versus elapsed time and is presented in Figure 4. It can be seen from Figure 4 that a majority of pile capacities increased 10% to over 120% per log cycle of time. Skov and Denver (1988) proposed a unitless log-linear Setup Factor, A, which is equal to the rate of increase of the setup ratio (Q/Q o ) per log cycle change of the elapsed time ratio (T/T o ): Q/Q o 1 = A log 10 (T/T o ) Where:Q = Pile capacity at time, T Q o = Pile capacity at initial time, T o T = Time elapsed since end of initial driving T o = Initial time elapsed since end of driving, a reference time before which there is no predictable Q o increase as a function of elapsed time. A = Setup Factor (log-linear)

8 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 7 PDA Capacity at BOR (kips)) BOR = 2 x EOD BOR = EOD PDA Capacity at EOD (kips) FIGURE 3 PDA Capacities (EOD vs. BOR) 3.0 Pile Setup Ratio (Q_BOR/Q_EOD)) % 50% 10% Elapsed Time [log (days)] FIGURE 4 Elapsed Time vs. Pile Setup Ratio By reanalyzing the data, averaging the setup ratio at the same elapsed time and using the above relation, Figure 4 is reduced as shown in Figure 5. As presented in the figure, the majority of the Setup Factors (based on T o = 1 day) fall in a range of 0.2 to 1.1, which is close to the findings of the UF research on North Florida soils.

9 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 8 Setup Ratio Increment (Q/Q0-1) Setup Factor A = 1.1 Setup Factor A = Elapsed Time Ratio, [log (T/T0)] FIGURE 5 Elapsed Time Ratio vs. Setup Ratio Increment DISCUSSION AND CONCLUSION From early FDOT experience, test piles usually have sufficient length to penetrate into the bearing layer and also usually exceed the required capacity based on SPT-97 analysis. However, more and more cases encountered recently, especially for high capacity piles, do not reach pile capacity at the end of initial driving as predicated by SPT-97. Since SPT-97 was developed based on conditions after pile freeze effects occur, it is not surprising to see lower capacities at the end of initial driving. However, SPT-97 analysis predicts the relatively longterm pile capacity very well. According to the research project conducted by UF on North Florida soils and the data collected from this project site, a conservative value for the Setup Factor, A, on the order of 0.2 can be used for sandy soils throughout Florida. To take advantage of pile freeze effects on pile design, it is recommended that during the design phase, the required long term pile capacity can be properly reduced considering a Setup Factor of 0.2 for the pile capacity at EOD conditions, and an appropriate construction schedule be arranged to allow sufficient time for setchecks and redrives to assess the Setup Factor A in the test pile program, so pile freeze effects can be included in determining the production pile length and the driving criteria. REFERENCES Axelsson, G. (1998). Long term set-up of driven piles in non-cohesive soils evaluated from dynamic tests on penetration rods. Proceedings of the First International Conference on Site Characterization. P.K. Robertson and P.W. Mayne, eds., Balkema, Brookfield, VT, 2,

10 By Ching Kuo, Guoan Cao, Amy L. Guisinger, and Paul Passe 9 Bartolomey, A.A. and Yushkov, B.S. (1985). Variation in time of capacity of pile foundation in clays. Proceedings of the Eleventh International Conference on Soil Mechanics and Foundation Engineering. Balkema, Brookfield, VT, 3, Chow, F.C., Jardine, R.J., Brucy, F., and Nauroy, J.F. (1998). Effects of time on the capacity of pipe piles in dense marine sand. Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Reston, VA, 124(3), Lukas, R.G. and Bushell, T.D. (1989). Contribution of pile freeze to pile capacity, Proceeding of the Congress, Foundation Engineering: Current Principles and Practices. Kulhawy, F.H., editor, ASCE, Reston, VA, 2, Majboor, B.M. (1996). Evaluation of time effects on skin resistance from dynamic and static pile data analysis. Master s Thesis. Department of Civil Engineering, University of Florida, Gainesville, FL. McVay, M.C., Schmertmann, J. and Bullock, P. (1999). Pile friction freeze: a field and laboratory study. Research Report submitted to Florida Department of Transportation. Department of Civil Engineering, University of Florida, Gainesville, FL. Schmertman, J.H. (1967). Guidelines for use in the soils investigation and design of foundations for bridge structures in the State of Florida. Research Report 121-A, Florida Department of Transportation. Schmertman, J.H. (1991). The mechanical aging of soils (the twenty-fifth Karl Terzaghi Lecture). Journal of Geotechnical Engineering, ASCE, Reston, VA, 117(9), Skov, R. and Denver, H. (1988). Time-dependence of bearing capacity of piles. Proceedings of the Third International Conference on the Application of Stress-Wave Theory of Piles. Fellenius, B.G., editor, BiTech Publishers, Vancouver, BC, York, D.L., Brusey, W.G.,Clemente, F.M., and Law, S.K. (1994). Setup and relaxation in glacial sands. Journal of Geotechnical Engineering, ASCE, Reston, VA, 120(9),

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