Chapter VIII. Summary and Conclusions

Discussion

Five tasks were planned to complete this phase of the ongoing research in laterally loaded piles. These tasks were:

1. Develop test hardware.
2. Design and construct a data acquisition system.
3. Calibrate all instrumentation.
4. Perform a pile group test.
5. Analyze data.

All of the tasks were accomplished with results being as good or better than planned. The model piles with their designed features, such as the pinned pile crown/load rod arrangement, can be adapted for further pile group testing. Changing spacing between the piles, for example, can be accomplished simply by fabricating a new load rod with smaller or greater distances between the pin holes.

The load distribution output obtained from the load rod was not conclusive. This probably was due to some degree to electronic noise in the data acquisition system. If the levels of strain in the loadrod were increased, either by testing at higher lateral loads, or by fabricating a new load rod with thinner inter-pile gage sections, the system noise would have a much smaller influence on the straingage output. If a new load rod were to be fabricated for further testing, it should be constructed in such a way that the axis on which the connecting pins act should be on the same plane as the centroidal axis of the inter-pile gage sections so that no bending stress is introduced during loading, thus keeping the total stress on the load rod at a lower level.

The data acquisition system proved to be capable of the design requirement in most aspects. Sampling at the rate of 4 Hertz was adequate for testing done in this project. With the capability to expand to 256 input channels, the system should function well for future testing. Several problems still exist with the system, such as the voltage saturation problem when the system is turned on. Hopefully a solution will be found, but this problem can be worked around for the time being. LabVIEW software for testing is a powerful tool and proved invaluable for making the data acquisition system work and in controlling the loading system.

The loading system worked quite well once operating procedures were figured out. The unequal loading by the hydraulic cylinders was overcome by monitoring the load cells. Perhaps this approach will work for all future applications. Further refinement could be made in this area for smoother operation. The load by-pass cables and the other linkage elements proved to be very versatile and were a necessity to make pile group loading possible while preventing possible damage to the pile group or other instruments. The LVDT hookup apparatus also proved to be quite versatile and functional.

Calibration procedures and software evolved during this project to the point where it is not a tedious task to calibrate any of the instrumentation. LabVIEW software and the calibration spreadsheet make data reduction simple, with regression analysis the only manual step in the process.

After 50 full cycles of testing, the testing procedure was refined to make this task fairly simple. Many obstacles were overcome, including a power outage, in getting the testing completed. The quality and quantity of data gathered demonstrate the abilities of all systems and show that if desired, the response of any pile, at any location along its length, at any load, and in any cycle can be determined. Group response also can be studied at the same time.

MATLAB software proved to be a valuable tool in the data reduction work. Once basic steps were learned, data analysis and output preparation were quickly accomplished.

Conclusions

Test output clearly showed that the lead pile takes more of the lateral load than the other piles, except after many cycles and at lower loads, where the trailing pile takes more of the load. As load levels increase, the lead pile again takes more load than the other piles. There was no clear trend showing how the other piles take up the remaining load. A statistical analysis of the load distribution data could yield more information in this area.

Bending moment data showed some definite trends. As the number of cycles increased, the point along the pile at which the maximum moment occurs moved down the pile. This result was expected. Another trend showed that for a single cycle, the depth at which the maximum moment occurred remained quite constant as the load increased. It also appeared that the inner piles had close to the same bending moment in both loading directions at higher loads. At lower loading levels, more moment occurs in the inner piles when they are trailing than when they are leading.

The comparison of test output with predictions made by Florida Pier and COM624P did not clearly show a correlation in either bending moment or pile top deflection.

Since the piles were sized using dimensionless Pi terms, results from this research can be applied to any full-scale pile foundation. The results are valid for predicting performance of prototype piles.

References

1. Brown, D. A. and L. A. Reese, 1985. Behavior of a large-scale pile group subjected to cyclic lateral loading. Minerals Management Service, U.S. Department of Interior, Department of Research, FHWA, U.S. Army Engineer Waterways Experiment Station. Report no. GR 85-12. 399 p.
2. McVay, M., D. Bloomquist, D. Vanderline, and J. Clausen. 1994. Centrifuge modeling of laterally loaded pile groups in sands. Geotechnical Testing Journal, GTJODJ17 (2):129-137.
3. McVay, M., R. Casper, and T. Shang. 1995. Lateral response of three-row groups in loose to dense sands at 3D and 5D pile spacing. Journal of Geotechnical Engineering 121(5):436-441.
4. Rao, S., V. Ramakrishna, and G. Raju. 1996. Behavior of pile-supported dolphins in marine clay under lateral loading. Journal of Geotechnical Engineering 122(8):607-612.
5. Rollins, K., K. Peterson, and T. Weaver. 1996. Full-scale pile group lateral load testing in soft clay. National Center for Earthquake Engineering Research Bulletin, 10(4):9-11.
6. Townsend, F., M. McVay, P. Ruesta, and L. Hoyt. 1997. Prediction and evaluation of a laterally loaded pile group at Roosevelt Bridge. Florida DOT Final Report. State Project No. 99700-3508-119. 381 p.
7. U.S. Department of Transportation. 1996. Design and construction of driven pile foundations. FHWA DTFH61-93-C-0015. 822 p.