Experimental Methods and Qualification of the Data
Authors: Bocgard Dewaine , Matlock Hudson 
Published: October 2008

Extreme care was taken in the design of axial strain measuring devices lateral total and pore pressure transducers custom cables and connectors and specially-stable instrument circuitry with the result that all instruments performed perfectly over the 2.5-year period that the pile was installed. The need to discriminate small differences in axial loads along the pile at shallow depths in order to establish accurate soil reaction versus displacement (t-z) curves led to the use of static load test data for full-scale calibration of the instruments along the upper half of the embedded length. The resulting calibration data exhibited a degree of linearity better than 0.1 percent. For the instruments along the bottom half of the pile such extreme precision in the load differences was not required to obtain an equal degree of precision in the calculated values of shear transfer. Exceptional stability was observed among the strain modules with no discernable drift in the units over the entire 2.5-year period. Variations with time in the residual axial loads along the pile were very regular and are related to the ongoing vertical consolidation of the upper 80 ft of sediments. Various other comparisons among the data taken with the instruments were made to verify the accuracy and reliability of the load and displacement data that were used to develop design correlations.

Introduction

Because of the economic importance of the field load test program extraordinary care was taken in the design and proof-testing of the transducers and instrumentation. As a result the instruments proved to be drift-free for the entire 2.5- year period during which field data was recorded. The primary objectives of the field tests included the observation of the distribution with depth of the axial shear transfer capacity. Careful attention was therefore given to obtaining the highest possible degree of precision and accuracy in the measurement of the axial thrust along the pile. A second objective of the research was the development of a rational method to relate the local shear transfer versus displacement (t-z) behavior to the geotechnical parameters of the clay. Therefore, accurate determination of the local pile-soil displacement along the embedded length was also necessary.

Calibration of Strain Modules and Extensometers

Due to the length of the instrumented section and the magnitudes of axial load involved suitable onshore facilities for performing a full-scale calibration of the pile were not available. During fabrication, the strain-sensing units in each 30-ft can were calibrated under axial loads of about 150 kips. However, most of the internal appurtenances were added after the calibration with the cross-sectional areas of the instrumented section increasing by amounts varying from 14 to 35 percent. The wall thickness of each can was measured ultrasonically before any instruments were installed. The cross-sectional areas that were added during fabrication included the strain modules the access tubes for the extensometers and tell-tales the reinforcing bars used for attaching the cables to the pile wall all internal and external cover plates and the cross-sectional areas of the fillet welds.