| It has long been established that piles driven into low permeability soils often gain capacity over time. This capacity gain, also termed "set-up" or "freeze", is believed to be controlled by two mechanisms: (i) the increase of effective stresses due to the dissipation of excess pore pressures built up during driving, and (ii) stress independent phenomena such as strength increase due to thixotropic bonding. Only recently efforts have been made to quantify time dependent pile capacity and develop a methodology of incorporating it into pile design through use of insitu testing.; A research project investigating time dependent pile capacity gain was supported by the Massachusetts Highway Department and carried out at the Geotechnical Engineering Research Laboratory of UMass Lowell. As the final stage of the project, an instrumented test pile cluster was installed and tested in a subsurface containing a significant deposit of Boston Blue Clay (BBC) in Newbury, Massachusetts. The test pile cluster consisted of the following major components: (1) three instrumented test piles, (2) surrounding ground instruments including piezometer field, accelerometers, and earth pressure cells, (3) an onsite data acquisition array, and (4) an extensive testing program including up-down and down-up static tests, and dynamic and statnamic tests. The testing program was designed to (i) observe the build-up and dissipation of excess pore pressure along and away from the pile's surfaces and (ii) determine the changes in the pile's capacity over time using dynamic and static load testing. In addition, radial total stresses were monitored at discrete locations within the Boston Blue Clay and silty sand layers on two of the three test piles. Coupled with the pore pressure readings, these measurements allowed for effective stresses to be examined.; Previous stages of the research including methodology, databases and model pile testing are reviewed. The current work concentrates on the full-scale testing results, emphasizing the data obtained during the tests and its reduction. The large number of instruments mounted on the piles and in the surrounding soil, along with the lengthy testing period, presented a major challenge of data management and reduction. After data reduction was completed, the excess pore pressure and pile capacity measurements, both overall and segmental, were analyzed using normalized relationships. The obtained relationships were compared to those developed by previous research efforts within this initiative, such as collection and analysis of several data sets of previous instrumented pile programs and the use of a Multiple Deployment Model Pile (MDMP).; The results of the instrumented test pile cluster showed that (i) the rates of radial excess pore pressure dissipation within the BBC were constant with distance from the driven piles, (ii) the rates of excess pore pressure dissipation and capacity gain within the BBC were nearly identical to the MDMP results, (iii) the time to 50% excess pore pressure dissipation and 75% capacity gain matched well with the MDMP results, and (iv) the overall rates of capacity gain showed excellent correlations with the previously compiled data. The matching full-scale pile and MDMP results verified the use of normalized time and size relationships in quantifying and predicting time dependent pile capacity gain using insitu testing, thereby establishing a practical method for incorporating time dependent capacity gain into pile design and construction monitoring. |
