Behavior of Large Capacity Jacked Piles =

This dissertation, "Behavior of Large Capacity Jacked Piles =" by Feng, Yu, 俞峰, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Behavior of Large Capacity Jacked Piles Submitted by YU Feng for the degree of Doctor of Philosophy at The University of Hong Kong in September 2004 Pile jacking is an environmentally friendly pile-installation system. It was introduced into Hong Kong relatively recently as an alternative to the traditional percussive and boring systems. Though jacked piles have been used for decades in Mainland China, their use has mainly been restricted to small-capacity piles in soft ground conditions. Reports on experimental studies are mostly confined to laboratory tests, and very few systematic field-tests have been carried out. A comprehensive field-test program of long steel H-piles jacked into stiff residual soils is presented in this thesis. The largest capacity machines then available worldwide were used in the tests. The jacking loads were up to 7400 kN. Six jacked piles were fully instrumented with strain gauges along the shaft for investigation of the load transfer behavior. In order to monitor the variations of pore pressure induced by jacking, piezometers were installed in the alluvial and completely decomposed granitic soil layers adjacent to each of the test piles. Changes in strains and pore pressures were recorded during various stages of installation and during the loading test, jacking of adjacent piles, reloading, and construction of superstructure. A number of instrumented H-piles installed by percussive hammer in similar soil iconditions were also monitored for comparison purpose. The results indicate that the mobilized unit shaft resistance during the installation of the jacked piles increased persistently with increasing penetration, while the base resistance varied according to the stiffness of the surrounding soil. Excess pore pressure induced by jacking was localized and closely related to the penetration. It dissipated rapidly in both the alluvium and completely decomposed granite, and the performance of the piles after installation was unaffected by reconsolidation. The pre-creep jacking procedure was very effective in reducing creep and residual settlement during the loading tests, hence increasing the pile capacities. Although the jacked piles were founded on softer strata than the driven piles, they exhibited similar capacities. Installation of adjacent piles mainly produced tensile stress along a completed jacked pile. Significant compressive residual stress due to jacking was locked in. This could be the reason why the shaft resistance induced by the applied load was considerable. A framework based on the spherical cavity expansion theory was developed to predict the pile-tip behavior with the knowledge of local overburden pressure and the N value of standard penetration test. The mean volumetric strain in the plastic zone was analytically determined by the finite strain theory. The dependency of soil strength on the relative density and stress level was taken into account. The cavity radius and subsequently the pile-tip settlement were evaluated by incorporating the principle of energy conservation. The model was used to simulate the load-test results derived from the jacked piles and also a large number of piles driven through various sandy soils. The consistency relies on the appropriate interpretation of field measurements as well as the evaluation of soil parameters. Word count: 476 Signature: ii DOI: 10.5353/th_b314