Termination Criteria for High-Capacity Jacked and Driven Steel H-Pilesin Hong Kong

This dissertation, "Termination Criteria for High-capacity Jacked and Driven Steel H-pilesin Hong Kong" by Joley, Lam, 林早妮, 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 Termination Criteria for High-capacity Jacked and Driven Steel H-piles in Hong Kong Submitted by Joley Lam for the degree of Doctor of Philosophy at The University of Hong Kong in June 2007 In recent years, steel H-piles with high yield strength have routinely been used in Hong Kong in foundation systems, but well-accepted installation procedures have yet to be developed for jacked and driven steel H-piles. Jacked piling was recently introduced into Hong Kong with no well-established termination criteria, and many construction problems have yet to be resolved. Field measurements indicate that pile buckling is caused by loss of lateral constraint after deep voids have been formed on both sides of the webs of H-piles. To address this problem, an analytical method has been developed for assessing the critical depth of void at which pile buckling may occur. A sand filling technique has also been developed to limit the void depth and hence prevent buckling of the jacked H-piles. A systematic study has been made to investigate the settlement behaviour of high-capacity jacked H-piles under various loading conditions in order to develop termination criteria for the piles to meet the stringent residual settlement limit for loading test in Hong Kong. The results indicate that preloading the piles by a load equal to 2.3 to 2.4 times the design working capacity (WL) for about half an hour during installation can effectively reduce the creep settlement of jacked piles installed in weathered granites, and hence reduce the residual settlement that would occur after completion of the loading test to a value small enough to meet the acceptance criterion. This is called "pre-creeping" in the thesis. It is also demonstrated that repeated jacking is needed during pile installation to avoid sudden and large pile settlement when the installed pile is subsequently reloaded during loading test or construction of superstructure. For the driven H-piles, the termination criteria established decades ago are incompatible with the advanced pile driving equipment and the pile acceptance criteria adopted nowadays, creating many pile driving problems in Hong Kong. Extensive PDA tests have been conducted to verify that the existing applicable range of the allowable set of 25 to 50 mm per 10 blows is of little use in controlling pile driving stress. The results indicate that piles with C + C values (in millimetre) p q greater than 1.2 times pile length L (in metre) are susceptible to high driving stress. A simple rule for controlling pile driving stress was therefore developed by limiting the C + C values of piles to below 1.2 L. p q It is proposed to adopt the energy approach formula, which uses the impact energy determined by PDA tests to address the limitations of the Hiley formula for calculating the final set table for driven piles in Hong Kong. The energy capacity predictions are reasonably accurate, as confirmed by loading test data from various sites in Hong Kong. Local contractors usually drive the piles to a very small final set to ensure that the installed piles will meet the stringent residual settlement limit. An analysis of the local load test data indicates that piles should achieve a predicted capacity of at least 2.3 to 2.8WL to achieve desirable results. This provides a practical rule for