An improved instrumentation method for PHC piles

This paper describes an instrumentation method to study the behaviour of prestressed high-strength concrete pipe piles (PHC piles) under working static and high-speed-train-induced cyclic loads. The instrumentation method refers to bond resistance-type strain gauges with steel plate inserts that are pre-installed in PHC piles during fabrication. To verify the feasibility of the instrumentation method, three instrumented PHC piles were tested in situ. The test results indicate that the proposed instrumentation method is inexpensive, reliable and promising for cyclic loading. More importantly, the method does not alter the load transfer mechanism of PHC piles. The survival rate of the resistance-type strain gauges installed on the tested piles was found to be higher than 90%. The cumulative settlements induced by high-speed-train-induced cyclic loads were significantly lower than those under working static loads (about 11·5% only), so may be practically neglected when assessing total or post-construction settlement. The unit shaft resistances decreased in clay layers (upper portion of the piles), but increased in sand layers (lower portion of the piles) up to 0·4 million loading cycles. The results from this study provide important technical information for the design of pile foundations supporting high-speed railway bridges.