Effects of polymer and bentonite support fluids on the performance of bored piles

Synthetic polymer fluids have been used for the construction of bored piles (drilled shafts) for more than two decades, but their effect on the performance of the completed piles is still a matter of debate. To investigate the effects of polymer and bentonite fluids on the behaviour of bored piles, a field trial comprising three full-scale instrumented test piles has been conducted at a site in East London. It was found that the two piles constructed using polymer fluids showed much stiffer load-settlement response than the one constructed using bentonite slurry. Surprisingly, an extended pile bore open time of up to 26 h was found to have no adverse effect on the piles if supported by polymer fluids. Based on the results of back-analyses using the load-transfer approach, polymer fluids were found to have little effect on the Woolwich and Reading Formations but a noticeable effect on the Upnor Formation - the mixed results are believed be due to the different soil mineralogies. It has also been shown that the common problem of 'soft toes' can be eliminated by adopting good construction practice including proper base checking and fluid cleaning or exchange procedures on site.