Ageing behaviour of polyacrylamide-based excavation fluids

Abstract

Solutions of synthetic polymers based on the chemistry of partially hydrolysed polyacrylamides (PHPAs) are increasingly being used as a replacement for the more conventional bentonite slurries in the construction of bored piles and diaphragm walls. On a construction site, these fluids may be in use for weeks to months and therefore their ageing behaviour is important. However, rather little has been published on the subject and the literature that is available is contradictory. To develop the understanding of ageing behaviour, a series of tests were undertaken on clean solutions of a commercial PHPA designed for civil engineering works. The tests showed that the PHPA fluids showed a gradual reduction in viscosity over the first days after mixing and that the rate of reduction was slightly dependent on the quality of the mix water. These findings are contrary to the rapid initial reduction in viscosity reported in some studies. An extended ageing test showed that the reduction in viscosity had effectively ceased at about 60 d from mixing with a significant proportion of the initial viscosity retained. These test results will allow foundation contractors to be more confident about the use of PHPA support fluids over extended times.