Permeation behaviour of PHPA polymer fluids in sand

Polymer solutions are now commonly used as excavation-support fluids for the construction of bored piles (drilled shafts) and diaphragm walls around the world. Fluids based on synthetic polymers have several advantages over the conventional bentonite-based slurries, including improved foundation performance, smaller site footprint and reduced environmental impact. However, the flow behaviour of polymer fluids in soils is not well understood, and this hinders the study of the stabilising mechanisms of the support fluids. This paper presents the results of an experimental study to investigate the permeation characteristics of polymer fluids in coarse soils at the macroscopic level. To this end, constant-head permeability tests were carried out using three gradings of sand and partially hydrolysed polyacrylamide solutions over a range of concentrations, with water as a reference fluid. The results show that, under steady-state flow conditions, the hydraulic conductivity of the sand reduced from about 10(-2) m/s when tested with water to values ranging from an order of 10(-4) to 10(-7) m/s when permeated with polymer solutions. This reduction in permeability by many orders of magnitude explains why polymer fluids can stabilise excavations in sand, while water alone cannot. In addition, the rheological properties of the polymer fluids derived from the permeability tests were found to differ from those measured in a rheometer. This suggests that the permeation behaviour of polymer fluids in sand cannot be predicted from rheometer measurements alone and that the effect of soil must be considered. The findings reported in this paper will provide the basis for future investigations into the stability of excavations stabilised with polymer fluids.