Rheological Properties of PHPA Polymer Support Fluids

Synthetic polymer fluids are becoming a popular replacement for bentonite slurries to support excavations for deep foundation elements. However, the rheological properties of the polymer fluids used in excavation support have not been studied in detail, and there is currently confusion about the choice of mathematical models for this type of fluid. To advance the current state of knowledge, a laboratory study has been performed to investigate the steady-shear viscosity and transient viscoelasticity of a polymer support fluid. It is found that over the shear-rate range measurable with the Fann viscometer, an industry standard instrument, the power-law model can be used to represent the results, whereas the Bingham plastic model will significantly overestimate the viscosity at low shear. When evaluated over a much wider shear-rate range with a cone-and-plate rheometer, the polymer fluids show signs of approaching limiting viscosities at the very low and high shear rates, and for this behavior the Carreau model is more appropriate. From a series of oscillatory tests, the viscoelastic properties of the polymer fluid have been shown to be very different from those reported for their bentonite counterparts. The key engineering implications of the rheological results have been discussed.