PHPAs are water-soluble synthetic polymers which carry a negative charge on the molecules. PHPAs now used in the foundation industry have a high molecular weight, so that when dissolved in water they form a non-Newtonian solution which may be used in replacement of bentonite slurry for excavation support. Unlike bentonite, polymer fluids do not form a gel when left undisturbed (nonthixotropic) and have negligible yield stress, although they can still have very high viscosity, up to 105  MPa⋅s at low shear rates. Polymer fluids have been found to offer many benefits such as smaller site footprint, ease of fluid mixing, and better concrete–sand interface resistance.