Accuracy of pore water pressure measurements using vibrating wire piezometers with fully grouted and bentonite chip seal borehole backfill methods

The fully grouted method (FGM) to backfill vibrating wire piezometers has become common geotechnical practice; however, the studies upon which acceptance of the FGM has been based considered very limited and specific, steady-state groundwater flow conditions that are not representative of many common conditions encountered in practice. Many geotechnical professionals have uncritically accepted recommendations to adopt the FGM and rely on the FGM in settings where accurate pore water pressure measurements are a matter of public safety, such as dams and hill slopes. The earlier published data that formed the basis for widespread adoption of the FGM did not consider rapid transient pressure changes, such as situations where pore water pressures resulting from application of external stresses (e.g., embankment construction) are induced in a low-permeability soil layer. This paper reports the results of steady state and transient finite element analyses of FGM installations in low permeability clay layers and provides insight into the applicability and limitations of the FGM and conditions where the FGM may result in significant errors in the unconservative direction. Results from field installations are presented that show the errors that can occur with the FGM. Practical recommendations are provided to guide practitioners in minimizing errors.