Characterizing Aquitard Properties from the response of Grouted Vibrating Wire Piezometers to Surface Loading

Abstract

Barometric Response Functions (BRF) are used to characterize the observed pore pressure response within grouted-in vibrating wire piezometers to changes in surface barometric pressure. The BRF facilitates determination of loading efficiency (λLE) which is a function of in situ compressibility. However, the mechanisms which control the characteristic shape of a BRF within a fully grouted borehole are not well understood. In this study, the transient pore pressure responses to both local instantaneous loading and transient barometric loading are used to improve our understanding of the BRF response. Two boreholes were each drilled to a depth of 200 m in a thick clay sequence in Southern Saskatchewan. One borehole was advanced through continuous coring while the other was drilled using rotary fluid circulation. Ten vibrating-wire piezometers (VWPs) were placed within each borehole at a 10m spacing. The pore pressure in all VWPs and barometric pressure was recorded concurrently for 3 years following installation. Multiple-linear regression was undertaken on both data sets to determine the BRF for each VWP. In addition, localized instantaneous surface loading was applied using heavy construction equipment. The coupled load-pore responses were simulated using a commercial coupled stress and water flow finite element model to evaluate the grout and formation hydraulic and mechanical properties. The BRF characteristics of the monitored depth profile were used to identify the limitations of linear-regression methods for determining λLE. Near-borehole influences, such as stress-release induced damage or mud filter-cake build-up, can influence the magnitude and timing of observed pore pressures. These limitations can be addressed by judicious selection of drilling methods, grouting procedures, and pressure sensor resolution. In addition, a more rigorous interpretation of the BRF can be used to obtain additional information about the in situ hydraulic and geomechanical properties of the aquitard. The rigorous analysis of measured pore pressure response to changes in external stress improves our understanding of in situ properties and the behavior of low-hydraulic conductivity and low-compressibility formations.