Session
Hydrogeology, Verbena Room
Abstract Summary
Grouted-in vibrating wire piezometers (VWPs) are now the most widely used method of collecting pore pressure data within the mining industry. Despite their widespread usage, there is a relatively poor understanding of the following:
- Their ability to accurately measure pore pressure in ultra-low permeability settings (<10-9 m/s) where grout permeability can be several orders of magnitude greater than formation permeability.
- The factors controlling the post-grouting equilibration time of the sensor. This can extend beyond 12-months in certain settings and limit their usefulness in time-critical applications.
- The representativeness of sensor readings in heterogenous fractured rock environments where precise sensor placement adjacent to target structures may not be achievable due to uncertainties in the field data and the practicalities of installation.
- The adequacy of conventional groundwater flow theory for explaining observed pore pressure changes in low permeability environments where poroelastic behaviour may have a considerable influence on the rate of pore pressure response.
This paper addresses these issues through a comprehensive literature review and illustrative numerical modelling exercise. Several sources of uncertainty are highlighted when measuring short-term pore pressure responses to hydraulic stress but these uncertainties are less prominent when monitoring pore pressure trends over longer time periods (e.g. months to years). It is proposed that conventional groundwater flow theory cannot explain the full range of VWP responses observed in low permeability environments. The impact of poroelasticity is likely to be greatest in the period immediately after a stress change when rapid increases and decreases in pore pressure may occur as the 3D stress field equilibrates. In the medium term, the poroelastic effects may lead to a faster rate of depressurisation in a low permeability formation.
The post-grouting pore pressure equilibration period is subject to numerous uncertainties and therefore difficult to predict, and accurate estimates of hydraulic properties cannot be obtained from post-grout recovery data. Slow post-grout recoveries (months to years) can, however, be used to infer extremely low permeability in the adjacent formation. The gathering of high-resolution permeability data during drilling is recommended to provide greater understanding of the factors controlling post-grout equilibration time and VWP responses in general.
Authors
- Mark Raynor | SRK United Kingdom
- Lenar Sultanov
- Houcyne El Idrysy
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