Over 20 years of GRS pore pressure investigations at the Mont Terri Rock Laboratory (MTRL) — an overview

Claystone is a fine-grained material whose hydro-mechanical behaviour is strongly influenced by pore pressure. There is a direct dependence of pore pressure on the mechanical processes within the rock matrix, and, conversely, pore pressure affects the time-dependent deformation of the rock itself. Conventional pore-pressure monitoring relies on borehole multi-packer probes. Inflatable packers isolate test intervals that are filled with native or conditioned water; the measured interval pressure then equilibrates with the formation over time. However, this method has notable drawbacks: drilling perturbs the local pore-pressure field through both stress redistribution and air flushing, and the magnitude of disturbance increases with borehole diameter. Additionally, packers must be inflated to pressures well above the formation pressure to prevent bypass flow, introducing further bias into the pressure field. To minimize these effects, GRS developed a mini-piezometer system for ~20 mm boreholes that operates without active pressurization. Based on the mechanisms of load transfer in claystone and the dependence of hydro-mechanical behaviour on the degree of saturation and clay content, the pore-pressure measurements contribute significantly to process understanding in the various experiments.