Grouted-in Pressure Transducers: Implications for Hydrogeological Testing and Monitoring in Bedrock

Grouted-in pressure transducers are commonly deployed at open pit and underground mines to monitor hydraulic heads in bedrock near the mine working and to provide information on the progress of dewatering/depressurization efforts. Grout mixtures typically have a lower hydraulic conductivity than permeable features that act as conduits for groundwater flow in bedrock. Therefore, a question often arises as to how grout properties affect measurements of hydraulic head when compared to installations with pressure transducers embedded in sand pack or installed in conventional standpipe piezometers. A series of numerical simulations have been completed to illustrate the hypothetical response of a grouted-in pressure transducers and the response of an “ideal” instrument that is not affected by grout properties. For the range of parameters tested, these simulations indicated that the grouted-in transducers resulted in a “lag” in pressure response of a few hours following initiation of the hydraulic stress compared to the response of the sand packed transducer. Thus, care needs to be exercised when estimating hydrogeological properties from tests that rely on a network of grouted-in transducers. The numerical simulations also suggested that approximately 1 day after the initiation of hydraulic stress, the readings recorded by a grouted-in transducer were essentially the same as readings that would be obtained from an “ideal” instrument. Longer-term monitoring of hydraulic heads behind the pit walls using such instruments thus appears to be a viable alternative at mine sites.