EGU22-5194, updated on 27 Mar 2022
https://doi.org/10.5194/egusphere-egu22-5194
EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

The poroelastic response of cracked Westerly granite to cyclical changes in load

Bobby Elsigood1, Nicolas Brantut1, Philip Meredith1, Tom Mitchell1, and David Healy2
Bobby Elsigood et al.
  • 1Earth Sciences, University College London, London, United Kingdom (bobby.elsigood.15@ucl.ac.uk)
  • 2School of Geosciences, University of Aberdeen, United Kingdom

The poroelastic behaviour of cracked rocks is expected to depend on the geometry and properties of the crack network. Any preferred orientation of microcracks produces anisotropy in physical rock properties, including poroelastic parameters. Under conventional triaxial loading there is an alignment of cracks parallel to the vertical direction of compression, leading to vertical transverse isotropy in the cracked rock.

Here, we repeatedly measured transversely isotropic poroelastic parameters during increasing amplitude cyclic loading in a sample of Westerly granite saturated with water. Independent step changes in confining pressure and differential stress were repeated at selected levels of differential stress to measure the change and reversibility in the transversely isotropic parameters throughout the loading and unloading cycles.

We used miniature differential pressure transducers which were located directly around the sample surface, allowing for direct measurement of the pore pressure in the sample. The direct measurements of pore pressure allow us to estimate undrained properties, including Skempton’s coefficients. Axial and radial strain gauges allow for the calculation of elastic moduli from the step changes in axial and radial stress. We determine the undrained moduli from the initial short-term response, and the drained moduli following pore pressure equilibration for each step change in stress.

Results show that the radial Skempton’s coefficient increases with increased differential stress, and the axial coefficient decreases and even becomes negative (where increases in axial stress cause a decrease in pore pressure) at high stress (i.e., about 80% of failure stress). During unloading, the measured Skempton coefficients are observed to be recovered, without hysteresis.

How to cite: Elsigood, B., Brantut, N., Meredith, P., Mitchell, T., and Healy, D.: The poroelastic response of cracked Westerly granite to cyclical changes in load, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5194, https://doi.org/10.5194/egusphere-egu22-5194, 2022.