BeeAx: A New Biaxial Apparatus to Investigate Shallow Brittle Rock Deformation and Frictional Sliding

The mechanisms that lead a fault in the brittle crust to catastrophically fail during fluid injection remain poorly understood. In traditional rock mechanics, significant effort has been devoted to developing fluid injection experiments that reactivate small specimen surfaces over their whole area. However, in natural settings, only a portion of the fault may initially experience an increase in fluid pressure and reactivation by enlarging the area of slip.

Therefore, it is important to explore further the processes that lead to nucleation on a localized patch subjected to fluid injection to involve a larger portion of the fault that are not initially subjected to the elevated fluid pressure. To do so, we developed a new bi-axial apparatus specifically designed to investigate the nucleation phase of fluid injection-induced earthquakes on large enough sample.

This apparatus accommodates slabs of rock with a sliding surface of 15 x 17 cm and 1 cm thick. It is equipped with two servocontrolled hydraulic rams capable of delivering horizontal and vertical forces up to 450kN. The fluid pressure (both upstream and downstream) and both rams are controlled by adapted Nova-Swiss hand pumps that are independently managed by EUROTHERM process controllers, which operate MAXXON brushless electric motors via ESCON digital servo-controllers. This setup enables precise servo-control of each piston and upstream and downstream pressure in either displacement or pressure mode, using feedback from LVDTs and pressure transducers, respectively.

We designed a new double L-shaped assembly that ensures a uniform shear stress distribution on the sliding surface prior to reactivation, as shown by finite element analysis. The assembly is equipped with eight piezoelectric transducers to record acoustic emissions (AE) and to conduct active velocity surveys, as well as strain and displacement gauges placed across the sliding surface to monitor local strains and finite displacements. The slip surface can be machined to form an area with an initial stress concentration around its tip so that slip can be initiated entirely by raising the fluid pressure in the ‘crack’. The sample material chosen is Pennant sandstone, a tight sandstone with high cohesive strength and very low porosity and permeability.

Here, we present a suite of preliminary experiments ranging from conventional stable/unstable frictional sliding to fluid injection-induced fault reactivation. Acoustic emissions were utilized to monitor the evolution of the seismic b-value and to locate AE events throughout the experiments.