The Swedish Nuclear Fuel and Waste Management Co. has carried out the Äspö Pillar Stability Experiment at the 450-m-level of the Äspö Hard Rock Laboratory. The stresses in the pillar were controlled by the geometry of the experiment drift, the spacing between the boreholes and heating of the surrounding rock. A large scale rock mass strength test was done at the end of the experiment. It was found that the observed fracturing was extensional in nature, that small confining pressure has a large impact on the fracture initiation and that the Mohr-Coulomb failure criteria gave the best estimate of the rock mass strength.
The Swedish Nuclear Fuel and Waste Management Company (SKB) is responsible for the disposal of spent nuclear fuel in Sweden. The fuel is to be placed in copper canisters that will be deposited in vertical 8-m-deep 1.8-m-diameter boreholes at 400- 700 m depth in crystalline rock. This will result in the formation of approximately 4500 rock mass pillars surrounding the emplacement boreholes. The stability of pillars in the mining industry is traditionally carried out using empirical methods. It is unknown if these methods are suitable for the design of a borehole emplacement pillar. Hence, SKB is conducting the Äspö Pillar Stability Experiment (APSE) to: 1) demonstrate our current capability to predict brittle failure (spalling) in a fractured rock mass, 2) demonstrate the effect of backfill (confining pressure) on the brittle failure response, and 3) compare the 2D and 3D mechanical and thermal predicting capabilities of existing numerical models. The rock mass that will be studied in the experiment is a 1-m-thick pillar between two vertical boreholes with practically the same geometry as the deposition holes described above but spaced only 1 m apart (Fig. 1).