The Swedish Nuclear Fuel and Waste Management Co. is carrying out the Äspö Pillar Stability Experiment at the 450-m-level of the Äspö Hard Rock Laboratory. The major objectives of this experiment are to demonstrate our understanding of brittle failure (spalling) in a fractured rock mass, and the effect of confinement (backfill) on the brittle failure process,. During the experiment displacements, temperature, and acoustic emission events will be monitored through a loading cycle that starts in prepeak, i.e., elastic range and ends in the post-peak range. The rock mass that will be studied is a 1-m-wide pillar between two 1.8- m-diameter vertical cylindrical boreholes. The stresses in the pillar are controlled by the geometry of the experiment drift, the spacing of the boreholes and finally heating of the surrounding rock. The pillar response has been predicted by several numerical codes and the results from those codes will be compared to each other and the actual measured results.
1. INTRODUCTION
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 ackfill (confining pressure) on the brittle failure response, and 3) compare the 2D and 3D mechanical and thermal predicting capabilities of xisting numerical models. The rock mass that will be studied in the experiment is a 1-m-thick pillar between two vertical boreholes with the same geometry as the deposition holes described above but spaced only 1 m apart.
Previous in-situ work on brittle failure focused on massive intact granite [1] The Äspö Hard Rock Laboratory (HRL) is located in fractured rock typically found in the Scandinavian Shield, and hence provides unique opportunities to study brittle failure in a wide range of in-situ conditions. The APSE will be carried out 450m below the ground surface and at this depth the rock mass response around tunnels at Äspö HRL due to the excavationinduced stresses is essentially elastic. This provides a unique opportunity to monitor the rock mass behaviour from the elastic state through the transitional zone characterized by increased micro cracking into the final brittle failure, spalling, stage. The monitoring during this process will provide an indication of the volumetric change of the rock mass during the spalling process. The objective with these measurements is to provide a better understanding of the initiation of the brittle failure process in a fractured rock mass.
