The Aspö Pillar Stability Experiment (APSE), at SKB's Hard Rock Laboratory, involved the development of a pillar by excavating two deposition holes, 1m apart. The evolution of damage in the pillar was monitored by an ultrasonic acquisition system providing Acoustic Emission (AE) and ultrasonic surveying. This paper builds on previous work that revealed that spalling in the unconfined deposition hole pillar wall was concentrated in two 0.5m areas at 1m and 3m depth which correlated to higher seismic b-values thought to be a result of the greater heterogeneity. The data set of 15,198 AEs cluster tightly around the damaged zones in sufficient quantity to allow a detailed temporal analysis of changes in the seismicity prior to spalling and assess the degree to which these changes correlate to changes to the heterogeneity, local stress regime failure in the integrity of the deposition holes. The temporal and spatial analysis of patterns in seismicity were analyzed, showing minima in b-value of induced event prior to observed periods of major damage development.

1 Introduction

Constraining the volumetric extent of excavation damage zones (EDZ) around engineered structures has benefitted from the application of remote, scaled seismic studies that passively monitor the medium using microseismic (MS), acoustic emissions (AE) and ultrasonic techniques. By mapping AE/MS locations, fracturing and rock deformation can be correlated with known changes in pressure and temperature conditions both spatially and temporally (Young et al. 2004).

Earthquakes, MS and AE, are self-similar phenomena, with scale invariability which obey a power law allowing b-value analysis in combination with fracture geometry to provide additional insights into the mechanics of rock response to changes in confining pressure and temperature.

1.1 The Äspö Pillar Stability Experiment

A typical nuclear waste disposal facility could require up to 4500 deposition holes to be excavated, therefore the spacing and geometry of these installations becomes of critical importance (Andersson et al. 2009).

The Äspö Pillar Stability Experiment (APSE) at SKB's Äspö Hard Rock Laboratory (HRL) was designed to examine the nature of failure processes when an ancient, deep, mildly anisotropic, fractured crystalline rock mass is subjected to both excavation and thermal-induced stresses. The development of damage and potential spalling was monitored through the remote monitoring of induced AE and ultrasonic data to determine changes in the velocity structure. The experiment was intended to demonstrate the effect of heating and subsequent cooling in the pillar between two adjacent deposition holes within a repository (Andersson & Martin 2009). One hole was simply excavated and monitored whilst the other was subject to backfill with a confining pressure of 0.8MPa.

A tunnel boring machine was used in the excavation of two 1.75m diameter 6m deep holes, the first, DQ0066G01 was confined with a water-filled bladder delivering around 0.8MPa of confining pressure. The second, separated by a 1m thick wall, DQ0063G01was left unconfined. Both were subjected to a two-month phase of heating from mid-May of 2004, after which the confined deposition hole was slowly depressurized.

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