Stochastic simulations of regional scale advective transport in fractured rock masses using block upscaled hydro-mechanical rock property data

A method for upscaling coupled hydro-mechanical properties in fractured rock from the small (metre) scale to the large (decametre to kilometre) scale has been developed and applied within a stochastic modelling framework. The method was implemented to assess the impact on estimates of advective travel times and transport pathways in the geosphere arising from uncertainties in the mechanical properties of a fractured host rock. A 2D hypothetical geological environment for a deep nuclear waste repository, characterized using rock and fracture property data from the site investigations at Sellafield, UK, was used for the assessment. Hydro-mechanical modelling was undertaken to obtain block scale equivalent hydraulic properties for discrete fracture networks described by power-law statistics for fracture length and density. Values of the mechanical properties for the host rock were employed to derive a basic statistical model of the uncertainty in the key fracture property values: joint compressive strength (JCS) and joint roughness coefficient (JRC). The block upscaling was carried out in two stages. First, mechanical modelling was undertaken to determine aperture distributions throughout each modelled fracture network using the distinct element code UDEC. The Barton-Bandis model for fracture closure was adopted in UDEC and each fracture network was modelled under a range of stress conditions with five (JCS, JRC) pairs that reflected the spread of mechanical properties observed in the actual field data. Second, mean block scale hydraulic conductivity tensors were determined for each stress and mechanical property combination by inverting the results from steady state hydraulic models for the mechanically controlled fracture networks. Finally, a Monte-Carlo simulation experiment was performed on a 2D profile model of the geosphere populated by sampling from the block upscaled hydraulic results. Particle tracking was applied to each realisation of the flow field in the geosphere to study the range of advective travel times from the repository to the seabed (biosphere). The simulated travel times were found to depend significantly on the rock and fracture mechanical properties with breakthrough times at the seabed ranging between 4000 years and 2,560,000 years. The spread of travel times in the simulation results demonstrate the significance of both hydro-mechanical rock properties and their spatial distribution. The very low joint compressive strength (JCS) of the uppermost formation and the assumed lateral uniformly distributed mechanical and hydraulic parameters of this formation are observed to provide the dominant control on the particle travel times. The present study suggests that observations of hydro-mechanical property data and their spatial distribution can offer useful additional information for developing models of the regional scale hydraulic behaviour of a fractured host rock mass. (C) 2009 Elsevier B.V. All rights reserved.