Rubble Characteristics Associated with Room Collapse at the Waste Isolation Pilot Plant: Impact of Salt Clast Shapes and Size Distributions on the Depositional Pore System

The fluid transport properties of rubble associated with disposal room collapse within layered salt deposits at the Waste Isolation Pilot plant (WIPP) in southeastern New Mexico, USA are of concern when evaluating repository performance. Although properties of crushed salt have been studied extensively, salt rubble characteristics are less well known, while also being more difficult to characterize and model given the broader associated range in clast sizes. We describe a methodology for assessing rubble porosity and three-dimensional internal structure through deposition simulations that use high-fidelity renditions of clast shapes and that consider a broad range in clast sizes. We demonstrate the application of this approach using clast size and shape data that we collected on run of mine (ROM) material from WIPP, which represents the best available proxy for rubble material. Clast sizes from sieve analysis span ~1–100 mm with a mass weighted mean value of 13 mm. Simulated rubble deposits have interclast porosity values ranging from 34.1–38.6 vol% (n = 6). The largest clasts in these simulations tend to be underlain by the largest pores. Thus, although these clasts act as barriers for flow, they also are associated with highly permeable but localized flow paths. Scenarios involving alternative input configurations reveal that porosity values are highly sensitive to clast size variability (~10 vol% greater for nearly uniform compared to highly variable clast size distributions), clast shape (~10–13 vol% greater when using ROM shapes compared to spheres), and the extent of induced rearrangement (~3 vol% lower with limited rearrangement and ~10 vol% lower for extensive rearrangement). Porosity also is strongly affected by frictional coefficient values (~10 vol% lower for μ = 0.05 compared to μ = 1.0), but less so for values within the range of uncertainty for salt clasts (~2 vol% lower for μ = 0.62 compared to μ = 1.0).