Selection and testing of rock for armoring uranium tailings impoundments

Pacific Northwest Laboratory has developed an approach for selecting and testing rock for its suitability and durability as armor for protecting decommissioned uranium mill tailings piles. A preliminary survey of the literature determined that existing techniques for testing rock durability were inadequate for evaluating long-term (approx.100 years) applications. Suites of rock samples with common lithologies and documented durations of exposure to weathering were then collected and submitted to three-axis ultrasonic testing in an attempt to develop a more reliable testing technique. We found little correlation between the duration of weathering and ultrasound velocity or attenuation in the rock. Through further study, we determined that the best screening approach incorporates common geomorphologic field collection techniques and laboratory tests. Suites of samples with known durations of exposure to weathering can be subjected to wet abrasion and wetting-drying tests to screen local rock types and select those with the greatest potential durability. Furthermore, the expected decrease of rock mass with environmental stresses (e.g., flood impingement and diurnal wetting-drying cycles) can be estimated using this approach. Further work was beyond the scope of this study. However, we are confident that this approach can determine the suitability for long-term armor of the most durable candidates more » using careful field sampling (including relative and absolute age-dating) and re-applying the laboratory tests. This approach, with supporting hydrologic analyses, should yield an expression of expected mass decrease as a function of time for the samples. 25 figs., 6 tabs. « less