CAREER: Understanding and Predicting the Dynamic Behavior of Mine Tailings Dam Materials

PROJECT SUMMARY Tailings dams are constructed to contain waste materials produced as a result of mining activities. Approximately one-third of the 1,555 tailings structures in the United States are high-hazard potential, where failure would result in loss of human life. Over the past 40 years, approximately 15 tailings dams have failed worldwide as a result of liquefaction of fine refuse due to earthquake shaking. Methods to analyze liquefaction resistance exist for soil, but mine tailings, including coarse and fine refuse, are inherently different than soil. Some efforts have been made towards understanding and predicting the cyclic behavior, liquefaction resistance, and shear strength of tailings materials, but a comprehensive understanding and unified approach towards the assessment of the dynamic behavior of mine tailings is lacking, partly due to the difficulties involved with testing and sampling of tailings dam structures. Therefore, a study is proposed to understand and predict the dynamic behavior of mine tailings dam materials. Laboratory resonant column testing will be used to measure the relationship between shear modulus, material damping, and shear strain. Laboratory cyclic triaxial testing will be used to determine liquefaction resistance in terms of the cyclic resistance ratio (CRR). The effect of parameters such as void ratio, fines content, age, water content, pore pressure and confining stress on the dynamic behavior of the materials will be investigated. Special measures will be taken to recover specimens for laboratory testing, including fixed piston sampling, and construction of work pads in the impoundments to reduce surface pressures of field equipment. Field standard penetration testing (SPT), cone penetrometer testing (CPT), and seismic testing will be performed, and methods will be developed to use overburden-corrected SPT blow count "Ni)60), CPT cone tip resistance (qcJ), and shear wave velocity (Vsi), to assess in situ liquefaction resistance. In situ vane shear data will be correlated with CPT data to develop a method to estimate peak and residual undrained shear strength of fine refuse for stability analyses. With respect to intellectual merit, this research will advance current understanding of the dynamic behavior of tailings dam materials. This is an area where relatively little is known, and current design methods applicable to soil are not considered appropriate for tailings materials without modification. By measuring the dynamic behavior of these materials, methods used for soils will be adapted, including in situ methods. Geoscientists will be able to better predict the response of tailings dams to dynamic excitation, which will result in improved estimates for earthquake-induced loading in terms of cyclic stress ratio (CSR). They will be able to better predict reductions in effective stress and reductions in strength due to earthquake-induced excess pore pressures and liquefaction, which will allow them to more accurately assess the stability of dams during earthquakes and under post-earthquake conditions. The application of in situ methods (Le. SPT, CPT, and Vs measurement) is within the capabilities of the typical practitioner, which will allow them to apply the relationships developed during this study to competently and cost-effectively assess undrained shear strength and CRR in coarse and fine refuse. With better estimates for shear strength, CSR, and CRR, factors of safety against liquefaction and slope stability failures will also be estimated more accurately. With respect to broader impact, application of the results of this project will allow tailings dams to be designed more safely, which will reduce property loss and loss of life due to earthquake-induced dam failures. Cased boreholes installed at two dams will open the door for future research because the boreholes can be instrumented with seismometers and the data can be used to calibrate site response models. The data acquired during this study will be added to the Network for Earthquake Engineering Simulation (NEES) repository for use by other researchers. Students from the historically impoverished region of Appalachia will participate at the university and K-12 levels, which will demonstrate the accessibility of a career in science and engineering to the students and ultimately benefit the economy of Appalachia. This research also ties into the missions and tasks of several other existing organizations and National Science Foundation (NSF) initiatives, and directly responds to the NSF's efforts through the 2003 International Workshop on Seismic Stability of Tailings Dams to better understand the dynamic behavior of tailings dams. A-I