Cyclic Behavior of Fine Coal Refuse and Seismic Stability of Coal Tailings Dams
Open Access
- Author:
- Salam, Sajjad
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 11, 2020
- Committee Members:
- Ming Xiao, Dissertation Advisor/Co-Advisor
Ming Xiao, Committee Chair/Co-Chair
Tong Qiu, Committee Member
Shimin Liu, Outside Member
Murali Haran, Committee Member
Patrick Joseph Fox, Committee Member
Patrick Joseph Fox, Program Head/Chair - Keywords:
- cyclic behavior
coal tailings
liquefaction
physical modeling
PM4Silt
seismic stability
stochastic modeling
aging
strain history - Abstract:
- Coal has been one of the main sources of energy in the world. The coal waste that is produced through coal extraction and processing is typically stored in form of slurry in tailings impoundments. The static and dynamic stability of these impoundments are of great importance, as their failure can result in significant spill, loss of human lives, and damages to the environment and infrastructure. The main objectives of this research were to (1) characterize the static and dynamic geotechnical properties of fine coal refuse (FCR), (2) investigate the cyclic behavior and liquefaction resistance of FCR and influencing factors such as strain history and aging, and (3) numerically assess the seismic stability of coal tailings dams incorporating the heterogeneity on FCR deposit in the field. To characterize the static and dynamic geotechnical properties of in situ FCR samples, representative FCR samples were taken from two coal slurry impoundments in the Appalachian coalfields in the USA. Standard penetration tests (SPT) were conducted in the field. Index properties, hydraulic conductivity, and classification of FCR were determined. Staged triaxial tests under consolidated undrained (CU) state and consolidated drained (CD) state were conducted to assess short-term and long-term shear behavior of FCR, respectively. Torsional resonant column tests were performed to determine shear stiffness properties of FCR. Cyclic direct simple shear (DSS) tests followed by static shearing were adopted to evaluate the cyclic and post-cyclic behavior of FCR under various cyclic stress ratios (CSR). To overcome the shortcomings of element testing methods, large-scale shake table testing was conducted. Furthermore, the effects of strain history and short-period aging on cyclic response and liquefaction resistance of FCR were investigated. The FCR specimen was slurry-deposited in a membrane-lined laminar shear box (L×W×H: 2.29 m × 2.13 m × 1.4 m). The FCR specimen was subjected to three shaking events. Instruments including piezometers and linear variable differential transformers (LVDTs) were used to measure the FCR’s dynamic response during shaking. A piezocone penetrometer (CPTu) was used to measure soil resistance and estimate cyclic behavior of the FCR specimen before and after each shaking test for time intervals up to 97 days. The cyclic behavior, liquefaction resistance, aging rate, and strength gain within the FCR were studied and compared with those of clean sands. Dynamic loadings such as earthquakes and blasting are among the main threats to the stability of tailings dams. Seismic stability analyses of tailings dams are further challenged by the uncertainty and variability of tailings properties. The influence of input motion characteristics and spatial variability in coal tailings (CT) properties on the seismic stability of a typical upstream-construction CT dam was investigated. First, the applicability of two advanced constitutive plasticity models, PM4Sand and PM4Silt, in simulating the cyclic behavior of CT was evaluated and a suitable model was selected. The undrained shear strength of CT was modeled as a spatially correlated Gaussian random field. Six input motions representing a variety of peak ground accelerations (PGA), equivalent number of cycles (ENC), and frequency content were selected for the dynamic analyses. The seismic stability of the CT dam with uniform properties (i.e. uniform models) was compared to the stochastic models. The uncertainty in seismic response of the studied dam caused by spatial variability in geotechnical properties was investigated. The necessity of post-seismic stability analysis in CT dams was discussed. The influencing factors on the seismic stability of CT dams were also characterized.