Insitu Characteristics of Fine Coal Refuse
Jedari, C., A. Palomino, and E. C. Drumm.  2018.  Proceedings Eighth International Conference on Case Histories in Geotechnical Engineering, accepted , March 24-27, 2019, Philadelphia PA.

Fine coal refuse (FCR) is a pre-combustion by-product of the processing of raw coal. This silt-sized material is typically mixed with water and a flocculant and hydraulically placed behind coal refuse impoundments. One of the most critical concerns about the FCR is whether or not the material is prone to flow, since flow or destabilization of the in-situ FCR at several slurry impoundments has resulted in fatalities and environmental degradation. The disposal process that accompanies the production of coal assumes hydraulically placed fine coal refuse will consolidate over time to gain sufficient shear strength to prevent destabilization. However, past failures demonstrate the assumption sufficient consolidation takes place is not always true. To investigate in-situ characteristics of FCR, a field exploration study was performed on an inactive coal refuse impoundment in the Appalachian region (USA). A series of standard penetration tests (SPT) along with cone penetration test (CPT) were conducted on the crest of an FCR impoundment and undisturbed Shelby tube samples were obtained from various depths within the impoundment. Consolidation test results suggest the in-situ FCR is underconsolidated, in spite of being subjected to large overburden pressures for several decades. Based on CPT results, the FCR responds as clay to silty-clay material with low tip resistance. The physical test results show the silt-sized FCR is predominantly (about ~40%) carbon, has low plasticity, and has liquidity index values less than one. Undisturbed samples reveal intermittent layering over short intervals, with thin layers of coarser FCR separated by layers of the finer under-consolidated FCR. The SPT blow counts (N-values) in the FCR range from 7 to 15 blows per foot suggest firm to stiff material consistency and does not seem to be consistent with the observed underconsolidated material which behaves as a lower consistency material. It appears thin layers of coarser FCR material produce blow counts that obscure the presence of the soft layers of under-consolidated FCR. This suggests the SPT is not an effective means to investigate these slurry placed materials, if the intent is to identify flowable, underconsolidated materials.