Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 98

of investigation for site characterization, design, construction, operation, and
closure of coal refuse impoundments.
Structural Stability
Structural stability is based on the same general principles as the stability
of water-storage dams, and structural failure generally occurs by the same
processes: slope instability, liquefaction (commonly due to seismic activity),
and foundation failure. Evaluation of slope stability relates the resisting forces
of the embankment and its materials to the driving forces of the impoundment.
Evaluation of the seismic stability of the embankment is based on the seismicity
of the site and the potential for the embankment material to liquefy or lose
strength during shaking.
The committee has identified the following factors that merit special
attention in stability assessment, in particular for upstream-constructed
embankments:
• Long-term durability of the coarse refuse. Since the coarse refuse is a
component of the embankment structure, its resistance to weathering or
durability should be assessed with respect to future settlement and shear
strength. As discussed in Johnson (1999) and Linsey et al. (1982), the long-
term performance of rock depends on the rock type as well as the site
climate and setting for which the rock is used. Differing rock types vary in
durability, with certain sedimentary rocks more susceptible to weathering
due to the presence of clay minerals, which expand with moisture. Where
coarse refuse is used for critical portions of the embankment, its durability
should be assessed for the presence of these weathering minerals.
• Shear strength and higher embankments. For rockfill embankments, the
shear strength characteristics of the granular materials in the embankment
may change with increasing stresses as individual particles crush or break
(Leps, 1970; Marsal, 1973; Wilson and Marsal, 1979). The shear strength
and performance of embankment materials for a smaller embankment, for
example, may not be the same as those for a high embankment because of
particle crushing. The performance of the coarse refuse should be
confirmed by shear strength testing under anticipated loading conditions.
• Increasing clay content in fine refuse. Fine refuse impoundments contain
increasing amounts of clay, because more coal seams are