Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 96

outs is not within this committee's Statement of Task, the hydrogeological
implications for impoundment siting should not be neglected in design.
A comprehensive systems approach should be applied to the entire
impoundment structure when analyzing potential failure modes. While it is
important to view the entire impoundment as a system, it is also useful to
review the modes of failure that can occur for major components of an
impoundment—the embankment and the basin. The next sections provide that
review.
EMBANKMENT FAILURE MODES
An embankment to contain coal mining waste is similar to an embankment
to store and contain mine waste in other extractive industries, where they are
usually called tailings dams. While the nomenclature is different in the coal
business—refuse impoundment versus tailings dam, fine refuse versus tailings,
and coarse refuse versus waste rock—the concept of impounding slurry behind
an engineered embankment is the same. Coal refuse impoundments are a subset
of a larger group of mine slurry impoundment systems called tailings dams.
It is common for tailings dams to be of the upstream, centerline, or
downstream types (see Chapter 3 ). While the design and construction of tailings
dams draws on the technology used for water-storage dams, tailings dams differ
from water-storage dams in a number of important ways. They store primarily
mine waste and only secondarily water. Tailings dams are often constructed
from components of the mine waste stream and are usually built by mine
operators over the life of the mine. Finally, the allowable seepage may be more
restricted than with water storage because of environmental concerns.
There have been a number of worldwide surveys of tailings dam failures
(e.g., ICOLD, 2001; Martin and Davies, 2000; USCOLD, 1994; Vick, 2000).
Because the range of experience in these reviews is much greater than that in
coal waste embankments alone, they provide a record of potential failure
modes, many of which are directly relevant to failure modes that could befall
coal waste embankments.
The U.S. Committee on Large Dams (USCOLD, 1994) defines incidents
as dam breaks or loss of impoundments leading to a release of tailings and
impoundment fluids; accidents that stressed the dam in some form without
release; and groundwater contamination. A primary factor differentiating
incident cause was the type of dam construction. The distinction of causes in
terms of their relative proportion differs between upstream-type and