Limiting Potential Failures - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 95

6
Limiting Potential Failures
A coal waste impoundment can be viewed as a system composed of a
series of elements that include the embankment, principal and emergency
spillways, runoff diversion structures, the basin, barriers, slurry delivery and
water recovery systems, and any additional drainage systems installed to protect
the integrity of the structure. Analysis of the ways individual elements can fail
when coupled with analysis of the impacts a specific failure or set of failures
will have on the entire system allows a designer to evaluate ways to mitigate
effects of individual failures. The objective is to design and assess facilities so
as to avoid failures that compromise the impoundment system integrity.
An embankment or basin can each fail in a number of ways. It is essential
to understand these failure modes and take appropriate measures to mitigate
them. This chapter examines embankment and basin failure modes and
mitigative measures. Particular emphasis is placed on basin failure modes and
mitigation activities, because the largest remaining uncertainties for
impoundments lie in the characterization of the basin area and in the mitigation
of risks associated with the breakthrough potential (see Chapter 3 ). Risk
assessment of new and existing impoundments is the first step toward risk
reduction. Once the impoundments with the highest risk are identified, various
methods can be explored to manage or reduce the level of risk.
Failure can be initiated through faulty construction and operation of coal
waste impoundments. The role of the impoundment operators is discussed,
including the establishment of best practices and management systems and
emergency planning and risk communication.
Creating an impoundment, particularly near the head of a valley, which is
usually a groundwater discharge zone, can also have significant consequences
for the local hydrogeological regime. In the worst case, changes in the local
hydrogeologic flow setting caused by an impoundment, could contribute to
mine-associated blow-outs that can cause flooding or other environmental
damage. Although a detailed analysis and discussion of blow-