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Large openpit mine closure considerations:

Dry pit or lake?

by Anne Parry and Jim Stefanoff

The Berkeley Pit    Planning for closure of a large openpit mine      could cease resulting in a pit lake with the
in Butte, MT is a       presents numerous challenges and requires     following potential benefits:
former openpit      complicated decision-making, especially when
copper mine         the minerology can generate acid rock drainage,       •	 Eliminates the need to treat pit related
that’s filled with  as is common with porphyry deposits all over the            water immediately following closure.
heavily acidic      world. Benefits of a pit lake could include the
water.              potential to reduce acid production, defer water      •	 Reduces acid production by minimizing
                    pumping and treatment costs, increase pit wall              oxidation of metal sulfides on pit walls
                    stability, and provide a convenient location to             (less oxygen would be available under
                    store site water. Careful study is recommended              the surface of the pit lake).
                    to determine if these benefits are real, or just
                    a myth. Should a pit lake be allowed to form          •	 Avoids or lowers pumping costs and
                    and to what depth? What is the expected water               other dewatering expenses.
                    quality compared to goals, and how can the
                    chemistry be controlled, if necessary? If a dry       •	 Stabilizes pit walls by applying a force
                    pit is maintained, how is that done, and how is             against the wall before pit wall pore
                    the extracted water managed? These and other                pressures increase.
                    considerations are described, along with options
                    and examples for each.                                •	 Provides water for migratory birds and
                                                                                other wildlife.
Anne Parry, P.E. and Jim Stefanoff,   Introduction
P.E., members SME, are president          Mine dewatering is              •	 Provides recreational opportunities for
of Parry Engineering in Salt Lake                                               nearby residents and travelers.
City, UT and principal engineer       needed in openpit and
with Jacobs Engineering in            underground mines to access     Discussion
Spokane, WA, respectively. Email      ore bodies and to maximize          These potential benefits may not be realized                    geotechnical stability during
                                      operations. Once an openpit     due to poorer-than-expected water quality, or
                                      mine closes, dewatering         pit wall stability issues. Saturation of the toe of
                                                                      potentially unstable slopes as the pit lake rises
                                                                      could result in geotechnical failures. If poor water
                                                                      quality develops in the pit lake, an untreated or
                                                                      poorly treated pit lake could result in bird kills
                                                                      and other risks. A well-known example of this

JANUARY 2020     Mınıng engıneerıng	                        
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