2014-02-13_REVISION - M1977493 ‎(2)‎

• Permit No: M1977493
• IBM Index Class Name: REVISION
• Doc Date: 2/13/2014
• Doc Name: Review assistance memorandum TR23
• From: Colorado Division of Water Resources
• To: DRMS
• Type & Sequence: TR23
• Email Name: ECS
• Media Type: D
• Archive: No

Review Assistance Memorandum — Mayflower Tailings Storage Facility (aka 5 Dam)
February 13, 2014
Page 4 of 6
It is also recommended that pore pressures in the slimes under the progressive construction of
new berms and beaches be closely monitored.
16. The scope of this seepage and stability analyses report did not include a comprehensive review
of the project history. It is recommended that the owner and the engineer closely review all past
records to identify any reports of unusual events that could affect the safety of the proposed
dam and impoundment. Such events could include plugged or failed drains, unusual or uneven
settlement, sinkholes or slumping on the dam or the beaches, elevated piezometer readings,
episodes of increased and /or cloudy seepage discharge, etc.
Operations and Maintenance Manual Review Comments
Following are comments documenting Jason Ward's review of the Operations and Maintenance
Manual, Revision 1.0 for Mayflower Tailing Storage Facility and 5 Dam, dated September 2013.
Recommendations are provided.
— Facility Overview
17. Page 3 -1, Paragraph 4, "The subdrain system will be extended as the dam is raised ", and "a
design of the subdrain system will be prepared when extension activities are required." The
method of tailings deposition results in an upstream raise of the dam and the need to extend the
subdrains is unclear. Figure 6 -1 in the associated Seepage and Stability Analyses Final Report
does not show a change in configuration of the downstream toe or depict extension of the drain
system during the "life of the mine" raise.
Recommendations:
a. Please clarify why the drain extension is needed during the raise of the dam.
b. An engineering design for the drain system extension should be submitted to DRMS for
review, prior to construction.
c. The design should include inspection and evaluation of the suitability for extension of the
existing drain system prior to construction. See related comment and recommendation in
Section 5.3.1.1.
18. Page 3 -2, Paragraph 5, "The historic decant system will be closed with grout in the future..."
Recommendations:
a. A plan for closure of the decant system should be submitted to DRMS for review, prior to
construction.
b. Results of the closure should be provided to the DRMS following construction.
Section 3.6.2 — Design Storms, Storage and Operational Considerations
19. Page 3 -8, Paragraph 2, third bullet, "It will be necessary to operate the water pool on the pond
to ensure that the water level does not exceed the prerequisite spillway crest."
Comments
a. It is unclear how operations will ensure that this level is not exceeded.
b. See comments in Section 5.2.1 for further discussion and recommendations.