2025-10-23_REVISION - M1982121

• Permit No: M1982121
• IBM Index Class Name: Revision
• Doc Date: 10/23/2025
• Doc Name: Interoffice Memo/Review Memo
• From: ZTT
• To: ACY
• Email Name: ACY
• Email Name: THM
• Media Type: D
• Archive: No

TR-9 Geotechnical Review Memo October 23, 2025

value being within standard published ranges, the adjustment of the cohesion to is acceptable and
reasonably representative based on the site conditions applied to the back analysis.

Seismic Parameters

As requested in the technical review memo dated April 25, 2025, GGI provided additional information,
methodology and rationale regarding the seismic parameters that were applied to the pseudo-static slope
stability analyses. Seismic applications to the geotechnical stability analysis are based off widely accepted
and standard engineering practices from the American Association of State Highway and Transportation
Officials (AASHTO) and the American Society of Civil Engineering (ASCE). The Division was able to
verify the seismic coefficient provided in the geotechnical model to site location and conditions.

Board Policies – Section 30, Table 1

Table 1. - Recommended Minimum Factors of Safety for Slope Stability Analyses for Operations and
Reclamation outlines the minimum requirements for factors of safety for the Division. Based on the
updated geotechnical analysis provided in TR-9, the resulting factors of safety (FoS) are summarized
below along with the recommended minimum FoS for critical structures using generalized, assumed, or
single test strength measurements as provided in Section 30 of the MLRB policies.

Condition East West Sect 30 - Critical
Static 1.75 1.51 1.5
Pseudostatic 1.38 1.15 1.3

When comparing the TR-9 geotechnical analysis provided to Section 30 of the MLRB policies for FoS,
one of the conditions is below the minimum outlined in Table 1. This is for the pseudostatic condition for
the west highwall. While this condition is below the minimum recommendation, Section 30.4 allows for
the Division to review geotechnical analyses on a case-by-case basis based on engineering analyses
performed in consideration of good practices as specified in relevant industry guidelines and/or
professional standards.

When reviewing the seismic condition modeled in the provided geotechnical analysis with TR-9, the size
of the modeled seismic event is probabilistically low. Additionally, the location of the mine is designated
as a low seismic hazard zone by AASHTO and ASCE guidelines. While the resulting FoS for this specific
condition is below the recommended values outlined in Table 1 of Section 30, the FoS still indicates
stability, within acceptable values of Table 1 of Section 30, and exceeds other relevant industry guidelines
and professional standards outlined in AASHTO and ASCE as well as Colorado Department of
Transportation. Based on the extensive engineering reporting and geotechnical analyses provided within
TR-6 and subsequently with TR-9, the resulting FoS are acceptable.

All adequacy items noted in the technical review memo dated April 25, 2025 have been satisfied and
there are no additional questions. This concludes my review of reviewed the provided GGI geotechnical
report, “Highwall Reclamation Stabilization” (Report), associated with Technical Revision No. 9 (TR-9)
for Mid-Continent Limestone Quarry. If you have any questions, feel free to contact me.

Sincerely,

Zach Trujillo
Senior Environmental Protection Specialist
[phone]
[e-mail]