2024-06-25_PERMIT FILE - C1980007 ‎(7)‎

• Permit No: C1980007
• IBM Index Class Name: Permit File
• Doc Date: 6/25/2024
• Doc Name: Exhibit 84 Geotechnical Assessment for Pillar Extraction in Sunset South Mains
• Section_Exhibit Name: Exhibit 84 Geotechnical Assessment for Pillar Extraction in Sunset South Mains
• Media Type: D
• Archive: No

April 22, 2024 Page 1-1
Agapito Associates, Inc.
1 INTRODUCTION
This report provides the assessment of the geotechnical parameters associated with partial
secondary pillar extraction in the E Seam in Sunset Mains between crosscuts 2 and 29 (see Figure
1-1). Following discussions with Arch West Elk (West Elk Mine), a comprehensive review of the
geotechnical environment, and the consideration of the mining constraints for the panel, Agapito
Associates, Inc. (Agapito) has made recommendations for maximum pillar extraction. Agapito
considered the following requirements:
 Pillar extraction mining will begin at crosscut 29.
 For the purpose of this assessment, it is assumed that pillar extraction will stop at 2 crosscut
at the outby end of Sunset Mains.
 In order to maintain a bleeder system around areas of gob and to inspect inby seals, two
life-of-mine roadways must remain travelable during and after pillar extraction activities.
 The mine plans to develop two additional rows of pillars along the south side of Sunset
Mains between crosscuts 2 and 13, and crosscuts 16 and 29 (see Figure 1-2).
 To avoid unstable remanent pillars, the pillar extraction should aim to mine as much of the
pillar as practically possible.
 If a large percentage of the pillar remains following extraction, the remaining pillar should
be classed as long-term stable.

The geotechnical environment was characterized from geological and geotechnical information
provided by West Elk Mine, including borehole logs from drillholes in the vicinity of the panel,
geological structure drawings, surveyed mine plan drawings, and overburden depth contour
boundaries. Agapito also drew on its extensive experience of ground condition characterization
and geotechnical design in the E Seam at the mine.
The pillar stability assessment utilized industry standard empirical pillar design formula and
criteria. The numerical model program LaModel was used to assess the detailed stress environment
during pillar extraction following extraction of the pillars in the panel.
Surface subsidence modeling was carried out using the Surface Deformation Prediction System
(SDPS) to estimate maximum surface displacement following pillar extraction in the panel.