2023-07-14_PERMIT FILE - C1981010 ‎(16)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 7/14/2023
• Doc Name: Summary of Geotechnical Design andf Operational Cnsiderations for Highwall Mining-I,J,N, and L Pits
• Section_Exhibit Name: Appendix X
• Media Type: D
• Archive: No

January 2, 2020 Page 2-1
Agapito Associates, Inc.
2 PHYSICAL AND GEOTECHNICAL SETTING

The coal deposits at Trapper occur in Cretaceous sediments of the Upper Williams Fork
Formation (Norwest 2000). These sediments consist primarily of sandstone, mudstone and coal,
with less frequent occurrences of sandy shale and siltstone. For each HWM area, cores from holes
drilled to support the design effort were recovered and tested to determine physical properties for
input to the geotechnical design. The physical property data sets for each area are discussed in the
individual reports. A summary of the physical properties for the major rock types is given in
Table 2-1. The test type headings and abbreviations are defined as follows:

UCS Unconfined compressive strength
E Young’s modulus
S Slake durability (second cycle, percent [%])
PLCS Axial point load compressive strength
PLT-D Diametral point load Is(50) index
Density Density (as tested)
Moisture Water content (by weight)

Table 2-1. Physical Property Summary of Major Rock Types
Rock Type
UCS
(psi)
E
(106 psi)
Slake
(%)
PLCS
(psi)
PLT-D

Moisture
(%)
Density
(pcf)
Carbonaceous Mudstone 1,960 0.14 29.1 1,240 n/a 13.1% 127.2
Mudstone 2,640 0.52 66.3 1,610 44.3 7.7% 145.5
Sandstone 3,610 1.10 67.6 3,060 106.3 7.9% 143.0
Siltstone 3,910 1.49 81.4 3,180 89.8 6.2% 150.3
psi = pounds per square inch; pcf = pounds per cubic foot; n/a = not available

The data in Table 2-1 show that the properties of the coal-bounding strata are similar,
though marginally weaker than those found at other surface mines of the Western United States
(US).

Slake durability tests were specified to provide a quantitative measure of floor trafficability
and as input to Coal Mine Roof Ratings (CMRR) (Mark, Molinda, and Barton 2002) used to
quantify roof stability. Table 2-2 provides the slake durability classification (resistance to
weathering) as proposed by Franklin and Chandra (1971). According to this classification
criterion, the mudstone, sandstone, and siltstone have high durability, while carbonaceous
mudstone, which occurs frequently in the immediate roof and floor, has low durability. In general,
should floor trafficability or poor roof conditions be encountered, leaving approximately 6 inches
in the roof and/or floor should improve the situation.

Historical data shows that jointing on the property is near-vertical, with two orthogonal
joint sets trending northwest and northeast (AAI 2004).