2021-02-05_PERMIT FILE - C1980007A ‎(5)‎

• Permit No: C1980007A
• IBM Index Class Name: Permit File
• Doc Date: 2/5/2021
• Doc Name: SUBSIDENCE EVALUATION
• Section_Exhibit Name: Exhibit 60E Subsidence Evaluation for the South of Divide and Dry Fork Mining Areas
• Media Type: D
• Archive: Yes

Subsidence Evaluation for the
Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 25
7.0 EFFECTS OF TOPOGRAPHY AND STRUCTURE ON
SUBSIDENCE PROCESSES
In contrast to subsidence of rock units as fixed-end, laterally constrained, multiple plates,
subsidence in steep topography may occur as non-fixed end,laterally unconstrained multiple plates
(rock units). This lack of lateral confinement may cause reversals of horizontal displacement and
excessive tensile strain may occur on steep slopes. Peng and Hsuing (1986) found that horizontal
displacement is affected by slopes greater than 20 percent. Displacements on steep slopes and
cliffs can cause cracks to open more along faults, fractures,and joints than would occur in subdued
topography where the rock units are laterally constrained. Therefore, steep slopes and cliffs,which
commonly are susceptible to rockfalls and landslides anyway, may become less stable when
undermined.
The topography is less rugged in the Southern Panels and Sunset Trail mining areas than in the
Box Canyon mining area, while the Apache Rocks West mining area is comparable. However,
there are steep slopes and local cliffs and ledges. Therefore, these steeper slopes and cliffs may
become less stable when they are undermined.
7.1 Effects of Topography on Subsidence Cracks
Cracks commonly are wider,deeper,and may remain open longer above rigid chain pillars or mine
boundaries on steep slopes where there is little or no lateral constraint. In addition, the direction
of mining relative to slope direction may control crack width,depth, and abundance. For example,
tension cracks were observed to be wider, deeper, and more abundant on steep canyon slopes that
faced in the direction of mining than they were on slopes facing in directions opposite the mining
direction(Dunrud and Osterwald 1980, p. 26-29; Gentry and Abel 1978, p. 203-204).
Cracks are projected to be the widest and deepest on the steep slopes, cliffs, and ridges adjacent to
and on either side of Minnesota Creek and its tributaries, as well as Lick Creek, South Prong, and
Deep Creek. Maximum crack depth on these steep slopes and cliffs is estimated to locally be from
15 to as much as 35 feet deep. Due to the lack of lateral constraint,these cracks may remain open
until they are filled by processes such as sheet wash and sedimentation.
7.2 Effects of Rugged Topography on Subsidence and Mine Stresses
The subsidence factor (a) reportedly can vary significantly in draws and on ridges in rugged
topography. Gentry and Abel (1978, p. 203-204) report that vertical displacement was 25 to 30
percent greater on a ridge than it was in an adjacent draw in the York Canyon(Raton,New Mexico)
longwall mining area (Figure 4). Based on this information, the subsidence factor is projected to
be closer to 0.6 in deep draws and closer to 0.8 on isolated points and ridges in the Southern Panels,
Apache Rocks West, and Sunset Trail mining areas. No significant similar influence is expected
in these mining areas because there are few, if any, isolated ridges.
Based on observations by Dunrud in the Somerset Mine in the mid-1970s, stresses tended to be
significantly higher beneath isolated ridges than they were beneath more uniform overburden of
similar thickness. For a similar mine geometry, roof falls,bumps (rock bursts), and floor heaving
were noticeably greater beneath the ridges than they were beneath more uniform overburden of
831-032.912 Wright Water Engineers, Inc.
December 2020